CN104328501B - TiAl single crystal alloy with fully controllable lamellar orientation and preparation method thereof - Google Patents

Abstract

The present invention discloses a TiAl intermetallic compound single crystal with fully controllable lamella orientation and a preparation method thereof. The composition expression of the TiAl alloy material of the present invention is Ti _a Al _b Nb _c (atomic percentage), wherein 42≤a≤55, 43≤b≤49, 2≤c≤9, and a+b+c=100. The preparation method of the TiAl alloy single crystal with controllable lamella orientation of the present invention is: electromagnetic induction suspension melting of TiAl alloy master alloy ingot in a water-cooled copper crucible, and casting into a directional solidification rod by suction casting; placing the test rod in a high-purity corundum crucible with a high-purity yttrium oxide coating sintered on the inner wall for Bridgman directional solidification; controlling the solid-state phase change process by changing the solidification parameters, so that the lamella orientation of the single crystal is fully controllable, and obtaining a TiAl intermetallic compound single crystal with a lamella orientation parallel to the growth direction.

Description

A TiAl single crystal alloy with fully controllable lamellar orientation and its preparation method

technical field

The invention belongs to the technical field of intermetallic compound materials, and in particular relates to a TiAl single crystal alloy with fully controllable lamellar orientation and a preparation method thereof.

Background technique

TiAl intermetallic compound is a new type of light-weight high-temperature structural material, its specific gravity is less than 50% of that of nickel-based superalloys, and it has high specific strength, high specific rigidity, corrosion resistance, wear resistance, high temperature resistance and excellent oxidation resistance. It is an ideal material to replace Ni-based superalloys due to its advantages such as creep resistance.

The mechanical properties of fully lamellar TiAl alloys are closely related to their lamellar orientation. Through the research on the full-sheet polysynthetic twinned crystal PST (Polysynthetic twinned crystal) with a single orientation, it is found that its strength and plasticity present obvious anisotropy. Due to the anisotropy of the full lamellar structure, when the lamellar orientation is appropriate, it is more suitable for the service conditions of aeroengine blades that require high temperature resistance and are only subjected to one-dimensional direction loads. It will undoubtedly be extremely beneficial if the TiAl alloy can be directional solidified to produce a fully lamellar engine blade, and the orientation of the lamellar structure is parallel to the axial direction of the blade (the growth direction of the crystal in directional solidification). Yamaguchi et al. systematically studied the effect of TiAl alloy sheet orientation on mechanical properties, and found that when the load direction is parallel to the sheet orientation, the yield strength and elongation achieve the best combination. Therefore, in order to further improve the performance of TiAl alloys, it is necessary to control the lamellar orientation of the final structure to obtain a TiAl intermetallic compound single crystal full lamellar structure whose orientation is consistent with the loading direction.

At present, the methods for controlling the lamellar orientation of TiAl alloy at home and abroad mainly include the seed crystal method and the non-seed crystal method that changes the solidification path. Yamaguchi, Johnson et al. used the α-phase solidification seed crystal method, selected Ti-Al-Si alloy as the seed crystal, and obtained a single crystal PST with the lamellar orientation completely parallel to the growth direction through the necking crystal selection method. The composition of the seed crystal is usually different from the composition of the master alloy, resulting in uneven composition and properties of the directionally solidified alloy, and the preparation process of the seed crystal is complicated. Therefore, the seed crystal method has obvious disadvantages.

In the non-seed method, there is no research at home and abroad to obtain a full-lamellar TiAl single crystal structure parallel to the growth direction. Under low G/V conditions, Lin Junpin et al. used the "double directional solidification" method for Ti-46Al-5Nb alloy to obtain a full-lamellar single-crystal structure parallel to the growth direction. They believe that the lower G/V process makes the β-phase dendrite spacing under suitable conditions, and the single-oriented α-phase parallel to the growth direction can be obtained through the all-peritectic reaction without solid-state phase transition β→α Generation of alpha variants in different phases allows for control over sheet orientation. This method requires two times of directional solidification of the same process, which is one more solidification process than the common non-seed method, which aggravates the pollution of the crucible material to the alloy, and is unfavorable to the industrialization of the directional solidification TiAl alloy.

Previous domestic and foreign studies on the control of lamellar orientation by the non-seed method are all about changing the solidification path, which cannot control the orientation of the single wafer layer, and have not obtained a single wafer layer structure that is completely parallel to the growth direction. In order to solve this technical problem, the solid-state directional phase transformation process of TiAl alloy becomes the key to controlling the lamellar orientation. It can be seen from the phase diagram that the solid-state phase transformations of β→α and α→α ₂ +γ must be experienced after solidification of the full-lamellar TiAl alloy. When the primary phase is β phase, the preferred growth direction is <001>, and its phase relationship is: {110} _β //{0001} _α //{111} _γ ^[25] , and the 12 variables of {110} _β Among them, 4 are parallel to the growth direction, and 8 are inclined at 45° to the growth direction ^[16,26] . Only 1/3 of the habit planes in the lamellar structures formed after solid-state phase transition are oriented parallel to the growth direction. Obviously, the orientation of the final lamellar structure of TiAl alloy depends not only on the growth direction of the primary β phase, but also on the subsequent solid-state phase transformation process. Therefore, the β→α solid-state phase transition process is also the key to controlling the lamellar orientation. So far, the studies on the orientation control of TiAl sheets have focused on the solidification process, but neglected the solid-state phase transition process after solidification.

Therefore, it is not only necessary to control the solidification process so that the primary phase of directional solidification is β phase, but also to control the nucleation and growth of the new phase and the phase boundary directional migration process during the directional solid phase transformation of TiAl alloy, so that it can Only the lamellar orientation at 0° to the growth direction is retained, and the control of the lamellar orientation of the TiAl alloy under the condition of continuous directional liquid-solid phase transition-solid phase transition is completed.

Contents of the invention

The object of the present invention is to provide a TiAl intermetallic compound single crystal with fully controllable lamellar orientation and a preparation method thereof by controlling the continuous directional liquid-solid phase transition-solid phase transition process.

A single crystal of TiAl intermetallic compound whose lamellar orientation is fully controllable. In atomic percent, its alloy composition expression is Ti _a Al _b Nb _c , where, 42≤a≤55, 43≤b≤49, 2≤ c≤9, a+b+c=100.

The preparation of the TiAl intermetallic compound single crystal with fully controllable lamellar orientation includes the following specific steps:

The first step: select Ti, Al, Nb pure metal raw materials with a purity of 99.999%, and mix them according to the alloy composition, and melt the master alloy in a cold crucible suspension melting furnace with a vacuum degree of less than 10 ^-3 Pa. After 3 to 4 The secondary smelting makes the alloy composition homogeneous, and sucks and casts it into a directionally solidified bar;

Step 2: Put the TiAl alloy test rod into a high-purity yttrium oxide-coated corundum crucible for directional solidification, vacuumize to 5×10 ^-3 Pa, and then fill the system with high-purity argon protective gas;

Step 3: Adjust the power of the induction power supply to heat the sample. The holding temperature is 1450-1650K, and the holding time is 15-30 minutes. The directional solidification starts, and the pulling rate of the directional solidification is controlled to be 5-20 μm/s; continue to grow until the sample At the length of 50 mm, the rapid quenching is started to perform rapid quenching treatment on the directional solidified sample, and the solid-liquid interface is retained.

The diameter of the directionally solidified bar in the first step is Φ (4-6mm)×100mm.

In the second step, the size of the high-purity yttrium oxide-coated corundum crucible is Φ(7-9mm)×100mm; the filling amount of high-purity argon protective gas is 0.04-0.06MPa.

The principle of the present invention: use the Bridgman directional solidification method to control the orientation of the TiAl alloy sheet, by changing the temperature gradient of the solidification parameter and the growth rate, firstly ensure that the primary phase is a full β phase, and secondly obtain a single crystal through competition and elimination of grains during the solidification process, and In the solidification process, there is a critical temperature corresponding to a specific pulling rate. At this pulling rate, the α phase with the final sheet orientation and growth direction at 45° is eliminated through phase boundary migration, so that the β→α phase transition obtained Among the 12 α variables, only the α phase whose final lamellar orientation is parallel to the growth direction is retained, thereby completing the control of lamellar orientation.

Compared with the prior art, the present invention has the following advantages: 1. Using the common Bridgman directional solidification method, by adjusting the solidification parameters, the continuous directional liquid-solid phase transition-directional solid phase transition is controlled to ensure the growth of all β phases and through solidification in the solid state The phase transformation controls the final lamellar orientation, and obtains a TiAl alloy single crystal structure in which the lamellar orientation is completely parallel to the growth direction.

2. The present invention effectively avoids the disadvantage of non-uniform composition and properties of the seed crystal method, and at the same time obtains an ideal lamellar-oriented single crystal structure in a single directional solidification process, which simplifies the process.

3. In the process of preparing TiAl alloy single crystal, the present invention can completely control the orientation of its single wafer layer under a certain range of solidification parameters. The invention provides a theoretical basis for the industrial application of the directionally solidified TiAl alloy.

Description of drawings

Fig. 1 is a partial Ti-Al binary alloy phase diagram in the prior art.

Fig. 2 is a microstructure diagram of the maximum longitudinal section (a) and sheet orientation (b) of the directionally solidified sample of the present invention.

Fig. 3 is a microstructure diagram of the longitudinal section of the competition section of the directional solidification sample of the present invention.

Fig. 4 is a microstructure diagram of the maximum longitudinal section (a) and lamellar orientation (b) of the directionally solidified sample of the present invention.

Fig. 5 is a microstructure diagram of the longitudinal section of the competition section of the directional solidification sample of the present invention.

Fig. 6 is a microstructure diagram of the maximum longitudinal section (a) and lamellar orientation (b) of the directionally solidified sample of the present invention.

Fig. 7 is the quenched solid-liquid interface of the directional solidification sample of the present invention.

Note: The direction of microstructure growth in Figure 2-7 is from right to left.

detailed description

The present invention provides a fully controllable TiAl intermetallic compound single crystal and its preparation method, the specific implementation of which is as follows:

(1) Select Ti-Al-Nb ternary alloy whose primary phase is all β phase. According to the multi-element alloy phase diagram and the principle of phase selection, as shown in Figure 1, by adjusting the ratio relationship between the atomic components, all the first precipitated phases are β phases. Specifically, increasing the content of Nb and reducing the relative proportion of Al form a wider β-phase region.

(2) According to the alloy composition obtained in 1), high-purity metal components are used to configure, and under the protection of high-purity Ar gas, the master alloy is melted by cold crucible electromagnetic levitation melting equipment. The master alloy is smelted several times to obtain a uniform master alloy ingot, which is suction-cast into a master alloy rod.

(3) Put the TiAl alloy rod into a corundum crucible with sintered high-purity yttrium oxide coating on the inner wall. The size of the corundum crucible is Φ(5-8mm)×100mm, put it into the Bridgman directional solidification furnace, and set the vacuum degree to 5×10- At 3Pa, fill it with 0.04-0.06MPa high-purity argon protective gas.

(4) Adjust the power of the induction power supply to heat the sample, the holding temperature is 1450-1650K, the holding time is 15-30min, and the directional solidification is started, and the directional solidification growth rate is controlled to be 5-20 μm/s;

(5) Continue to grow at a certain rate until the length of the sample is 50 mm, start rapid quenching to perform rapid quenching on the directional solidified sample, and retain the solid-liquid interface.

The present invention will be further described below in conjunction with specific examples.

Example 1

The alloy composition used in the experiment is Ti ₄₇ Al ₄₅ Nb ₈ (atomic percentage at%), and the purity of its metal components is 99.999%. Under the protection of high-purity Ar gas, a cold crucible electromagnetic The suspension melting equipment melts the master alloy. A uniform master alloy ingot was obtained after 4 times of smelting, and suction-cast into a Φ4×100mm master alloy bar. Put the TiAl alloy test rod into a corundum crucible coated with high-purity yttrium oxide for directional solidification experiment, vacuumize to 5×10-3Pa, and then fill the system with 0.05MPa high-purity argon protective gas. , adjust the power of the induction power supply to heat the sample, the holding temperature is 1550K, the holding time is 25min, the directional solidification starts, and the growth rate of the directional solidification is controlled to be 5μm/s; when the drawing length reaches the sample length of 50mm, the rapid quenching is started The sample was subjected to rapid quenching to retain the solid-liquid interface. Characterize the microstructure of the largest longitudinal section of the cylindrical sample, observe and analyze the solidified pre-precipitated phase, grain size and lamellar orientation at the pulling rate, as shown in Figure 2(a) and Figure 2(b) As shown, it was found that a TiAl alloy single crystal with lamellar orientation parallel to the growth direction was obtained. When the growth rate is as small as 5 μm/s, the enrichment of solute can be fully diffused, the growth can be carried out stably, and the grains have sufficient time to grow, so the obtained grains are relatively coarse until single crystal growth is obtained.

Fig. 3 is the microstructure of the competitive segment of directional solidification at 5 μm/s. Since in the β→α solid-state phase transition, due to the difference in the degree of mismatch between the two interfaces forming 0° and 45° sheets, the mobility of different phase boundaries is different, so there is a critical pulling rate of 5 μm/s, in Below this pulling rate, the growth driving force of 0° grains is larger after nucleation of α grains with 0° and 45° lamellar orientations, and finally the 45° grains are eliminated, and a single crystal with lamellar orientations parallel to the growth direction is obtained. .

Example 2

Using the same alloy composition and method as in Example 1, the holding temperature is 1550K, the holding time is 25min, and directional solidification begins, and the directional solidification growth rate is controlled to be 15 μm/s; as shown in Figure 4 (a) and Figure 4 (b) It shows that the β→α solid-state phase transformation at this pulling rate retains the α phase with a 45° lamellar orientation, so the final structure is a single crystal with a 45° lamellar orientation.

Fig. 5 is the microstructure of the competitive segment of directional solidification at 15 μm/s. At this pulling rate, the driving force of the 45° grain solid phase deformation nucleus is greater than that of the 0° grain, so that the 0° grain cannot grow, and a TiAl alloy single crystal with a lamellar orientation and a growth direction of 45° is obtained.

Example 3

Using the same alloy composition and method as in Example 1, the holding temperature is 1550K, the holding time is 25min, directional solidification begins, and the growth rate of directional solidification is controlled to be 20 μm/s; as shown in Figure 6(a) and Figure 6(b) It was shown that a single crystal with a lamellar orientation at 45° to the growth direction was obtained.

Figure 7 shows the solid-liquid interface retained by the rapid quenching treatment. The dendrite growth morphology is 4-fold symmetric, with obvious secondary dendrites and a 90° vertical relationship with the primary dendrite stem. It can be inferred that during the directional solidification process Among them, the β phase of the cubic crystal system is the primary phase.

Example 4

Using the same method as in Example 1, the alloy composition used is Ti ₅₅ Al ₄₃ Nb ₂ , the holding temperature is 1650K, the holding time is 30 minutes, and the growth rate of directional solidification is 5 μm/s to obtain TiAl with lamellar orientation parallel to the growth direction. Alloy single crystal.

Example 5

Using the same method as in Example 1, the alloy composition used is Ti ₄₈ Al ₄₃ Nb ₉ , the holding temperature is 1450K, the holding time is 30 minutes, and the directional solidification growth rate is 10 μm/s, and the lamellar orientation and the growth direction are 45°. TiAl alloy single crystal.

Example 6

Using the same method as in Example 1, the alloy composition used is Ti ₅₁ Al ₄₅ Nb ₆ , the holding temperature is 1650K, the holding time is 15 minutes, and the directional solidification growth rate is 5 μm/s to obtain TiAl with lamellar orientation parallel to the growth direction. Alloy single crystal.

Example 7

Using the same method as in Example 1, the alloy composition used is Ti ₄₂ Al ₄₉ Nb ₉ , the holding temperature is 1550K, the holding time is 25 minutes, and the directional solidification growth rate is 5 μm/s, and the TiAl with lamellar orientation parallel to the growth direction is obtained. Alloy single crystal.

Claims (6)

1. the fully controllable TiAl intermetallic compound monocrystalline of a kind of lamellar orientation, it is characterised in that with atomic percentage, its Alloying component expression formula is Ti_aAl_bNb_c, wherein, 42≤a≤55,43≤b≤49,2≤c≤9, a+b+c=100, wherein, by with It is prepared by lower step：

The first step：Ti, Al, Nb simple metal raw material that purity is 99.999% are chosen, is matched according to alloying component, Vacuum is less than 10^-3Master alloy melting in the cold crucible levitation melting stove of Pa, alloying component is homogenized through 3 ~ 4 meltings, and Directional solidification bar is cast in suction；

Second step：TiAl alloy coupon is put into solidification is oriented in the corundum crucible of high-purity yttrium oxide coating, be evacuated to 5 ×10^-3Pa, then protect gas to high-purity argon is filled with system；

3rd step：Regulation inductive source power sample is heated, holding temperature be 1450 ~ 1650K, soaking time be 15 ~ 30min, starts directional solidification, when controlling directional solidification withdrawing rate for 15 μm/s, at continued propagation to specimen length 50mm, opens Dynamic fast quenching carries out rapid quenching to directional solidification sample, retains solid liquid interface, obtains lamellar orientation with the direction of growth in 45 ° TiAl alloy monocrystalline；When directional solidification withdrawing rate is controlled for 20 μm/s, the list that lamellar orientation and the direction of growth are in 45 ° is obtained It is brilliant.

2. the fully controllable TiAl intermetallic compound monocrystalline of lamellar orientation as claimed in claim 1, it is characterised in that first Directional solidification diameter of rod is Φ in step（4~6mm）×100mm.

3. the fully controllable TiAl intermetallic compound monocrystalline of lamellar orientation as claimed in claim 1, it is characterised in that second The corundum crucible size of high-purity yttrium oxide coating is Φ in step（7~9mm）×100mm；High-purity argon protection gas charge is 0.04 ~ 0.06MPa。

4. the preparation method of the fully controllable TiAl intermetallic compound monocrystalline of a kind of lamellar orientation as claimed in claim 1, It is characterised in that it includes following specific steps：

The first step：Ti, Al, Nb simple metal raw material that purity is 99.999% are chosen, is matched according to alloying component, Vacuum is less than 10^-3Master alloy melting in the cold crucible levitation melting stove of Pa, alloying component is homogenized through 3 ~ 4 meltings, and Directional solidification bar is cast in suction；

Second step：TiAl alloy coupon is put into solidification is oriented in the corundum crucible of high-purity yttrium oxide coating, be evacuated to 5 ×10^-3Pa, then protect gas to high-purity argon is filled with system；

3rd step：Regulation inductive source power sample is heated, holding temperature be 1450 ~ 1650K, soaking time be 15 ~ 30min, starts directional solidification, when controlling directional solidification withdrawing rate for 15 μm/s, at continued propagation to specimen length 50mm, opens Dynamic fast quenching carries out rapid quenching to directional solidification sample, retains solid liquid interface, obtains lamellar orientation with the direction of growth in 45 ° TiAl alloy monocrystalline；When directional solidification withdrawing rate is controlled for 20 μm/s, the list that lamellar orientation and the direction of growth are in 45 ° is obtained It is brilliant.

5. the preparation method of the fully controllable TiAl intermetallic compound monocrystalline of lamellar orientation as claimed in claim 4, it is special Levy and be, directional solidification diameter of rod is Φ in the first step（4~6mm）×100mm.

6. the preparation method of the fully controllable TiAl intermetallic compound monocrystalline of lamellar orientation as claimed in claim 4, it is special Levy and be, the corundum crucible size of high-purity yttrium oxide coating is Φ in second step（7~9mm）×100mm；High-purity argon protection gas fills Enter amount for 0.04 ~ 0.06MPa.