Method of thermal treatment of cast slabs from hypereutectic intermetallide alloys based on phases γ-tial+α2-ti3al

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy. proposed method of treatment of said allows completely hardening in β-phase containing alloying elements, at least, boron and elements stabilising β-phase comprises cooling the blanks from temperatures of β-phase region. Blanks are cooled immediately after gardening or after heating and curing at temperatures of β-phase region. Note here that blanks are cooled to temperatures of (α+γ)- or (α+β+γ)-phase depending on size in air or in air in container to form thermodynamically nonequilibrium structure but at the rate smaller than that of cooling at tempering of alloy selected composition. The, blanks are cooled from temperatures of (α+γ)- or (α+β+γ)-phase region to room temperature together with the furnace or cooled in air with subsequent annealing at temperatures of (α+γ)- or (α+β-γ)-phase region and cooling after annealing together with the furnace.

EFFECT: higher performances and processing ductility.

5 cl, 5 dwg, 7 tbl, 3 ex

 

The technical FIELD TO WHICH the INVENTION RELATES.

The invention relates to methods of heat treatment of castings from zaevtektoidnyh intermetallic alloys based on the phase γ-TiAl and α2-Ti3Al. The method can be used to obtain these materials in an industrial scale castings with a regulated structure. In particular, the method can be used when machining castings used for the manufacture of parts for gas turbine engines and land-based power plants.

PRIOR art

Zaevtektoidnyh (figure 1) intermetallic alloys based on the phase γ-TiAl and α2-Ti3Al (next (γ+α2-the alloys are characterized by high heat resistance, heat resistance, resistance to oxidation and combustion, high modulus, continued until the temperature of the brittle-viscous transition (≈800°C). Potential operating temperature (γ+α2)alloys are 600...800°C possible applications - first of all, the aviation and aerospace industry.

Excellent high temperature properties (γ+α2)alloys due to an ordered atomic structure phase γ-TiAl and α2-Ti3Al due to the presence of highly directional covalent bonds between atoms of titanium and aluminum. However, it is fundamental when the other low-temperature brittleness and low technological plasticity of ingots from (γ+α 2)alloys, which limits their application in melted condition, and processing capabilities castings pressure, thermo-mechanical methods and cutting. Moreover, in the initial state castings from (γ+α2)alloys usually have a coarse lamellar structure, texture and chemical of micrometrology because of the strong dendritic segregation, which further impair the technological properties of these alloys. These structural defects are manifested to a greater extent in peritectically solidified alloys (peritectically reactions L+β⇒α, L+α⇒γ) and to a lesser extent in alloys solidified completely through the β-phase (L⇒β), bypassing protectionsee reaction (figure 1). In particular, the dendritic porosity characteristic peritectically solidified alloys, alloyed refractory elements. In this case, chemical microinhomogeneity of the material is almost a fatal flaw.

The presence of alloying elements shifts the line of the phase diagram presented in figure 1; however, in addition to the main phase γ-TiAl and α2-Ti3Al alloys can be additional phases. In particular, when the doping (γ+α2)alloys β-stabilizing elements in the alloy is present in the β-phase (ordered at temperatures below ≈1100°C with the formation of superstructures is B2).

To improve the technological and operational properties of castings from (γ+α2)alloys typically use thermal and/or thermo-mechanical processing [1-8]. This treatment in combination with a specific doping aimed at obtaining a homogeneous fully lamellar, or a duplex structure consisting of colonies γ/α2plates and equiaxial γ-grains. It should be noted that alloys with duplex structure, when the volume fraction of equiaxial grains is ~50%, markedly inferior in heat resistance and fracture toughness of the alloys with a fully lamellar structure. Reduce the heat resistance of the duplex condition due to the fact that the mechanisms of creep and diffusion creep and slippage on the limits of equiaxial grains, greatly facilitated in comparison with a fully lamellar structure. Reduction of fracture toughness in the case of duplex structures due to the reduced density of preventing crack propagation of coherent and polygenetic γ/γ, γ/α2boundaries.

The known method of thermomechanical processing of ingots from (γ+α2)alloys, allowing you to grind colony/grain and thus to improve the technological properties of the workpiece [1]. The method includes high temperature extrusion or upsetting of the billet in the shell of quasiisometry is practical or isothermal conditions. This thermomechanical processing involves significant effort and without regard to alloy composition and microstructure of the original cast billets does not always ensure uniformity of microstructure and consequently the quality of the deformed workpiece material, which is also often destroyed during deformation. In addition, this treatment almost impossible for workpieces of complex shape. It is therefore extremely relevant in respect of ingots from (γ+α2)-alloys is the problem of exclusion of thermomechanical processing and use only heat treatment in combination with certain alloying alloy.

There is a method of heat treatment of castings (γ+α2)alloys having the composition of Ti-(47-48)Al-2(Cr or Mn)-2Nb (at.%) [2]. Alloys belong to peritectically solidified alloys and ingot have coarse lamellar structure with an extended area of the columnar crystals. Due to the relatively low content of alloying elements in the alloy and through the use of in the processing of cast billet homogenization annealing is possible to reduce the effects of acute dendritic segregation. Annealing at temperatures of (α+γ)phase field makes it possible to obtain a molded workpiece duplex structure with grains/colonies d 100 μm and a volume fraction costal is affected by ~50%. In this state, the alloys have the flexibility to δ=1.5 to 2% at room temperature [2]. However, as noted above, in the presence of ~50% volume fraction of equiaxial grains is markedly reduced creep resistance at T>700°C, and fracture toughness, which limits the scope of castings treated in this way. In addition, preparations have relatively low strength, usually at the level of σIn=400-500 MPa [2], which also limits the scope of their application.

Known methods of heat treatment castings high alloy (γ+α2)alloys, which mainly composed of Ti-(38-46)Al-(5-10)Nb (at.%) [3, 4]. These alloys may optionally contain various elements such as chromium, zirconium, tantalum, lanthanum/scandium/yttrium, vanadium, iron/molybdenum, tungsten, manganese, boron and/or carbon. Processing of cast billet is heated to the temperature T>900°C and holding at that temperature over 60 minutes followed by slow cooling at a speed selected in the range of 0.5...20°C/min, Almost all of the alloys specified range, except close to the alloys based on Ti-46Al-(5-10)Nb, harden, bypassing protectionsee reaction that causes the workpieces relatively low level of dendritic segregation. However, this heat treatment does not provide General of the Elenia patterns and to improve the technological and operational properties of the workpiece of these alloys are subjected to hot pressing at temperatures T≥1250°C [5]. As already noted, the hot pressing involves considerable effort and almost impossible for workpieces of complex shape.

There is a method of heat treatment of castings (γ+α2)alloys having the composition of Ti-46Al-8(Ta or Nb) (at.%) [6], including heating and holding at a temperature α-phase region, followed by quenching in oil or air, leading to the development of a massive transformation α⇒γMacwhere γMacmassive γ-phase, and final annealing at a temperature of (α+γ)phase region. Such heat treatment in combination with the specified alloying elements (especially tantalum) provides getting crushed microstructure plate type throughout the volume of the workpiece. However, the microstructure is chemically microscopically, due to the high level of dendritic segregation in the workpiece due to the facilities of the alloys to peritectically solidified alloys and high content of refractory alloying element. In addition, obtained after quenching and annealing the microstructure thermodynamically non-equilibrium, resulting in instability of the lattice parameters of γ-TiAl phase upon heating and exposure of the workpiece in the temperature range of 800-1100°C [7].

A method of processing of ingots from (γ+α2)-alloys solidified completely is via β-phase, containing as alloying elements, at least boron and elements that stabilize the β-phase, such as niobium, molybdenum, tungsten [8].

The method includes cooling the cast billets from temperature β-phase region to room temperature at a speed lying in the range of quenching speeds. The hardening speed is provided due to the small size of cast billets and cooling water-cooled copper crucible. Cooling is carried out directly after solidification of the cast billets.

After annealing at temperatures of (α+γ)or (α+β+γ)-phase region (depending on the content of β-stabilizing elements) and aging in the cast billet homogeneous microstructure with the size of colonies/grains d≈5 to 50 μm, which ensures its superior technological plasticity.

The total crushing of the microstructure of castings is provided during β⇒α transformation due to the increased (due to the presence of borides) the rate of formation of nuclei of α-phase and a low linear growth rate OS-grains during subsequent cooling, aided by the presence of alloying elements with low diffusion mobility of atoms (Nb, Mo, W) and a large number of metastable and stable at relatively high content of β-stabilizing elements) β-phase. The presence of a high the lines share the metastable β-phase is due to the high cooling rate of the cast billets from temperature β-phase region. It should be noted that, compared with the method in [7], where the quenching of the α-phase region is massive α⇒γwt.the transformation in the way during β⇒α transformation is more effective overall refinement of the microstructure of cast billets of (γ+α2)alloy.

However, the use of high (hardening) of the speed of cooling from the temperature β-phase region leads to the formation of the blank from (γ+α2)alloy is thermodynamically non-equilibrium microstructure. Imbalance is manifested: (1) expressed microsegregation refractory alloying elements; 2) immature lamellar γ/α2the microstructure in regions with a high content of alloying elements, where diffusion is slowed down; 3) formation of a lamellar structure with a thickness of plates λ=10-100 nm, a high dislocation density and high levels of stress along the plate boundaries; 4) fixing a large number of metastable β - and α2phase; (5) high dislocation density (ρ~1010cm-2in the volume of plates/grains, which is 1-2 orders of magnitude higher than in the case of slow cooling.

To resolve these manifestations of nonequilibrium patterns required annealing at temperatures higher than the temperature of the (α+γ)/(α+β+γ)-phase region, i.e. at temperatures of α/(α+β)-or even β is the gas region; however, the microstructure in fact returned to its original state with the larger size of colonies/grains, which reduces the technological properties of the material. In the way annealing for most workpieces alloys is performed at 1250°C or lower temperature, which, as mentioned above, corresponds to (α+γ)or (α+β+γ)-phase region [8]. Such annealing does not resolve emerging thermodynamically non-equilibrium state, particularly in the case of a heat (γ+α2)alloys. Save thermodynamically nonequilibrium microstructure has no effect on technology of plasticity, which is improving thanks to the overall refinement of the microstructure of the castings, but has a negative impact on the operating properties of the produced workpieces, such as low temperature flexibility, heat resistance, creep), fatigue strength and other

Also known is a method of thermal processing of ingots from zaevtektoidnyh intermetallic alloys based on the phase γ-TiAl+α2-Ti3Al, solidified completely through β-phase containing as alloying elements boron, niobium and molybdenum, comprising cooling the workpiece from the temperature β-phase region, annealing at temperatures of (α+γ)or (α+β+γ)-phase region, and then (α2+γ)-phase region [9].

Total ismalic is the structure of castings, as in example [8], is provided in the course of solid-phase β⇒α transformation, but the resulting size of colonies/grains in the ingot of the investigated alloys is considerably larger (d=50-60 µm)than in the previous example. This can be explained by slow cooling rate of the castings from the temperature β-phase region due to their relatively large size. The heat treatment of the castings in the example of [9] is that possible to completely dissolve the β-phase. As a result, the investigated alloys after heat treatment showed a relatively high strength and superior in comparison with other cast (γ+α2)-alloys, ductility at room temperature.

Meanwhile, thermal processing, the proposed method [9], does not allow you to grind the structure of the castings to the size of colonies/grains less than 50 microns. Analysis examples [8, 9] shows that in order to obtain as small colonies/grains in the cast billet β-hardening (γ+α2)-alloy need to provide a certain level of kinetics of solid-phase β⇒α transformation, which depends on the cooling conditions of cast billets from temperature β-phase region and on the content of alloying elements - boron and β-stabilizing niobium, molybdenum and other Sizes of castings (⌀95×170 mm)used in example [9], allowed the to avoid quenching effects upon cooling from the single-phase β-region and related negative consequences, marked with the criticism of the method [8], consisting in substantial thermodynamic nonequilibrium resulting microstructure [8]. However, the decrease in the number of alloying elements - boron, niobium, molybdenum and others, getting in the cast billet microstructure even specified in [9] the size of colonies/grains will be impossible due to the fact that the kinetics of β⇒α transformation will change in an adverse manner: to increase the linear speed of growth of α grains with β⇒α transformation that will lead to rostochi/grains in the cast billet. With the reduction of the sizes of cast billets when β⇒α and subsequent phase transformations, as in example [8], the occurrence of quenching effects.

The objective of the invention is the expansion of technological capabilities of the method with increasing technological plasticity of ingots from (γ+α2)alloys and further improve their performance mechanical properties.

Technical result provided by the invention, is expressed in the optimization of heat treatment castings depending on alloy composition and size of the workpiece.

Applying the method of thermal processing of ingots from zaevtektoidnyh intermetallic alloys based on the phase γ-TiAl+α2-Ti3Al, solidified completely through the β-phase, aderrasi as alloying elements, at least boron and elements that stabilize the β-phase, which includes the cooling of the workpieces from the temperature β-phase region to room temperature.

The inventive method differs from the known method that the cooling of the billet is subjected directly after curing or after heating and holding at temperature β-phase region, and before the temperature of the (α+γ)or (α+β+γ)-phase region of the workpiece is cooled depending on the size of the blanks in the air or forced air, or air in the container with the formation of a thermodynamically non-equilibrium structure, but with a speed less than the speed of cooling during hardening of the selected composition of the alloy, then from temperature (α+γ)or (α+β+γ)-phase region to room temperature, the workpiece is cooled together with the furnace or continue to cool in air with subsequent annealing at temperatures of (α+γ)or (α+β+γ)-phase region and the cooling after annealing with the oven.

The objective is also achieved in the following cases:

- billet of the alloy on the basis of the phase γ-TiAl+α2-Ti3Al containing elements (at.%): Ti - 42-45 Al - 3-6 Nb - 1-2 Mo - 0.1 to-0.5 V, is cooled to temperatures of (α+γ)or (α+β+γ)-phase region at a rate of 0.5...10°C/sec;

- billet of the alloy on the basis of the phase γ-TiAl+α2-Ti3Al containing elements (at.%): Ti - 42-45 Al - 2-3 Nb - 02-1 Mo - 0.1 to 0.5 V, is cooled to temperatures of (α+γ)or (α+β+γ)-phase region with a speed of 5...20°C/sec;

when heating the workpiece to a temperature β-phase region, the exposure is carried out for 104-15 minutes after full warm-up;

- cooling of the workpiece in the furnace of a temperature of (α+γ)or (α+β+γ)region, including after annealing at these temperatures, carried out at a speed of not more than 0.1°C/sec.

The EXPLANATION of the INVENTION

The invention consists in the choice of the optimal heat treatment castings from (γ+α2)alloys, including cooling of the workpiece with high speed, hereinafter for brevity called "rapid cooling", when the most critical for the formation of a homogeneous structure with a relatively small size colonies/grain phase transformation of β⇒α. Rapid cooling can be carried out: 1) on the air without the use of additional techniques and tools; 2) force on the air, if you want high speed cooling; 3) in air using a container, if you want low speed cooling. In each case, for the selected alloy composition technique of accelerated cooling takes into account the size of the workpiece and is selected so that the cooling rate of the workpiece was less than the cooling rate CR is hardened.

Accelerated cooling from the temperature β-phase region up to temperature (α+γ)or (α+β+γ)-phase region provides a favorable kinetics of phase transformations and overall refinement of the microstructure, which contributes to the high rate of heterogeneous nucleation of α-grains due to the presence of borides and relatively low linear growth rate a-grains during cooling of the workpiece due to the alloying elements with low diffusion mobility of atoms, for example, niobium, molybdenum, and the presence of metastable or stable, depending on the content of β-stabilizing elements, β phase.

In the present method the resulting accelerated cooling from the temperature β-phase region up to temperature (α+γ)or (α+β+γ)-phase region is thermodynamically non-equilibrium microstructure substantially eliminated by using two alternative methods:

1) cooling temperature (α+γ)or (α+β+γ)-phase region to room temperature in the furnace;

2) cooling from temperatures of (α+γ)or (α+β+γ)-phase region to room temperature at the selected speed in the air, subsequent annealing at temperatures of (α+γ)or (α+β+γ)-phase region and the cooling after annealing from this temperature to room temperature in the furnace.

Thus, the technique, which is to slow the, together with the furnace, cooling the preform to room temperature from the temperature (α+γ)/(α+β+γ) phase region at the end when the accelerated cooling from the temperature β-phase region is equivalent to annealing the workpiece at temperatures of (α+γ)or (α+β+γ)-phase region, before rapidly cooled from the temperature β-phase region to room temperature; while in the second case, after annealing is also needed is the slow cooling of the workpiece in the furnace.

Getting in the cast billet microstructure with small size colonies/grains and achievement with thermodynamically non-equilibrium microstructure, which can be eliminated at a later stage heat treatment is possible because in the present method at a cooling temperature (α+γ)or (α+β+γ)-phase region offers reception (operation) type of normalization. Normalization is a form of heat treatment, close to the hardening [10]that for (γ+α2)-alloys as well as steels, involves cooling the workpiece in the air.

It is well known that the normalization used as thermal processing of steel billets, leads to an overall reduction of at austenitization due to phase recrystallization. Unlike steels General refinement of the structure in (γ+α2)-alloys when COI is the whether this acceptance is achieved, as noted, due to the favorable kinetics of β⇒α transformation, i.e. the mechanism of the General crushing of the microstructure of (γ+α2)-alloys is different than in steels.

The use of this technique in relation to (γ+α2)alloys, which are characterized by high sensitivity of the microstructure to the cooling rate and even a slight change, provides a fundamentally new result, since, on the one hand, allows to limit the growth of α-grains, and thus the final size of the colonies/grains in the cast billet, and on the other hand, does not create a material substantially non-equilibrium microstructure, fatal during further heat treatment at temperatures of (α+γ)or (α+β+γ)-phase region.

With a slow, together with the furnace, cooling the workpiece from the temperature (α+γ)or (α+β+γ)-phase region to room temperature or annealing the workpiece at these temperatures and the subsequent slow with the oven, cooled to room temperature, the following occurs:

1) in the workpiece are all normal diffusive phase transformations, in particular, the formation of the lamellar microstructure is completed in areas with a high content of refractory alloying elements;

2) disappears excess dislocation density formed because of the relatively quick chilled the I temperature β-phase region;

3) formed lamellar microstructure becomes more balanced - contains a relatively low dislocation density and contains almost no plates nanoscale thickness (λ=10-100 nm);

4) fully or substantially eliminates the effects of dendritic segregation;

5) significantly reduced the volume fraction of metastable β - and α2-phases.

Measurement of the lattice parameters of γ-TiAl phase, performed for treated by the present method blanks before and after exposure at T=1000°C, showed that the parameter of tetragonality is practically unchanged. This indirectly indicates thermodynamic stability of the microstructure (γ+α2)alloys processed by the present method. Rapid cooling can be carried out immediately after solidification of cast billets or after heating and holding the billet at the temperature β-phase region. The first option seems the most reasonable in the case where the workpiece is a casting, similar in form to the final product. The second option is most suitable in the case of bulk melting of the ingot, when in fact, in order to provide the required cooling rate from the temperature β-phase region is not strictly necessary, since most of the potential products of (γ+α2-Slavophile relatively small cross-section, and the required cooling rate from the temperature β-phase region can be achieved by repeated heating of the blanks cut from the bulk of the ingot and having dimensions close to the final product.

The technique of accelerated cooling of the workpiece in each case chosen experimentally depending on the degree of doping (γ+α2)alloy and the size of the workpiece. For example, alloys with a relatively high content of niobium and molybdenum (3-6 at.% Nb, 1-2 at.% Mo) should be cooled on the so-called calm air, without the use of forced cooling, since the doping of such refractory elements significantly influences the development of diffusion in the material and can lead to quenching effects. Alloys with a relatively low content of niobium and molybdenum (2-3 at.% Nb, 0.2 to 1 at.% Mo) should be cooled in still air, if the thickness of the workpiece is not great, or use forced cooling, if the workpiece has a relatively large thickness. If the alloy is additionally doped with a small amount, for example, chromium, which is a weak β-stabilizer and having close to titanium atomic radius, the diffusion characteristics of the material, and accordingly, the temperature and the kinetics of phase transformations, changed slightly, so Eminema technique of cooling the workpiece may be the same as in the case of the alloy without chromium. In case of additional alloying elements, which is a strong α - or β-stabilizers, such as silicon or vanadium, the technique of cooling air is also picked experimentally from the condition exclusions quenching cooling rates.

In the claimed invention in the process of developing the basic techniques of the method was experimentally determined cooling technique castings of alloys that can be used, for example, in the manufacture of the blades of the low-pressure turbine or compressor blades of a high pressure gas turbine engine.

Measurements made using a pyrometer, allowed to quantify the approximate range of optimal cooling rates blanks. For (γ+α2)alloys, relatively high-alloyed refractory elements containing 3-6 at.% niobium, 1-2 at.% molybdenum, 0.1-0.5 at.% boron is approximately 0.5...10°C/sec; for a relatively low-alloy, containing 2-3 at.% niobium, 0.2 to 1 at.% molybdenum, 0.1-0.5 at.% boron, which is about 5...20°C/sec. Naturally, the size of colonies/grains produced in the workpiece, will somewhat vary depending on the alloy composition and cooling rate: with increase in the degree of alloying of β-stabilizing elements and boron, as well as the speed increases ohla the Denia size of colonies/grains in the workpiece will be reduced.

The dwell time at the temperature β-phase region in the case of heating to these temperatures cast billets, pre-cooled in an arbitrary manner, after full warm-up of the workpiece, it is advisable to choose a small - 10-15 minutes because it predplavlenie temperature and diffusive transformations at these temperatures are developing very quickly.

Annealing the workpiece depending on the composition of the alloy, it is advisable to carry out at the temperature of the upper part (α+γ)or (α+β+γ)-phase region (depending on alloy composition)that will reduce the annealing time.

Cooling of the workpiece in the furnace of a temperature of (α+γ)or (α+β+γ)region, including after annealing at these temperatures, it is recommended to exercise at a speed of not more than 0.1°C/sec. In particular, such a cooling rate used after full annealing of steels [10].

When using such cooling rates all the above processes, eliminating non-equilibrium microstructure of the workpiece are the most complete.

As a result, when using all the techniques of the proposed method achieves not only the improvement of the technology of plasticity, but also the improvement of operational properties (γ+α2)alloys as low temperature flexibility, heat resistance (creep resistance), fatigue properties and other

All PE Chislennye advantages are achieved when machining castings in fact a whole class of intermetallic alloys, namely zaevtektoidnyh alloys on the basis of the phase γ-TiAl+α2-Ti3Al, solidified completely through β-phase containing as alloying components Bor and β-stabilizing elements, because the cooling rate is selected depending on the alloy composition. Thus the workpiece can be of different sizes within a size that ensures the selected speed air cooling, including the use of forced cooling or cooling in the container. If the cast bar has a relatively large sizes, not allowing cooling air to the selected speed even when using forced cooling, the ingot can be cut into smaller blanks having dimensions that will provide the selected cooling rate after heating and holding at temperature β-phase region, as stipulated by the techniques of the proposed method.

Considering the above advantages can be concluded that by means of the proposed method the objective of the invention is to expand the technological capabilities of the method with increasing technological plasticity of ingots from (γ+α2)alloys and further improve their performance mechanical properties, can be solved successfully.

LIST of DRAWINGS AND PHOTOGRAPHS EXPLAINING the ESSENCE of INVENTIONS

IG. Binary phase diagram of the state of the system Ti-Al in the area of equiatomic stock;

Figure 2. Photomicrographs of (a, b) billet alloy Ti-45Al-5Nb-1Mo-V (at.%), processed by the present method;

Figure 3. Photomicrographs of (a, b) billet alloy Ti-44Al-2,5Nb-0,3Mo-0.2B (at.%), processed by the present method; 4. A histogram of the size distribution of colonies obtained for the microstructure of the billet of the alloy Ti-44Al-2,5Nb-0,3Mo-0.2B (at.%) before (a) and after (b) heat treatment by the present method.

Figure 1 shows the binary phase diagram of the state of the system Ti-Al in the area of equiatomic composition explaining two possibilities for crystallization (γ+α2)alloys. Left arrow crystallization of alloys ingots is carried out via β-phase (L⇒β), right arrow-through protectionsee reaction (L+β⇒α, L+α⇒γ).

On figa, 3A images obtained using scanning electron microscopy mode back-scattered electrons. On figb, 3b image obtained using transmission electron microscopy. All images obtained by transmission electron microscopy, - svetlopoli image, made in the reflecting position of g(yγ)=<111>.

Presented in figure 2, 3 microstructure, as well as a histogram of the size distribution of colonies, presented the figure 4, explained in the description of examples of a specific implementation method.

The following examples do not exhaust all the possibilities of the method of thermal processing of ingots from zaevtektoidnyh intermetallic alloys based on the phases of y-TiAl and α2-Ti3Al in respect of specific alloy compositions and dimensions of the processed workpieces.

EXAMPLES of SPECIFIC IMPLEMENTATION METHOD

Example 1

As the source material was taken from the ingot of the alloy with a nominal composition of Ti-45Al-5Nb-1Mo-0,2B (in at.%). The ingot produced by vacuum arc remelting. The original dimensions of the ingot after cutting Gating parts and machining was ⌀120×180 mm Analysis of the chemical composition of the alloy showed its proximity to the nominal composition of the alloy throughout the volume of the ingot.

From an ingot carved castings of the same size is 60×13×5 mm, which was then subjected to heat treatment. Temperature control blanks, cooled in air, was performed using the pyrometer.

For carrying out the heat treatment used 2 furnace company ATS series 3350 with cantaloupe heaters.

The heat treatment by the present method, 1 and 2, used for castings of alloy Ti-45Al-5Nb-1Mo-0,2B, and description respectively received in the blanks of the microstructure are shown in table 1.

Table 1
thermal processingDescription of the microstructure: D-size colonies, d is the grain size, ρ is the dislocation density, cm-2
Mode 1:≈95% vol. - lamellar colonies (D≈30 µm) ≈5% vol. - globular (γ+β)-structure (d=2-15 μm), the content of β-phase<1-3 vol.%, p=108-5×109cm-2
1) heating and curing in an oven at T=1440°C (10 min)
2) the cooling air in the container with the speed of 3-10°C/s to room temperature,
3) place in oven and annealing at T=1250°C (1 hour),
4) cooling after annealing together with the furnace at a velocity of ≈0.1°C/sec.
Mode 2:≈93% - lamellar colonies (D≈30 µm) ≈7% vol. - globular (γ+β)-structure (d=2-15 μm), the content of β-phase ≤1-3 vol.%, the dislocation density ρ=108-
5×109cm-2
1) Heating and curing in an oven at T=1440°C (10 min)
2) the cooling air in the container with the speed of 3-10°C/sec to a temperature of 1250°C,
3) placement in another furnace, the temperature is in which To 1250°C,
4) cooling together with the second oven with a speed of ≈0.1°C/s to room temperature.

The microstructure of the processed workpiece was performed on a scanning electron microscope (Leo 1550 (Zeiss SMT) using back-scattered electrons, allowing you to see the different phases and segregation of elements. The microscope was equipped with an adapter for energy dispersive analysis. The study of the dislocation microstructure was performed on a transmission electron microscope JEM-2000EX with accelerating voltage of 200 kV. The average size of colonies and the grain size was determined by the intercept method based on the images obtained by scanning electron microscope. When determining the average size in the calculation was taken not less than 400 colonies. The dislocation density was evaluated in the γ-TiAl phase by electron-microscopic images in the usual manner. The results of the analysis of the microstructure are presented in table 1.

Figure 2 (a, b) presents the microstructure of a sample cut from billet alloy, subjected to a heat treatment for mode 1 in accordance with the claimed method. Carrying out this heat treatment leads to the formation of a homogeneous microstructure with an average size of lamellar colonies D 30 μm and a volume fraction of globular (γ+β)-component of about 5%. The dislocation density in the volume to which lone is ρ=10 8-5×109cm-2(table 1).

Heat treatment for mode 2 in accordance with the inventive method leads to the formation of identical microstructure in the alloy (table 1). This microstructure due to the said identity is not illustrated.

The blank produced in mode 1, studied from the viewpoint of the stability parameter of the tetragonality C/a γ-TiAl phase under operating conditions. To do this, the heat-treated according to mode 1 of the workpiece cut samples with a size of about 1×1×0.5 cm, which was subjected to aging at T=1000°C for 5 hours.

Potential operating temperature (γ+α2)alloys are 60...800°C. Possible applications (γ+α2)alloys is, first of all, the aviation and aerospace industry. Exposure was carried out at 1000°C, i.e. at a guaranteed higher temperature.

Measurement of the lattice parameters of γ-TiAl phase, performed by x-ray diffraction was performed using a Juiceα-radiation. Setting the tetragonality C/a was calculated by the x-ray spectra using peaks (002) and (200). Measurements showed that the parameter with the/and remained stable within the measurement error. The obtained data are shown in table 2, are indirect evidence of thermodynamic stability condition billet alloy, thermally-on is designed by the present method, that is, the achievement of the technical result of the invention.

Table 2
The condition of the alloy Ti-45Al-5Nb-1Mo-0.2BThe option of tetragonality, C/a
Processed by the present method (mode 1)1,0122±0,0003
Processed by the present method (mode 1) + annealing at T=1000°C (5 h)1,0124±0,0003

Of thermally treated in mode 1 billet alloy cut samples for subsequent mechanical testing. Samples of the workpiece is thermally treated in mode 2, not cut and therefore not tested, because the microstructure in the blanks obtained for modes 1 and 2 was identical.

Evaluation of mechanical properties was performed on short-term tensile tests and long-term 100-hour tests. For short-term tests used flat samples with a size of 20×5×2 mm sample Surface was mechanically polished and polished before testing. When measuring short-term properties tested three samples at the point at room temperature and two samples at the point - at elevated temperatures. When measuring long so the spine was tested on two samples per pixel. Mechanical tensile testing was performed in air at temperatures T=20, 700 and 750°C with an initial strain-rate ε'≈10-3with-1. For tensile tests used test machine Instron company. Tests on long 100-hour durability was carried out on the cars of Russian production 2147 P-30/1000. Testing machines - 3-the lever, the lever arm is 7:1. Testing for long-term strength was carried out at loads in the range of 300-550 MPa and temperature T=700 and 750°C. the Tests were carried out on the air. The elongation of the sample was measured after the test is completed by changing the total length of the sample, referred to the length of the working part of the sample.

Tests for fracture toughness was carried out at room temperature by the method of three-point bending. This used the rectangular specimens with Chevron notch, size 4.5×5.5×32 mm3experienced axis 2 sample by state. Fracture toughness KQ(MPa×m1/2) was calculated using the maximum load Fmaxfrom the following equation: KQ=Fmax×Ymin/B×W1/2, where W is the height, the thickness of the sample, Ymin- the minimum value of the dimensionless coefficient factor stress intensity.

For tests on high-cycle fatigue used standard samples with the dimensions of the working portion ⌀5×10 mm Ispy the project was carried out at room temperature on the basis of N=10 7cycles at a frequency of 10 Hz. The maximum tensile load was Pmax=400 MPa, and the minimum Pmin=70 MPa.

Table 3 presents the mechanical properties of tensile, creep rupture strength, fracture toughness and fatigue properties of specimens cut from heat treated billet alloy Ti-45Al-5Nb-lMo-0,2B, at different temperatures tested. It is seen that the samples treated by the present method, show a relatively high mechanical properties.

Table 3
Mode t/oMechanical properties tensile, creep rupture strength, fracture toughness and high-cycle fatigue
20°C700°C750°C
KQ,Mphm1/2σ-1MPa N=107cyclesδ, %σIn, MPaδ, %σIn, MPaMPaδ, %σIn, MPaσ100 the , MPa
No. 127>4001,056802,3700>5003,0680>370

Thus, the processing conditions of the present method provide an effective total crushing of the microstructure of cast billets of (γ+α2)-alloy and high level of mechanical properties.

Example # 2

This example differs from example 1 in that here was taken a relatively low-alloy with a nominal composition of Ti-44Al-2,5Nb-0,3Mo-0,2B (in at.%). Heat treatment was carried out under the same conditions, and using forced air cooling (air flow) and a somewhat lower temperature annealing at temperatures of (α+γ)phase field.

Figure 3 (a, b) presents the microstructure of a sample cut from billet alloy, subjected to a heat treatment in accordance with the claimed method. Thermal treatment leads to the formation of a homogeneous microstructure with an average size of lamellar colonies D≈35 μm and a minor volume fraction of globular (γ+β)-component. The density of dislocations in the bulk of the colonies status is made by ρ=10 8-109cm-2.

The heat treatment used to cast billets of alloy Ti-44Al-2,5Nb-0,3Mo-0,2B, and description of the microstructure (Fig 3 a, b) are given in table 4.

Table 4
thermal processingDescription of the microstructure: D - size colonies, d is the grain size, β is the dislocation density, cm-2
1) heating and curing in an oven at T=1440°C (10 min)≈99% vol. - lamellar colonies (D≈35 µm)
2) forced air cooling with a rate of 10-20°C/s to room temperature,≈1% vol. - globular (γ+β)-structure (d=2-15 μm), the content of β-phase - ≤0,5%vol.,
3) place in oven and annealing at T=1220°C (1 hour),ρ=108-109cm-2
4) cooling with the oven at a velocity of ≈0.1°C/sec.

Figure 4 presents histograms of the size distribution of colonies in the samples of the alloy Ti-44Al-2,5Nb-0,3Mo-0,2B, have not been subject to and subjected to a heat treatment. It is seen that heat treatment ensures the formation of smaller and is narodnoi microstructure with less than in the initial state, the average size of the colonies.

Table 5 presents the mechanical properties of tensile, creep rupture strength, fracture toughness and fatigue properties of specimens cut from heat treated billet alloy Ti-44Al-2,5Nb-0,3Mo-0,2B, at different temperatures tested. The processing conditions of the present method provide an effective total crushing of the microstructure of cast billets of (γ+α2)alloy with a low content of alloying elements, allowing you to get to the workpiece with a high level of mechanical properties.

Table 5
Mechanical properties tensile, creep rupture strength, fracture toughness and high-cycle fatigue
20°C700°C750°C
KQMPa×m1/2σ-1MPa N=107cyclesδ, %σIn, MPaδ, %σIn, MPaσ100h, MPaδ, % σIn, MPaσ100h, MPa
25>4001,16302,7650>4503,5640>350

Example # 3

This example differs from example 1 in that the heat treatment of cast billets of alloy Ti-45Al-5Nb-1Mo-0,2B (in at.%) carried out only in mode 2; in addition, the workpiece was cooled immediately after its solidification. In the manufacture of cast billets (casting) used a centrifugal casting installation company "Linn". Procurement (casting) had a cylindrical shape with dimensions ⌀16×70 mm

The heat treatment and the description obtained in the workpiece microstructure are presented in table 6. It is seen that heat treatment leads to the formation of the same microstructure as in example 1 by the present method, therefore it is not shown in the figure.

Table 6
thermal processingDescription of the microstructure: D - size colonies, d is the grain size, ρ is the dislocation density, the m -2
1) cooling of the workpiece (casting) directly after melting temperatures of T≈1250°C in air with a speed of 5-10°C./sec,≈90% vol. - lamellar colonies (D 30 µm) ≈10 vol.% - globular (γ+β)-structure (d=2-15 µm)
2) the cooling of the workpiece (casting) on temperature T≈1250°C with a velocity of ≈0.1°C/sec with oven.the content of β-phase (≤2-3 vol.%, ρ=108-5×109cm-2

Table 7 presents the mechanical properties of tensile, creep rupture strength, fracture toughness and fatigue properties of specimens cut from heat treated billet casting) alloy Ti-45Al-5Nb-1Mo-0,2B, at different temperatures tested.

Table 7
Mechanical properties tensile, creep rupture strength, fracture toughness and high-cycle fatigue
20°C700°C750°C
KQMPa×m1/2σ-1MPa N=107cyclesδ, % σIn, MPaδ, %σIn, MPaσ100h, MPaδ, %σIn, MPaσ100h, MPa
26>4001,06702,5680>5003,2650>350

The heat treatment by the present method, including including stepwise cooling to room temperature directly after manufacture of the workpiece (casting), provide an effective total crushing of the microstructure of cast billets of (γ+α2)alloy, which allows you to get to the workpiece with a high level of mechanical properties. It should be noted that the obtained properties close to the properties of the workpiece processed in accordance with the inventive method presented in example 1.

Thus, the processing conditions of the present method provide an effective total crushing of the microstructure of cast billets of (γ+α2)alloy with high and with low content of logiraamat, that allows you to get to the workpiece relatively high level of mechanical properties. This is possible thanks to the achievement of most equilibrium and homogeneous predominantly lamellar microstructure with small size colonies/grains.

SOURCES of INFORMATION

1. Semiatin, S.L., J.C. Chesnutt, Austin .et al. Processing of intermetallic alloys // Proceedings of the 2nd International Symposium "Structural Intermetallics", editors M.V. Nathal, R. Darolia, C.T. Liu et al., the Minerals Metals and Materials Society. 1997. P.263-276.

2. Kim, Y-W. and D.M. Dimiduk Designing gamma TiAl alloys: fundamentals, strategy and production // Proceedings of the 2nd International Symposium "Structural Intermetallics", editors M.V. Nathal, R. Darolia, C.T. Liu et al., the Minerals Metals and Materials Society. 1997. P.531-543.

3. Appel F., J.D.H. Paul, M. Oehring patent Application No. 20090151822 "Titanium aluminides alloys", AC22C2100FI, 18.06.2009.

4. Appel F., J.D.H. Paul, M. Oehring patent Application No. 20100000635 "Titanium aluminides alloys", AC22F118FI, 7.01.2010.

5. Appel F., M. Oehring, J.D.H. Paul Nano-scale design of TiAl alloys based on β-phase decomposition // Adv. Eng. Mater. 2006. V.8. P.371-376.

6. Saage H, Huang AJ, Hu D, Loretto MH, Wu X. Microstructures and tensile properties of massively transformed and aged Ti46A18Nb and Ti46A18Ta alloys // Intermetallics. 2009. V.17. P.32-38.

7. Imayev V, Gaisin R., Wunderlich R., R. Valiev, Fecht H-J., Formation and stability of near convoluted structure obtained in the Ti-46Al-8Ta alloy via air quenching and ageing, // Adv. Eng. Materials. 2010. V.12. N1-2. P.30-34.

8. Imaev V.M., Khismatullin YEAR, Imaev P.M. Microstructure and technological plasticity of cast intermetallic alloys on the basis of Y-TiAl // FMM. 2010. T. N4. S-443.

9. Oleneva TI, Imaev V.M., Imaev P.M., Khismatullin YEAR Structures and mechanical properties of cast β-solidifying gamma titanium aluminides, alloyed Nb, Mo, In // all-Russian youth school-conference "Modern problems of physical metallurgy", dedicated to the 100th anniversary of the Department of metallurgy of non-ferrous metals and Alloys: proceedings, hezode, rahasia. 2009. Pp.28-30.

10. Novikov I.I. Theory of heat treatment of metals // Moscow "metallurgy". 1986. 480 S.

1. The method of thermal processing of ingots from zaevtektoidnyh intermetallic alloys based on the phase γ-TiAl+α2-Ti3Al, solidified completely through β-phase containing alloying elements, at least boron and elements that stabilize the β-phase, which includes the cooling of the workpieces from the temperature β-phase region, characterized in that the cooling of the billet is subjected directly after curing or after heating and holding at temperature β-phase region, and before the temperature of the (α+γ)or (α+β+γ)-phase region of the workpiece depending on the size of the cooled air, or forced air, or air in the container with the formation of thermodynamically nonequilibrium patterns and with a speed less than the speed of cooling during hardening of this alloy composition and forth from temperature (α+γ)or (α+β+γ)-phase region to room temperature, the workpiece is cooled together with the furnace or continue to cool in air with subsequent annealing at temperatures of (α+γ)or (α+who+γ)-phase region and the cooling after annealing with the oven.

2. The method according to claim 1, characterized in that the billet of the alloy on the basis of the phase γ-TiAl+α2-Ti3Al containing, at.%: Ti - 42-45, Al - 3-6, Nb - 1-2, Mo - 0.1 to-0.5 V, is cooled to temperatures of (α+γ)or (α+β+γ)-phase region at a rate of 0.5...10°C/s

3. The method according to claim 1, characterized in that the billet of the alloy on the basis of the phase γ-TiAl+α2-Ti3Al containing, at.%: Ti - 42-45, Al - 2-3, Nb - 0,2-1, Mo - 0.1 to-0.5 V, is cooled to temperatures of (α+γ)or (α+β+γ)-phase region with a speed of 5...20°C/s

4. The method according to claim 1, characterized in that upon heating the blanks to a temperature β-phase region, the exposure is carried out for 10-15 min after full warm-up.

5. The method according to claim 1, characterized in that the cooling of the workpieces in the furnace of a temperature of (α+γ)or (α+β+γ)-phase region, including after annealing at these temperatures, carried out at a speed of not more than 0.1°C/s



 

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