RU2503738C2 - METHOD OF THERMAL TREATMENT OF CAST SLABS FROM HYPEREUTECTIC INTERMETALLIDE ALLOYS BASED ON PHASES γ-TiAl+α2-Ti3Al - Google Patents

Abstract

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

Description

FIELD OF THE INVENTION

The invention relates to methods for heat treatment of cast billets from hypereutectoid intermetallic alloys based on γ-TiAl and α ₂ -Ti ₃ Al phases. The method can be used to obtain cast billets with a regulated structure from these materials on an industrial scale. In particular, the method can be used in the processing of cast billets used for the manufacture of parts for gas turbine engines and ground-based power plants.

BACKGROUND OF THE INVENTION

Hyeutectoid (Fig. 1) intermetallic alloys based on γ-TiAl and α ₂ -Ti ₃ Al phases (hereinafter (γ + α ₂ ) alloys) are characterized by high heat resistance, heat resistance, oxidation and combustion resistance, high elastic modulus, which remains brittle-viscous transition temperature (≈800 ° C). The potential working temperatures of (γ + α ₂ ) alloys are 600 ... 800 ° C, and the possible fields of application are, above all, the aircraft industry and the aerospace industry.

The excellent high-temperature properties of (γ + α ₂ ) alloys are due to the ordered atomic structure of the γ-TiAl and α ₂ -Ti ₃ Al phases due to the presence of a sharply directed covalent bond between titanium and aluminum atoms. However, this is also a fundamental reason for the low temperature brittleness and low technological ductility of cast billets from (γ + α ₂ ) alloys, which limits both their use in the cast state and the possibility of processing cast billets by pressure, thermomechanical methods, and cutting. In addition, in the initial state, cast billets from (γ + α ₂ ) alloys usually have a coarse lamellar structure, texture and are chemically microuniform due to strong dendritic segregation, which further impairs the technological properties of these alloys. These structural deficiencies are manifested to a greater extent in peritectically hardening alloys (with the participation of peritectic reactions L + β⇒α, L + α⇒γ) and to a lesser extent in alloys hardening completely through the β phase (L⇒β), bypassing peritectic reactions (figure 1). In particular, dendritic segregation is characteristic of perectectically hardening alloys alloyed with refractory elements. In this case, the chemical microinhomogeneity of the material is a practically irreparable defect.

The presence of alloying elements biases the lines of the phase diagram shown in figure 1; in addition to the main phases of γ-TiAl and α ₂ -Ti ₃ Al, additional phases can be present in the alloys. In particular, upon alloying (γ + α ₂ ) alloys with β-stabilizing elements, the β phase is present in the alloy (being ordered at temperatures below ≈1100 ° C with the formation of B2 superstructure).

To improve the technological and operational properties of cast billets from (γ + α ₂ ) alloys, thermal and / or thermomechanical treatment is usually used [1-8]. Such treatment, in combination with a certain doping, is aimed at obtaining a homogeneous completely lamellar or duplex structure consisting of colonies of γ / α ₂ plates and equiaxed γ-grains. It should be noted that alloys with a duplex structure, when the volume fraction of equiaxial grains is ~ 50%, are noticeably inferior in terms of heat resistance and fracture toughness to alloys with a fully lamellar structure. The decrease in heat resistance in the duplex state is due to the fact that creep mechanisms — diffusion creep and slipping along equiaxed grain boundaries — are significantly facilitated in comparison with a fully lamellar structure. The decrease in fracture toughness in the case of a duplex structure is due to the reduced density of the coherent and semi-coherent γ / γ, γ / α ₂ boundaries preventing the propagation of cracks.

A known method of thermomechanical processing of cast billets from (γ + α ₂ ) alloys, which allows to grind the colony / grain and thus improve the technological properties of the billet [1]. The method includes high-temperature pressing or settling of the preform in the shell in quasi-isothermal or isothermal conditions. Such thermomechanical processing is associated with considerable labor costs and without taking into account the composition of the alloy and the microstructure of the initial cast billet, it does not always ensure the uniformity of the resulting microstructure and, accordingly, the quality of the deformed billet material, which is also often destroyed during deformation. In addition, such processing is practically impossible for workpieces of complex shape. Therefore, the problem of eliminating thermomechanical treatment and using only heat treatment in combination with a certain alloying of the alloy is extremely urgent with respect to cast billets of (γ + α ₂ ) alloys.

A known method of heat treatment of cast billets from (γ + α ₂ ) alloys having the composition Ti- (47-48) Al-2 (Cr or Mn) -2Nb (at.%) [2]. Alloys belong to peritectically hardening alloys and in the ingot have a coarse lamellar structure with an extended zone of columnar crystals. Due to the relatively low content of alloying elements in the alloy and due to the use of homogenizing annealing during processing of the cast billet, it is possible to reduce the effects of acute dendritic segregation. Annealing at temperatures of the (α + γ) -phase region allows one to obtain a duplex structure in a cast billet with a grain / colony size d 100 100 μm and a volume fraction of components of ~ 50%. In this state, the alloys have ductility up to δ = 1.5–2% at room temperature [2]. At the same time, as already noted above, in the presence of ~ 50% of the volume fraction of equiaxed grains, the creep resistance at T> 700 ° C and the fracture toughness are noticeably reduced, which limits the scope of cast billets processed by this method. In addition, the workpieces have a relatively low strength, usually at the level of σ _B = 400-500 MPa [2], which also limits the scope of their application.

Known methods of heat treatment of cast billets from highly alloyed (γ + α ₂ ) alloys having the main composition Ti- (38-46) Al- (5-10) Nb (at.%) [3, 4]. These alloys may additionally contain various elements such as chromium, zirconium, tantalum, lanthanum / scandium / yttrium, vanadium, iron / molybdenum, tungsten, manganese, boron and / or carbon. The processing of cast billets consists in heating to a temperature T> 900 ° C and holding at this temperature for more than 60 minutes, followed by slow cooling at a speed selected in the range of 0.5 ... 20 ° C / min. Almost all alloys of this range, with the exception of those close to alloys based on Ti-46Al- (5-10) Nb, solidify bypassing peritectic reactions, which leads to a relatively low level of dendritic segregation in the workpieces. However, this heat treatment does not provide a general refinement of the structure and, to improve the technological and operational properties of the billets of these alloys, are hot pressed at temperatures T≥1250 ° C [5]. As already noted, hot pressing is associated with considerable labor costs and is practically impracticable for workpieces of complex shape.

A known method of heat treatment of cast billets from (γ + α ₂ ) alloys having the composition Ti-46Al-8 (Ta or Nb) (at.%) [6], including heating and holding at the temperature of the α-phase region, followed by quenching into oil or in air, leading to the development of a massive transformation of α⇒γ _wt , where γ _wt is the massive γ phase, and the final annealing at the temperature of the (α + γ) phase region. Such a heat treatment in combination with alloying with the indicated elements (especially tantalum) provides for obtaining a crushed plate-type microstructure throughout the entire volume of the workpiece. However, the microstructure is chemically microinhomogeneous, due to the high level of dendritic segregation in the workpiece due to the alloys belonging to peritectically hardened alloys and the high content of the refractory alloying element. In addition, the microstructure obtained after quenching and annealing is thermodynamically nonequilibrium, which manifests itself in the instability of the lattice parameters of the γ-TiAl phase during heating and holding the workpiece in the temperature range 800–1100 ° C [7].

A known method of processing cast billets of (γ + α ₂ ) alloys, completely solidifying through the β phase, containing as alloying elements at least boron and elements stabilizing the β phase, such as niobium, molybdenum, tungsten [8 ].

The method includes cooling the cast billet from the temperatures of the β-phase region to room temperature with a speed lying in the range of quenching speeds. Hardening speeds are ensured due to the small size of the cast billet and cooling on a water-cooled copper crucible. Cooling is carried out directly after the solidification of the cast billet.

After annealing at temperatures of the (α + γ) - or (α + β + γ) -phase region (depending on the content of β-stabilizing elements) and aging in the cast billet, a homogeneous microstructure with a colony / grain size of d≈5 ... 50 μm is obtained , which provides its improved technological plasticity.

The overall refinement of the microstructure of the cast billets is ensured during the β⇒α transformation due to the increased (due to the presence of borides) α-phase nucleation rate and a low linear grain growth rate upon subsequent cooling, which is facilitated by the presence of alloying elements with reduced diffusion mobility of atoms ( Nb, Mo, W) and a large number of metastable and stable (with a relatively high content of β-stabilizing elements) β-phase. The presence of a high volume fraction of the metastable β-phase is caused by the high cooling rate of the cast billet from the temperatures of the β-phase region. It should be noted that in comparison with the method [7], where upon quenching from the α-phase region, massive α⇒γ _wt. transformation, in the method during β⇒α transformation, a more efficient overall grinding of the microstructure of the cast billet from the (γ + α ₂ ) alloy is achieved.

At the same time, the use of high (quenching) cooling rates from temperatures of the β-phase region leads to the formation of a thermodynamically nonequilibrium microstructure in the workpiece from the (γ + α ₂ ) alloy. Nonequilibrium is manifested: 1) in pronounced microsegregations of refractory alloying elements; 2) in an unformed lamellar γ / α ₂ microstructure in areas with a high content of alloying elements, where diffusion is slowed down; 3) in the formation of a plate structure with a plate thickness of λ = 10-100 nm, an increased dislocation density and a high level of stress along the plate boundaries; 4) in fixing a large amount of metastable β - and α ₂ phase; 5) in an increased dislocation density (ρ ~ 10 ¹⁰ cm ^-2 ) in the volume of plates / grains, which is 1-2 orders of magnitude higher than in the case of slow cooling.

To eliminate the above manifestations of the structural disequilibrium, annealing is necessary at temperatures higher than the temperatures of the (α + γ) / (α + β + γ) -phase region, i.e. at temperatures of the α / (α + β) -or even β-phase region; however, the microstructure actually returns to its original state with a larger colony / grain size, which reduces the technological properties of the material. In the method, annealing for most alloy preforms is performed at 1250 ° C or lower temperature, which, as mentioned above, corresponds to the (α + γ) -or (α + β + γ) -phase region [8]. Such annealing does not eliminate the arising thermodynamically nonequilibrium state, especially in the case of highly doped (γ + α ₂ ) alloys. The preservation of the thermodynamically nonequilibrium microstructure does not affect the technological plasticity, which is improved due to the general grinding of the microstructure of the cast billets, but negatively affects the operational properties of the obtained billets, such as low temperature ductility, heat resistance (resistance, creep), fatigue strength, etc.

There is also known a method of heat treatment of cast billets from hypereutectoid intermetallic alloys based on γ-TiAl + α ₂ -Ti ₃ Al phases, which completely solidify through the β phase, containing boron, niobium and molybdenum as alloying elements, including cooling the billet from β- temperatures phase region, annealing at temperatures of the (α + γ) - or (α + β + γ) -phase region, and then the (α ₂ + γ) -phase region [9].

The general refinement of the structure of cast billets, as in the example of [8], is provided during the solid phase β⇒α transformation, but the obtained colony / grain size in the ingots of the studied alloys is noticeably larger (d = 50-60 μm) than in the previous example. This can be explained by the slow cooling rate of cast billets from the temperatures of the β-phase region due to their relatively large sizes. The heat treatment of cast billets in the example [9] is carried out so that, if possible, completely dissolve the β-phase. As a result, the studied alloys after heat treatment showed a relatively high strength and improved plasticity at room temperature in comparison with other cast (γ + α ₂ ) alloys.

Meanwhile, the heat treatment modes proposed in the method [9] do not allow grinding the structure of cast billets to colony / grain sizes less than 50 microns. An analysis of examples [8, 9] shows that in order to obtain the smallest possible colony / grain size in the cast β-hardening (γ + α ₂ ) alloy billet, it is necessary to provide a certain kinetics of the solid-phase β⇒α transformation, which depends on the cooling conditions of the cast billet from the temperatures of the β-phase region and from the content of alloying elements — boron and β-stabilizing niobium, molybdenum, etc. The dimensions of cast billets (⌀95 × 170 mm) used in example [9] made it possible to avoid quenching effects during cooling from single-phase β -regions and related with this, the negative consequences noted during the criticism of the method [8], which consist in a significant thermodynamic nonequilibrium of the resulting microstructure [8]. However, with a decrease in the number of alloying elements — boron, niobium, molybdenum, etc., it will be impossible to obtain a microstructure in the cast billet even with the colony / grain size specified in [9] because the kinetics of β⇒α transformation will unfavorably change: the linear velocity will increase growth of α-grains during β⇒α transformation, which will lead to growth. colony / grains in the cast billet. With a decrease in the size of the processed cast billet at β⇒α and subsequent phase transformations, as in the example of [8], quenching effects may occur.

The objective of the invention is to expand the technological capabilities of the method while increasing the technological ductility of cast billets from (γ + α ₂ ) alloys and further improving their operational mechanical properties.

The technical result provided by the invention is expressed in the optimization of the heat treatment of cast billets depending on the selected alloy composition and the size of the billet.

The method of heat treatment of cast billets from hypereutectoid intermetallic alloys based on γ-TiAl + α ₂ -Ti ₃ Al phases, completely solidifying through the β phase, containing at least boron and β phase stabilizing elements as alloying elements, comprising cooling the workpieces from temperatures of the β-phase region to room temperature.

The inventive method differs from the known method in that the preforms are subjected to cooling directly after solidification or after heating and holding at temperatures of the β-phase region, and to the temperatures of the (α + γ) - or (α + β + γ) -phase region of the preform, depending on the size of the workpieces in air or forcedly in air or in air in a container with the formation of a thermodynamically nonequilibrium structure, but with a speed lower than the cooling rate when quenching the selected alloy composition, further on temperatures (α + γ ) - or (α + β + γ) -phase region to room temperature, the workpieces are cooled together with the furnace or continue to cool in air, followed by annealing at temperatures of the (α + γ) - or (α + β + γ) -phase region and cooling after annealing with the furnace.

The goal is also achieved in the following cases when:

- billets from an alloy based on γ-TiAl + α ₂ -Ti ₃ Al phases containing elements (in at.%): Ti - 42-45 Al - 3-6 Nb - 1-2 Mo - 0.1-0, 5 V, cooled to temperatures of the (α + γ) - or (α + β + γ) -phase region at a rate of 0.5 ... 10 ° C / s;

- billets from an alloy based on γ-TiAl + α ₂ -Ti ₃ Al phases containing elements (in at.%): Ti - 42-45 Al - 2-3 Nb - 0.2-1 Mo - 0.1- 0.5 V, cooled to temperatures of (α + γ) - or (α + β + γ) -phase region at a rate of 5 ... 20 ° C / s;

- when heating the workpiece to temperatures of the β-phase region, the exposure is carried out for 104-15 minutes after complete heating;

- cooling the billet in the furnace from temperatures of (α + γ) - or (α + β + γ) -regions, including after annealing at these temperatures, is carried out at a speed of not more than 0.1 ° C / s.

SUMMARY OF THE INVENTION

The essence of the invention lies in the selection of optimal heat treatment conditions for cast billets from (γ + α ₂ ) alloys, including cooling the billet with a sufficiently high speed, hereinafter referred to as “accelerated cooling” for brevity, the most critical for the formation of a homogeneous structure with a relatively small colony size / grains phase transformation β⇒α. Accelerated cooling can be carried out: 1) in air without the use of additional techniques and means; 2) forcibly in air, if an increased cooling rate is required; 3) in air using a container if a reduced cooling rate is required. In this case, for each selected alloy composition, the accelerated cooling technique takes into account the size of the workpiece and is selected so that the cooling rate of the workpiece is less than the cooling rate during quenching.

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

In the inventive method, the result of accelerated cooling from the temperatures of the β-phase region to the temperatures of the (α + γ) - or (α + β + γ) -phase region, the thermodynamically nonequilibrium microstructure is largely eliminated by using two alternative methods:

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

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

Thus, the technique consisting in slowly, along with the furnace, cooling the preform to room temperature from the temperatures of the (α + γ) / (α + β + γ) phase region upon reaching the latter with accelerated cooling from the temperatures of the β-phase region is equivalent to annealing the preform at temperatures of the (α + γ) - or (α + β + γ) -phase region, previously rapidly cooled from the temperatures of the β-phase region to room temperature; in the second case, after annealing, slow cooling of the billet in the furnace is also necessary.

The preparation of a microstructure with a small colony / grain size in the cast billet and the achievement of a thermodynamically nonequilibrium microstructure, which can be eliminated at the subsequent heat treatment stage, are possible due to the fact that in the inventive method, when cooled to temperatures (α + γ) - or (α + β + γ) -phase region, a method (operation) of the normalization type is proposed. Normalization is a type of heat treatment close to quenching [10], which for (γ + α ₂ ) alloys, as well as for steels, involves cooling the workpiece in air.

It is well known that normalization, used as a heat treatment of steel billets, leads to a general refinement of the structure during austenitization due to phase recrystallization. Unlike steels, the general refinement of the structure in (γ + α ₂ ) alloys using this technique is achieved, as noted, due to the favorable kinetics of the β⇒α transformation, i.e. the mechanism of general grinding of the microstructure in (γ + α ₂ ) alloys is different than in steels.

The use of this technique in relation to (γ + α ₂ ) alloys, which are characterized by a high sensitivity of the microstructure to the cooling rate and even to its insignificant change, provides a fundamentally new result, since, on the one hand, it allows limiting the growth of α grains, and hence the final size of the colonies / grains in the cast billet, and on the other hand, does not create a substantially nonequilibrium microstructure in the material that cannot be eliminated during further heat treatment at temperatures of the (α + γ) - or (α + β + γ) -phase region.

During slow, together with the furnace, cooling of the workpiece from temperatures of the (α + γ) - or (α + β + γ) -phase region to room temperature, or during annealing of the workpiece at these temperatures and subsequent slow, together with the furnace, cooling to room temperature the following happens:

1) all normal diffusion phase transformations take place in the workpiece, in particular, the formation of a plate microstructure is completed in areas with a high content of refractory alloying elements;

2) the excess density of dislocations formed due to relatively rapid cooling from the temperatures of the β-phase region disappears;

3) the forming plate microstructure becomes more equilibrium - it contains a relatively low density of dislocations and practically does not contain plates of nanoscale thickness (λ = 10-100 nm);

4) the consequences of dendritic segregation are completely or substantially eliminated;

5) the volume fraction of metastable β and α ₂ phases is significantly reduced.

Measurement of the lattice parameters of the γ-TiAl phase, performed for the workpiece processed by the present method before and after exposure at T = 1000 ° C, showed that the tetragonality parameter is practically unchanged. This indirectly indicates the thermodynamic stability of the microstructure of (γ + α ₂ ) alloys processed by the present method. Accelerated cooling can be carried out immediately after the solidification of the cast billet or after heating and holding the billet at temperatures of the β-phase region. The first option seems to be most appropriate when the preform is a casting that is close in shape to the final product. The second option is most suitable in the case of volumetric ingot smelting, when there is no strict need to ensure the required cooling rate from temperatures of the β-phase region, since most potential products from (γ + α ₂ ) alloys have a relatively small cross section and the required speed cooling from the temperatures of the β-phase region can be achieved by reheating a workpiece cut from a bulk ingot and having dimensions close to the final product.

The technique of accelerated cooling of a workpiece in each case is selected experimentally, depending on the degree of alloying of the (γ + α ₂ ) 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 in the so-called calm air, that is, without the use of forced cooling, since alloying with such refractory elements significantly affects the development of diffusion in the material and can lead to hardening effects. Alloys with a relatively low content of niobium and molybdenum (2-3 at.% Nb, 0.2-1 at.% Mo) should be cooled in calm air if the thickness of the workpiece is not large, or use forced cooling if the workpiece has a relatively large thickness. If the alloy is additionally alloyed with a small amount of, for example, chromium, which is a weak β-stabilizer and has an atomic radius close to titanium, then the diffusion characteristics of the material, and accordingly the temperature and kinetics of phase transformations, will change slightly, therefore, the applied workpiece cooling technique may be the same. as in the case of an alloy without chromium. In the case of additional doping with elements that are strong α- or β-stabilizers, such as, for example, silicon or vanadium, the air cooling technique is also selected experimentally from the condition that quenching cooling rates are excluded.

When creating the invention during the development of the main methods of the method, the cooling technique for casting billets from alloys was experimentally determined, which can be used, for example, in the manufacture of low pressure turbine blades or high pressure compressor blades of a gas turbine engine.

Measurements made using a pyrometer made it possible to quantify the approximate range of optimal workpiece cooling rates. For (γ + α ₂ ) alloys, relatively high-alloyed with refractory elements containing 3-6 at.% Niobium, 1-2 at.% Molybdenum, 0.1-0.5 at.% Boron, this is about 0.5 ... 10 ° C / s; for relatively low alloy alloys containing 2-3 at.% niobium, 0.2-1 at.% molybdenum, 0.1-0.5 at.% boron, this is about 5 ... 20 ° C / sec. Naturally, the size of the colonies / grains obtained in the preform will vary slightly depending on the alloy composition and cooling rate: with an increase in the degree of doping with β-stabilizing elements and boron, as well as an increase in the cooling rate, the size of the colonies / grains in the preform will decrease.

It is advisable to choose a holding time at temperatures of the β-phase region in the case of heating a cast billet preliminarily chilled to these temperatures after complete heating of the billet - 10-15 minutes, since these pre-melting temperatures and diffusion transformations develop very quickly at these temperatures.

It is advisable to anneal the billet depending on the alloy composition at the temperature of the upper part of the (α + γ) - or (α + β + γ) -phase region (depending on the alloy composition), which will reduce the annealing time.

It is recommended to cool the billet in the furnace from temperatures of the (α + γ) - or (α + β + γ) -region, including after annealing at these temperatures, at a speed of not more than 0.1 ° C / s. In particular, such a cooling rate is used after complete annealing of steels [10].

When using such cooling rates, all the processes noted above, eliminating the nonequilibrium microstructure of the workpiece, proceed most fully.

As a result, when using all the methods of the proposed method, not only is the improvement of technological plasticity achieved, but also the increase of such operational properties of (γ + α ₂ ) alloys, such as low-temperature plasticity, heat resistance (creep resistance), fatigue properties, etc.

All these advantages are achieved when machining cast billets of virtually a whole class of intermetallic alloys, namely hypereutectoid alloys based on γ-TiAl + α ₂ -Ti ₃ Al phases that completely solidify through the β phase, containing boron and β-stabilizing elements as alloying components , since the cooling rate is selected depending on the composition of the alloy. In this case, the workpieces can have different sizes within the dimensions that provide the selected cooling rate in air, including using forced cooling or cooling in the container. If the cast billet has relatively large sizes that do not allow cooling in air at a selected speed even when using forced cooling, then such an ingot can be cut into smaller billets having dimensions that will provide the selected cooling rate after heating and holding at temperatures of the β-phase region that is provided by the methods of the proposed method.

Given the above advantages, we can conclude that by the proposed method the task of the invention is to expand the technological capabilities of the method while increasing the technological ductility of cast billets from (γ + α ₂ ) alloys and further improving their operational mechanical properties, can be successfully solved.

LIST OF DRAWINGS AND PICTURES EXPLAINING THE SUMMARY OF THE INVENTIONS

Figure 1. Binary phase diagram of the state of the Ti-Al system in the region of equiatomic composition;

Figure 2. Photos of the microstructure (a, b) of a workpiece from an alloy Ti-45Al-5Nb-1Mo-0.2B (at.%) Processed by the present method;

Figure 3. Photos of the microstructure (a, b) of a workpiece from an alloy Ti-44Al-2.5Nb-0.3Mo-0.2B (at.%) Processed by the present method; Figure 4. A histogram of the colony size distribution obtained for the microstructure of a Ti-44Al-2.5Nb-0.3Mo-0.2B alloy billet (at.%) Before (a) and after (b) the heat treatment according to the claimed method.

Figure 1 presents a binary phase diagram of the state of the Ti-Al system in the region of equiatomic composition, explaining two possibilities for crystallization of (γ + α ₂ ) alloys. To the left of the arrow, the crystallization of alloy ingots is carried out completely through the β-phase (L⇒β), to the right of the arrow through peritectic reactions (L + β⇒α, L + α⇒γ).

On figa, 3A, the images were obtained using scanning electron microscopy in the mode of back-scattered electrons. On figb, 3b images were obtained using transmission electron microscopy. All images obtained by transmission electron microscopy are bright-field images taken in the reflective position g (yγ) = <111>.

The microstructures shown in FIGS. 2 and 3, as well as the histogram of the colony size distribution, shown in FIG.

The following examples do not exhaust all the possibilities of the method of heat treatment of cast billets from hypereutectoid intermetallic alloys based on the y-TiAl and α ₂ -Ti ₃ Al phases in relation to specific alloy compositions and sizes of the processed billets.

EXAMPLES OF SPECIFIC EXECUTION OF THE METHOD

Example No. 1

An alloy ingot of a nominal composition Ti-45Al-5Nb-1Mo-0.2B (in atomic%) was taken as the starting material. The ingot was made by vacuum-arc remelting. The initial dimensions of the ingot after cutting the gate part and machining were ⌀120 × 180 mm. Analysis of the chemical composition of the alloy showed its proximity to the nominal composition of the alloy over the entire volume of the ingot.

Cast billets of the same size — 60 × 13 × 5 mm — were cut from the ingot, which were then subjected to heat treatment. The temperature control of the workpieces cooled in air was carried out using a pyrometer.

For the heat treatment, 2 ATS 3350 series ovens with cantalum heaters were used.

The heat treatment modes according to the claimed method, 1 and 2, used for cast billets of Ti-45Al-5Nb-1Mo-0.2B alloy, and a description of the microstructure obtained in the billets, respectively, are given in table 1.

Table 1 Heat Treatment Modes Description of the microstructure: D is the size of the colonies, d is the grain size, ρ is the dislocation density, cm ^-2 Mode 1: ≈95 vol.% - lamellar colonies (D≈30 microns), ≈5 vol.% - globular (γ + β) -structure (d = 2-15 microns), β-phase content - <1-3 vol.% , p = 10 ⁸ -5 × 10 ⁹ cm ^-2 1) heating and aging in an oven at T = 1440 ° C (10 min), 2) cooling in air in a container at a speed of 3-10 ° C / s to room temperature, 3) placement in the furnace and annealing at T = 1250 ° C (1 hour), 4) cooling after annealing together with the furnace at a rate of ≈0.1 ° C / s. Mode 2: ≈93 vol.% - lamellar colonies (D≈30 microns), ≈7 vol.% - globular (γ + β) -structure (d = 2-15 microns), β-phase content - ≤1-3 vol.% , dislocation density ρ = 10 ⁸ -
5 × 10 ⁹ cm ^-2 1) Heating and holding in a furnace at Т = 1440 ° С (10 min), 2) air cooling in the container at a speed of 3-10 ° C / s to a temperature of "1250 ° C, 3) placement in another furnace, the temperature of which is 1250 ° C, 4) cooling together with the second furnace at a rate of ≈0.1 ° C / s to room temperature.

The microstructure analysis of the processed workpiece was carried out on a Leo-1550 scanning electron microscope (Zeiss SMT) in the backscattered electron mode, which allows one to see various phases and segregation of elements. The microscope was equipped with an attachment for energy dispersive analysis. The study of the dislocation microstructure was carried out on a JEM-2000EX transmission electron microscope with an accelerating voltage of 200 kV. The average colony size and grain size were determined by the secant method from the images obtained on a scanning electron microscope. When determining the average size, at least 400 colonies were taken into account. The dislocation density was estimated in the γ-TiAl phase according to electron microscopy images by the usual method. The results of the analysis of the microstructure are presented in table 1.

Figure 2 (a, b) shows the microstructure of a sample cut from an alloy billet subjected to heat treatment according to mode 1 in accordance with the inventive method. Carrying out such 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 of the colonies is ρ = 10 ⁸ -5 × 10 ⁹ cm ^-2 (Table 1).

The heat treatment according to mode 2 in accordance with the inventive method leads to the formation of an identical microstructure in the alloy (table 1). This microstructure due to the marked identity is not illustrated.

The preform obtained according to regime 1 was studied from the point of view of stability of the tetragonality parameter of the c / a γ-TiAl phase under operating conditions. For this purpose, samples with a size of about 1 × 1 × 0.5 cm were cut from a heat-treated according to mode 1, which were subjected to exposure at T = 1000 ° C for 5 hours.

Potential working temperatures of (γ + α ₂ ) alloys are 60 ... 800 ° С. Possible applications of (γ + α ₂ ) alloys are, above all, the aircraft industry and the aerospace industry. The exposure was carried out at 1000 ° C, i.e. at guaranteed higher temperature.

The lattice parameters of the γ-TiAl phase were measured using x-ray diffraction using CoK _α radiation. The tetragonality parameter c / a was calculated from X-ray spectra using the (002) and (200) peaks. The measurements showed that the parameter c / a remained stable, within the limits of the measurement error. The obtained data, shown in table 2, indirectly indicate the thermodynamic stability of the state of the alloy billet, thermally processed by the present method, that is, to achieve the technical result of the invention.

table 2 The state of the alloy Ti-45Al-5Nb-1Mo-0.2B The tetragonality parameter, s / a Processed by the claimed method (mode 1) 1.0122 ± 0.0003 Processed by the present method (mode 1) + annealing at T = 1000 ° C (5 hours) 1.0124 ± 0.0003

Samples for subsequent mechanical tests were cut from the heat-treated mode 1 alloy preform. Samples from the preform thermally treated according to regime 2 were not cut out and, accordingly, were not tested, since the microstructure in the preforms obtained according to regimes 1 and 2 was identical.

The mechanical properties were evaluated by short-term tensile tests and lengthy 100-hour tests. For short-term tests, flat samples with a part size of 20 × 5 × 2 mm were used. The surface of the samples was mechanically ground and polished before testing. When measuring short-term properties, three samples per point were tested at room temperature and two samples per point at elevated temperatures. When measuring long-term strength, two samples per point were tested. Mechanical tensile tests were carried out in air at temperatures T = 20, 700, and 750 ° C with an initial strain rate of ε ≈ 10 ⁻³ s ⁻¹ . For tensile tests, an Instron test machine was used. Tests for a long 100-hour strength were carried out on Russian-made machines 2147 P-30/1000. Testing machines - 3-lever, lever arm - 7: 1. Long-term strength tests were carried out at loads in the range of 300-550 MPa and temperatures T = 700 and 750 ° C. The tests were carried out in air. The elongation of the sample was measured after completion of the test by changing the total length of the sample, relative to the length of the working part of the sample.

The fracture toughness test was carried out at room temperature using the three-point bending method. For this, rectangular samples with a chevron notch, dimensions 4.5 × 5.5 × 32 mm ^{3 were used} ; the axis was tested for 2 samples per state. The fracture toughness K _Q (MPa × m ^1/2 ) was calculated using the maximum load F _max from the following ratio: K _Q = F _max × Y _min / B × W ^1/2 , where W is the height, B is the thickness of the sample, Y _min - the minimum value of the dimensionless coefficient of the stress intensity factor.

For tests on multi-cycle fatigue, standard samples with a working part of ⌀5 × 10 mm were used. The tests were carried out at room temperature based on N = 10 ⁷ cycles at a frequency of 10 Hz. The maximum tensile load was P _max = 400 MPa, the minimum - P _min = 70 MPa.

Table 3 presents the mechanical tensile properties, long-term strength, fracture toughness and fatigue properties of samples cut from a heat-treated billet of Ti-45Al-5Nb-lMo-0.2B alloy at various test temperatures. It can be seen that the samples processed by the present method show relatively high mechanical properties.

Table 3 T / o mode Mechanical tensile properties, long-term strength, fracture toughness and multi-cycle fatigue 20 ° C 700 ° C 750 ° C K _Q , MPahm ^1/2 σ _-1 MPa N = 10 ⁷ cycles δ,% σ _V , MPa δ,% σ _V , MPa MPa δ,% σ _V , MPa σ _100h , MPa No. 1 27 > 400 1.05 680 2,3 700 > 500 3.0 680 > 370

Thus, the processing modes of the present method provide effective overall grinding of the microstructure of the cast billet from the (γ + α ₂ ) alloy and an increased level of mechanical properties.

Example No. 2

This example differs from example 1 in that a relatively low alloy alloy of a nominal composition Ti-44Al-2.5Nb-0.3Mo-0.2B (in atomic percent) was taken here. Heat treatment was carried out according to one regime using forced cooling in air (in an air stream) and a slightly lower annealing temperature at temperatures of the (α + γ) phase region.

Figure 3 (a, b) shows the microstructure of a sample cut from an alloy billet subjected to heat treatment in accordance with the claimed method. Heat treatment leads to the formation of a homogeneous microstructure with an average size of lamellar colonies D≈35 μm and an insignificant volume fraction of the globular (γ + β) component. The dislocation density in the volume of the colonies is ρ = 10 ⁸ -10 ⁹ cm ^-2 .

The heat treatment modes used for the cast billet of the Ti-44Al-2.5Nb-0.3Mo-0.2B alloy, and a description of the obtained microstructure (Fig. 3 a, b) are given in Table 4.

Table 4 Heat Treatment Modes Description of the microstructure: D is the size of the colonies, d is the grain size, β is the density of dislocations, cm ^-2 1) heating and aging in an oven at T = 1440 ° C (10 min), ≈99 vol.% - lamellar colonies (D≈35 microns), 2) forced cooling in air at a rate of 10-20 ° C / s to room temperature, ≈1 vol.% - globular (γ + β) -structure (d = 2-15 μm), β-phase content - ≤0.5 vol.%, 3) placement in the furnace and annealing at T = 1220 ° C (1 hour), ρ = 10 ⁸ -10 ⁹ cm ^-2 4) cooling together with the furnace at a rate of ≈0.1 ° C / s.

Figure 4 presents the histograms of the size distribution of the colonies in samples of the alloy Ti-44Al-2.5Nb-0.3Mo-0.2B, not subjected and subjected to heat treatment. It can be seen that heat treatment provides the formation of a finer and more uniform microstructure with a smaller average colony size than in the initial state.

Table 5 presents the mechanical tensile properties, long-term strength, fracture toughness and fatigue properties of samples cut from heat-treated preforms of Ti-44Al-2.5Nb-0.3Mo-0.2B alloy at various test temperatures. The processing modes of the present method provide effective overall grinding of the microstructure of the cast billet from the (γ + α ₂ ) alloy with a low content of alloying elements, which allows to obtain a high level of mechanical properties for the processed billet.

Table 5 Mechanical tensile properties, long-term strength, fracture toughness and multi-cycle fatigue 20 ° C 700 ° C 750 ° C K _Q , MPa × m ^1/2 σ _-1 MPa N = 10 ⁷ cycles δ,% σ _V , MPa δ,% σ _V , MPa σ _100h , MPa δ,% σ _V , MPa σ _100h , MPa 25 > 400 1,1 630 2.7 650 > 450 3,5 640 > 350

Example No. 3

This example differs from example 1 in that the heat treatment of the cast billet of the Ti-45Al-5Nb-1Mo-0.2B alloy (in at.%) Was carried out only according to mode 2; in addition, the preform was cooled immediately after it solidified. In the manufacture of cast billets (castings), a Linn centrifugal casting plant was used. The blank (casting) had a cylindrical shape with dimensions ⌀16 × 70 mm.

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

Table 6 Heat Treatment Modes Description of the microstructure: D — colony size, d — grain size, ρ — dislocation density, cm ^-2 1) cooling the preform (casting) immediately after its smelting to a temperature of T≈1250 ° C in air at a speed of 5-10 ° C / s, ≈90 vol.% - lamellar colonies (D »30 microns), ≈10 vol.% - globular (γ + β) -structure (d = 2-15 microns), 2) cooling the workpiece (casting) from a temperature of T≈1250 ° C at a speed of ≈0.1 ° C / s together with the furnace. β-phase content - ≤2-3 vol.%, ρ = 10 ⁸ -5 × 10 ⁹ cm ^-2

Table 7 presents the mechanical tensile properties, long-term strength, fracture toughness and fatigue properties of samples cut from heat-treated blanks (castings) of Ti-45Al-5Nb-1Mo-0.2B alloy at various test temperatures.

Table 7 Mechanical tensile properties, long-term strength, fracture toughness and multi-cycle fatigue 20 ° C 700 ° C 750 ° C K _Q , MPa × m ^1/2 σ _-1 MPa N = 10 ⁷ cycles δ,% σ _V , MPa δ,% σ _V , MPa σ _100h , MPa δ,% σ _V , MPa σ _100h , MPa 26 > 400 1,0 670 2,5 680 > 500 3.2 650 > 350

The heat treatment modes according to the claimed method, including including step-by-step cooling to room temperature immediately after the manufacture of the billet (casting), provide effective overall grinding of the microstructure of the cast billet from the (γ + α ₂ ) alloy, which allows to obtain a high billet for the workpiece level of mechanical properties. It should be noted that the obtained properties are close to the properties of the workpiece processed in accordance with the inventive method presented in example 1.

Thus, the processing modes of the present method provide effective overall grinding of the microstructure of the cast billet from the (γ + α ₂ ) alloy with both high and low content of alloying elements, which allows to obtain a relatively high level of mechanical properties for the workpiece. This is possible due to the achievement of the most equilibrium and homogeneous predominantly lamellar microstructure with a small colony / grain size.

INFORMATION SOURCES

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Claims (5)

1. The method of heat treatment of cast billets from hypereutectoid intermetallic alloys based on γ-TiAl + α ₂ -Ti ₃ Al phases, completely solidifying through the β phase, containing alloying elements, at least boron, and elements stabilizing the β phase, including cooling the preforms from the temperatures of the β-phase region, characterized in that the preforms are cooled immediately after solidification or after heating and holding at temperatures of the β-phase region, to temperatures of (α + γ) - or (α + β + γ) -phase dependency areas and the size is cooled in air, or forcibly in air, or in air in a container with the formation of a thermodynamically nonequilibrium structure and at a speed lower than the cooling rate when quenching a given alloy composition, and then from temperatures (α + γ) - or (α + The β + γ) phase region to room temperature the preforms are cooled together with the furnace or continue to cool in air, followed by annealing at temperatures of the (α + γ) - or (α + β + γ) phase region and cooling after annealing with the furnace. 2. The method according to claim 1, characterized in that the alloy billets based on γ-TiAl + α ₂ -Ti ₃ Al phases, containing, at.%: Ti - 42-45, Al - 3-6, Nb - 1 -2, Mo - 0.1-0.5 V, cooled to temperatures of the (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 billets from an alloy based on γ-TiAl + α ₂ -Ti ₃ Al phases, containing, at.%: Ti - 42-45, Al - 2-3, Nb - 0 , 2-1, Mo - 0.1-0.5 V, cooled to temperatures of the (α + γ) - or (α + β + γ) -phase region at a rate of 5 ... 20 ° C / s. 4. The method according to claim 1, characterized in that when the billets are heated to temperatures of the β-phase region, the exposure is carried out for 10-15 minutes after complete heating. 5. The method according to claim 1, characterized in that the cooling of the workpieces in the furnace from temperatures of the (α + γ) - or (α + β + γ) -phase region, including after annealing at these temperatures, is carried out at a speed of not more 0.1 ° C / s.

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