CN105088120A - Widmannstatten structure titanium alloy with composite laminated structure and preparation method thereof - Google Patents

Abstract

The invention discloses a Widmanstatten titanium alloy with a compound lamellar structure and a preparation method thereof, and belongs to the technical field of optimizing the comprehensive performance of titanium alloys. First select a titanium alloy blank with a Widmanstatten structure of ordinary lamellar structure, and then heat treat it. The heat treatment temperature is within the range of 10-20°C below the complete transformation point of the α/β phase of the titanium alloy; Cool to room temperature by air cooling or air cooling to obtain a Widmanstatten titanium alloy with a composite lamellar structure. The Widmanstatten structure has fine secondary α sheets distributed between the coarse α sheets. This special structure can improve the tensile properties and fracture toughness of the Widmanstatten structure and reduce its fatigue crack growth rate da/dN at the same time. Therefore, the mechanical properties of the Widmanstatten structure are further comprehensively optimized, the existing limitations of improving the cooling rate to optimize the properties of the Widmanstatten structure with a common lamellar structure are broken through, and the application potential of titanium alloys with a Widmanstatten structure is improved.

Description

A kind of Widmanstatten titanium alloy with compound lamellar structure and preparation method thereof

technical field

The invention relates to the technical field of optimizing the comprehensive properties of titanium alloys through heat treatment, in particular to a titanium alloy with a Widmanstatten structure with a composite lamellar structure and a preparation method thereof. The titanium alloy with optimized mechanical properties will have a wider application in the aerospace field. scope of application.

Background technique

Due to its high specific strength and excellent corrosion resistance, titanium alloys have been widely used in the fields of aviation, aerospace and ships. The equiaxed structure of titanium alloy has excellent tensile properties and high cycle fatigue properties, and is currently the most widely used microstructure type of titanium alloy. However, titanium alloys with a Widmanstatten structure with coarse original β grains have better damage tolerance performance than equiaxed or bimodal structures, that is, lower fatigue crack growth rate (da/dN) and higher fracture toughness (K _IC ). Aeronautical structure design began to list titanium alloys with Wei's structure as the material selection range to improve the service life and reliability of the structure.

The Widmanstatten structure of the existing titanium alloy has a common lamellar structure, which is obtained by heat treatment of the alloy above the complete transition point T _β of the α/β phase. The tensile strength and The plasticity is low, and the method of increasing the cooling rate after solid solution can increase the tensile strength and reduce da/dN to a certain extent, but at the same time it is accompanied by the loss of tensile plasticity and fracture toughness, and the reduction effect on da/dN exists There must be a limit.

In the selection of structural design materials, it is expected that the material will have lower da/dN, higher K _IC and better tensile properties. Therefore, it is urgent to adjust and optimize the properties of the material through heat treatment.

Contents of the invention

Aiming at the problem of further optimizing the Widmanstatten structure of titanium alloy to improve its comprehensive performance, the present invention provides a titanium alloy with a composite lamellar structure and a preparation method thereof, which reduces the da/ dN, increase K _IC , and the tensile strength and plasticity of the material can be improved to a certain extent.

For realizing the above object, technical scheme of the present invention is:

A method for preparing a titanium alloy with a Widmanstatten structure of a composite lamellar structure. The method first selects a titanium alloy blank with a Widmanstatten structure of a common lamellar structure, and then heat-treats it. The heat treatment temperature is the titanium alloy α/ Within the range of 10-20 °C below the complete transformation point of the β phase; after the heat preservation, the titanium alloy billet is cooled to room temperature by air cooling or air cooling to obtain a Widmanstatten titanium alloy with a composite lamellar structure.

The holding time for heat treatment of titanium alloy billet t (min) = η × δ _max , δ _max is the maximum cross-sectional thickness or diameter of the billet expressed in millimeters (mm), η is the heating coefficient, and the value of η is 0.4 to 0.9 , t(min) refers to the time value expressed in minutes.

The Widmanstatten structure with ordinary lamellar structure means that β phase is distributed between the α sheets, and the titanium alloy blank with Widmanstatten structure with ordinary lamellar structure is obtained as follows:

The titanium alloy billet with equiaxed or two-state microstructure is heat treated at a temperature in the range of 20°C to 100°C above the complete transformation point of the alloy α/β phase, and the holding time is t(min)=η×δ _max , δ _max is the maximum cross-sectional thickness or diameter of the blank expressed in millimeters (mm), η is the heating coefficient, and the value of η is 0.3 to 0.8, and t (min) refers to the time value expressed in minutes; the titanium alloy blank is kept warm After cooling to room temperature in the air, a titanium alloy blank with a Widmanstatten structure of ordinary lamellar structure is obtained.

The titanium alloy prepared by the above method has a Widmanstatten structure of composite sheet structure, that is, fine secondary α sheets are distributed between coarse α sheets, and the volume ratio of the secondary α sheets is 60-80%.

The advantages of the present invention are:

1. In the present invention, a compound lamellar structure can be obtained by heat-treating the Widmanstatten structure within a specific temperature range in the α+β two-phase region, and controlling the cooling rate after the heat treatment. This special structure can improve the Widmanstatten structure. The tensile properties, fracture toughness and at the same time reduce its da/dN, thereby further comprehensively optimizing the mechanical properties of the Widmanstatten structure, breaking through the existing limitations of optimizing the properties of the Widmanstatten structure with a common lamellar structure by increasing the cooling rate , which improves the application potential of titanium alloys with Widmanstatten structure.

2. The present invention heats the Widmanstatten structure with ordinary lamellar structure below the complete transition point of α/β phase and cools it at a certain speed. During the cooling process, the β phase between the original coarse α sheets transforms into fine secondary Alpha sheet. The fine secondary α sheets between the coarse sheets in the composite lamellar structure refine the microstructure size of the Widmanstatten structure, improve the tensile strength and plasticity of the material, and fatigue cracks expand to the secondary α sheet area , the fine secondary α sheet further increases the tortuosity of crack propagation, which will further reduce da/dN.

3. The titanium alloy with compound lamellar structure Widmanstatten structure obtained in the present invention, compared with common lamellar structure Widmanstatten structure, has the strength, plasticity and toughness of compound lamellar structure Widmanstatten structure simultaneously. Optimization, in which the improvement of tensile plasticity is more obvious, about 30-40%. In terms of the anti-fatigue crack growth ability of the material, it has broken through the limit of the effect of only increasing the cooling rate on the reduction of da/dN. In the low ΔK range, the da/dN of the composite lamellar structure is only about 50% of that of the ordinary structural lamellar. The service life of the material can be greatly improved, the maintenance cycle of the structural parts can be increased, and the cost can be reduced.

Description of drawings:

Figure 1 shows a typical titanium alloy equiaxed structure and Widmanstatten structure with ordinary lamellar structure; among them: (a) equiaxed structure; (b) Widmanstatten structure with ordinary lamellar structure.

Figure 2 is the comparison of da/dN of TC4 titanium alloy with equiaxed structure and Widmanstatten structure with ordinary lamellar structure (R=0.1).

Fig. 3 is the micromorphology of the Widmanstatten structure with thicker lamellar layers obtained by furnace cooling and slow cooling.

Fig. 4 is a comparison of da/dN (R=0.1) of Widmanstatten structures with two sheet thicknesses obtained by furnace cooling (F.C.) and air cooling (A.C.).

Fig. 5 is a comparison of da/dN (R=0.1) of Widmanstatten structures with two α sheet thicknesses obtained by water cooling (W.Q.) and air cooling (A.C.).

Fig. 6 is a schematic diagram of the microstructure of a common lamellar structure of a titanium alloy.

Fig. 7 is the composite sheet structure and its microscopic appearance obtained by heat treatment; wherein: (a) the schematic diagram of the composite sheet structure; (b) the scanning electron microscope microscopic appearance of the composite sheet structure.

Fig. 8 is a comparison of da/dN between the composite lamellar structure obtained at different heat treatment temperatures and the common lamellar structure (R=0.1).

Detailed ways:

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments. In the following examples and comparative examples, the original titanium alloy billet is cylindrical, and the maximum cross-sectional diameter is 120 mm.

Comparative example 1

In this comparative example, heat treatment is performed on TC4 titanium alloy whose nominal composition is Ti-6Al-4V, and the transformation point of the alloy is 975±5°C. The heat treatment system is 920 ° C for 1 hour and then air cooling (AC), to obtain a titanium alloy with an equiaxed structure, as shown in Figure 1(a). The heat treatment system is 1020°C for 1 hour and then air-cooled to obtain a titanium alloy with a Widmanstatten structure with a common lamellar structure, as shown in Figure 1(b). Table 1 compares the mechanical properties of the equiaxed structure and Widmanstatten structure of the titanium alloy, and Figure 2 shows the da/dN comparison between the two. It can be seen that compared with the equiaxed structure, the Widmanstatten structure with ordinary lamellar structure has slightly lower tensile strength and plasticity, but has excellent damage tolerance performance, namely lower da/dN and higher K _IC .

Table 1 Comparison of tensile properties and fracture toughness of Ti-6Al-4V alloy equiaxed structure and Widmanstatten structure

Comparative example 2

This comparative example is to adjust and optimize the material properties of the TC4 titanium alloy whose nominal composition is Ti-6Al-4V by increasing the cooling rate after solution treatment. The heat treatment system of TC4 titanium alloy is furnace cooling after holding at 1020°C for 1 hour. Figure 3 is the microscopic appearance of the Widmanstatten structure with relatively thick sheets obtained by furnace cooling and slow cooling. The thickness of the α sheet in this structure is about 5-10 μm. Morphologically, the thickness of the α sheet in this tissue is about 2 μm. It can be seen that the titanium alloy is subjected to solid solution heat preservation treatment above the complete transformation point of the α/β phase, and then cooled at a certain speed. The thickness of the sheet obtained at different cooling speeds is also different.

Figure 4 is a comparison of da/dN of Widmanstatten structures with two different thicknesses of α sheets obtained by furnace cooling (F.C.) and air cooling (A.C.). When the cooling rate is faster, the da/dN is lower. However, when water cooling (W.Q.) with a higher cooling rate is further used to obtain finer acicular martensite sheets, the da/dN of the material is significantly higher than that of air cooling. As shown in Figure 5, the acicular martensite has Higher da/dN is associated with its brittle deformation characteristics.

When the fatigue crack propagates in the Widmanstatten structure, the crack tends to propagate along the direction perpendicular or parallel to the α sheet, forming a crack propagation path with a step shape. The more steps are formed during the crack propagation process, the more tortuous the crack path is. The higher the degree of crack closure, the lower the da/dN of the material. Decreasing the α-sheet thickness increases the number of steps formed during crack propagation and reduces da/dN. By increasing the cooling rate of heat treatment, da/dN can be reduced within a certain range, but when the cooling rate reaches or exceeds water cooling, a brittle martensitic structure will be formed, and the da/dN of the material will be significantly increased at this time, and K _IC is significantly lower. Therefore, if the formation of brittle martensite can be avoided while reducing the thickness of the α sheet, the da/dN of the material can be further reduced. However, the existing method of simply increasing the cooling rate to reduce the thickness of the α sheet will inevitably form brittle martensite after reaching a certain limit.

It can be seen that the method of increasing the cooling rate to further reduce the Widmanstatten structure da/dN has a certain limit, and is often accompanied by the loss of other mechanical properties. Table 2 shows the influence trend of increasing the cooling rate on the mechanical properties of the titanium alloy Widmanstatten structure.

Table 2 Effect trend of increasing the cooling rate after solution heat treatment on the mechanical properties of titanium alloy Widmanstatten structure

The schematic diagram of the microstructure of the ordinary Widmanstatten microstructure obtained by air cooling and furnace cooling in Comparative Example 1-2 is shown in Figure 6, and the black line between the α sheets in Figure 6 is the residual β phase. When the Widmanstatten structure undergoes plastic deformation, dislocations easily pass through the juxtaposed α sheets along a straight line, thereby reducing the tensile strength, and the da/dN value of the material is also high. The β phase between the α sheets will not optimize the tensile properties and da/dN values. On the contrary, the β phase between the α sheets is a weak area in the plastic deformation of titanium alloys. Optimizing the microstructure of the region can play a role in optimizing the mechanical properties.

In the following examples, according to the principle of phase transformation of titanium alloys, a heat treatment method in the α+β two-phase region for the Widmanstatten structure of titanium alloys is designed to obtain titanium alloys with composite lamellar structures. The acquisition steps are mainly divided into two stages: the first stage first obtains the Widmanstatten structure with ordinary lamellar structure; the second stage heat-treats the common lamellar structure Widmanstatten structure to obtain the compound lamellar structure Widmanstatten structure. details as follows:

The first stage:

The titanium alloy billet with equiaxed or two-state microstructure is heat treated at a temperature in the range of 20°C to 100°C above the complete transformation point of the alloy α/β phase, and the holding time is t(min)=η×δ _max , δ _max is the maximum cross-sectional thickness or diameter of the blank expressed in millimeters, η is the heating coefficient, the value of η is 0.3 to 0.8, and t (min) refers to the time value expressed in minutes. After the titanium alloy billet is kept warm, it is cooled to room temperature in the air to obtain a Widmanstatten structure with a common lamellar structure.

second stage:

The titanium alloy billet with the Widmanstatten structure of ordinary lamellar structure is heat-treated at a temperature within the range of 15±5°C below the complete transition point of the alloy α/β phase, and the holding time is t(min)=η×δ _max , δ _max is the maximum cross-sectional thickness or diameter of the blank expressed in millimeters, η is the heating coefficient, and the value of η is 0.4 to 0.9, and t (min) refers to the time value expressed in minutes. After the titanium alloy billet is kept warm, it is cooled to room temperature by air cooling or air cooling to obtain a Widmanstatten structure with a composite lamellar structure.

Example 1

This embodiment is to control and optimize the TC4 titanium alloy whose nominal composition is Ti-6Al-4V. The titanium alloy with ordinary lamellar structure obtained in Comparative Example 1 was heat-treated at 920°C, 940°C, and 960°C, and then air-cooled to room temperature after being kept for 1 hour. When the Widmanstatten structure is heat-treated in the two-phase region, phase transformation occurs first on both sides of the β phase, and part of the α phase transforms into a β phase. After a certain period of time, it is cooled at a certain speed, and the β phase between the original α sheets transforms into fine particles. The secondary α-sheets obtained in the composite sheet structure, that is, there are fine secondary α-sheets between the original coarser α-sheets. The schematic diagram of the composite sheet structure is shown in Figure 7(a) , and Figure 7(b) is the scanning electron microscope micrograph of the composite lamellar structure obtained by heat-treating the Widmanstatten structure. The fine secondary α lamellae between the coarse lamellae in the composite lamellar structure refine the microstructure size of the Widmanstatten structure, which can improve the tensile properties of the material. And when the fatigue crack extends to the secondary α-sheet region, the fine secondary α-sheet further increases the tortuosity of crack propagation, which will further reduce da/dN.

In the Widmanstatten structure with complex lamellar structure, the ratio of the original coarse α-sheet to the fine secondary α-sheet is an important factor affecting the mechanical properties. When heat treatment is carried out at different temperatures in the α+β two-phase region, the higher the temperature is, the higher the proportion of fine secondary α sheets will be. Only when the fine secondary α sheets reach a higher proportion can the mechanical properties be significantly improved. optimization effect. Figure 8 shows the da/dN comparison between the composite lamellar structure and the ordinary lamellar structure obtained by the three heat treatment temperatures of 920°C, 940°C, and 960°C respectively for the Widmanstatten structure of TC4 alloy. It can be seen that when the heat treatment temperature reaches 960°C , the da/dN of the composite sheet structure is significantly lower than that of the ordinary sheet structure, because the proportion of fine secondary α sheets obtained by heat treatment at 960°C is sufficiently high, and the volume fraction of fine secondary α sheets is about 70%.

Table 3 is the comparison of the mechanical properties of the Widmanstatten structure of the optimized composite lamellar structure of the TC4 alloy in this example and the common lamellar structure. It can be seen that the tensile strength, tensile plasticity, fracture toughness and fatigue resistance of the composite lamellar structure The crack propagation ability is better than that of ordinary lamellar structure.

Table 3 Comparison of mechanical properties of Widmanstatten tissue with composite lamellar structure and common lamellar structure

Example 2

This embodiment is to control and optimize the TA15 titanium alloy, and the temperature of the α/β phase complete transformation point of the TA15 titanium alloy is 985±5°C. Firstly, the TA15 titanium alloy billet with equiaxed structure was heat treated at 1030° C., and kept for 1 hour to obtain a titanium alloy with ordinary lamellar structure. It is then heat-treated at a temperature of 970°C and held for 1 hour to obtain a titanium alloy with a Widmanstatten structure with a composite lamellar structure. About 70%.

Table 4 compares the mechanical properties of the Widmanstatten structure between the optimized composite lamellar structure and the common lamellar structure of TA15 titanium alloy. It can be seen that the tensile strength, tensile plasticity, fracture toughness and fatigue crack growth resistance of the composite lamellar structure The ability is better than ordinary lamellar tissue in an all-round way.

Table 4 Comparison of mechanical properties of TA15 titanium alloy Widmanstatten structure with composite lamellar structure and common lamellar structure

It can be seen that the Widmanstatten structure with composite lamellar structure can not only be obtained in the TC4 alloy, but also the Widmanstatten structure of titanium alloys with other components can be obtained at a temperature of 10-20°C below the complete transformation point of the α/β phase of the alloy at a certain temperature. Cooling at a high speed can obtain a performance-optimized composite lamellar structure.

Claims (6)

1. a kind of preparation method of the Widmanstatten structure titanium alloy with composite lamellar structure is characterized in that: the method at first selects the titanium alloy blank of the Widmanstatten structure with common lamellar structure, then it is carried out heat treatment, heat treatment temperature It is in the range of 10-20°C below the α/β phase complete transformation point of the titanium alloy; after the titanium alloy blank is kept warm, it is cooled to room temperature by air cooling or air cooling to obtain a Widmanstatten titanium alloy with a composite lamellar structure. 2. the preparation method of the Widmanstatten structure titanium alloy with composite lamellar structure according to claim 1, is characterized in that: the holding time t (min)=η * δ _max of heat treatment is carried out to titanium alloy billet, δ _max is the maximum cross-sectional thickness or diameter of the blank expressed in millimeters (mm), η is the heating coefficient, and the value of η is 0.4 to 0.9, and t (min) refers to the time value expressed in minutes. 3. the preparation method of the titanium alloy with the Widmanstatten structure of compound type lamellar structure according to claim 1, is characterized in that: the titanium alloy blank acquisition mode of the Widmanstatten structure with common lamellar structure is as follows: The titanium alloy billet with equiaxed or two-state microstructure is heat treated at a temperature in the range of 20°C to 100°C above the complete transformation point of the alloy α/β phase, and the holding time is t(min)=η×δ _max , δ _max is the maximum cross-sectional thickness or diameter of the blank expressed in millimeters (mm), η is the heating coefficient, and the value of η is 0.3 to 0.8, and t (min) refers to the time value expressed in minutes; the titanium alloy blank is kept warm After cooling to room temperature in the air, a titanium alloy blank with a Widmanstatten structure of ordinary lamellar structure is obtained. 4. according to the preparation method of the described Widmanstatten structure titanium alloy with composite lamellar structure of claim 1 or 3, it is characterized in that: described Widmanstatten structure with ordinary lamellar structure refers to the distribution between α sheets With β phase. 5. A titanium alloy with a composite lamellar structure prepared by the method of claim 1. 6. The titanium alloy with a Widmanstatten structure of a composite lamellar structure according to claim 5, characterized in that: the titanium alloy has a Widmanstatten structure of a composite lamellar structure, that is, distributed between thick α sheets There are fine secondary α-sheets, and the volume ratio of the secondary α-sheets is 60-80%.