A heat treatment method for improving the strength-ductility matching of metastable β titanium alloys

Abstract

The present invention relates to a heat treatment method for improving the strength-ductility matching of metastable β titanium alloys, comprising the following steps: (1) heating a forged blank of a metastable β titanium alloy with a basketweave structure to a solution holding temperature in the two-phase region; (2) directly furnace cooling the solution-treated alloy to 500°C - 650°C, and then taking it out for air cooling; (3) aging the alloy treated by solution treatment, furnace cooling and air cooling at 500°C - 650°C for 4 h - 8 h, and finally taking it out for air cooling. By means of two-phase region solution treatment, furnace cooling + air cooling, and aging + air cooling, and through the selection of heat treatment temperature, time and cooling method, the present invention can finely regulate the microstructure of metastable β titanium alloys, obtain a heterogeneous structure of micron-scale strip-shaped primary α + nano-scale needle-shaped secondary α + residual β phase, so that the metastable β titanium alloy has an excellent matching of high strength, high ductility and high fracture toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of metal mechanical heat treatment, and particularly to a heat treatment method for improving the strength-plasticity matching of metastable β titanium alloy. Background Art

[0002] Titanium alloys have characteristics such as high specific strength, resistance to low and high temperatures, good fatigue resistance and corrosion resistance, and are thus widely used in fields such as aerospace and marine ships. As a special type of titanium alloy, metastable β titanium alloy is characterized by containing an appropriate amount of β-stabilizing elements (such as molybdenum, niobium, etc.), which enables the alloy to maintain the β-phase structure at room temperature. However, there are some problems in the strength-plasticity matching of metastable β titanium alloy. Especially in a high-stress environment, the plastic deformation ability is insufficient, which to a certain extent limits its wider application.

[0003] The presence of α-phase in metastable β titanium alloy has an important influence on the strength and plasticity of the material. According to the morphology and distribution characteristics of α-phase, it can be divided into four categories: Widmanstätten structure, equiaxed structure, duplex structure and basket-weave structure, etc. The Widmanstätten structure exhibits very high fracture toughness, but low fatigue performance and plasticity; the strength of the duplex structure is slightly lower than that of the basket-weave and duplex structures, but the plasticity is better; the mechanical properties of the duplex structure are similar to those of the equiaxed structure, and the fracture toughness is slightly higher; the comprehensive performance of the basket-weave structure is between the equiaxed structure and the Widmanstätten structure, with excellent fracture toughness and creep strength, and the plasticity is lower than that of the equiaxed and duplex structures, but higher than that of the Widmanstätten structure. Traditional metastable β titanium alloy has certain limitations in strength-plasticity matching. Especially in a high-stress environment, its plastic deformation ability often cannot meet the requirements of practical applications. The development of high-performance titanium alloys with high strength, high plasticity and high fracture toughness is beneficial to ensuring the safe service of titanium alloy structural parts and further expanding the application of titanium alloys.

[0004] At present, the heat treatment methods for metastable β titanium alloy mainly include solution treatment, single-stage aging, two-stage aging and double annealing, etc. Solution treatment is to heat the alloy to the β-phase region or the α+β two-phase region for solution treatment to prepare the microstructure for the subsequent aging treatment. Aging treatment is to keep the alloy at an intermediate temperature for a period of time after solution treatment to promote the precipitation of secondary α-phase, thereby improving the strength of the material. However, the strength difference between the hard secondary α-phase and the soft β-phase often causes strain incompatibility, making it easy to generate stress concentration at the interface of the two phases, leading to the formation of pores or even microcracks, resulting in insufficient plasticity of the alloy. Therefore, in the heat treatment process of metastable β titanium alloy, how to effectively regulate the proportion and morphology of α-phase and β-phase to obtain better strength-plasticity matching has always been the focus and difficulty of research. Summary of the Invention

[0005] The present invention aims to provide a heat treatment method for improving the strength-ductility matching of metastable β titanium alloys, and by optimizing the heat treatment process parameters, excellent matching of the strength, plasticity and fracture toughness of metastable β titanium alloys is achieved.

[0006] In a first aspect, the present invention provides a heat treatment method for improving the strength-ductility matching of metastable β titanium alloys, comprising the following steps:

[0007] Step 1: Two-phase region solution treatment

[0008] Place the metastable β titanium alloy with a basket weave structure in a heat treatment furnace, and perform solution heat preservation at a two-phase region temperature for 0.5 to 1.5 hours;

[0009] Step 2: Furnace cooling and air cooling treatment

[0010] Directly furnace cool the alloy after solution treatment to 500 °C to 650 °C, and then take it out of the furnace for air cooling;

[0011] Step 3: Aging treatment

[0012] Place the alloy after solution treatment, furnace cooling and air cooling in a heat treatment furnace at a certain temperature, perform aging heat preservation for 4 to 8 hours, and then take it out of the furnace for air cooling.

[0013] Furthermore: In the said step 1, heat preservation is carried out at a temperature 25 °C to 50 °C below the β phase transformation point for 0.5 to 1.5 hours. The main purpose of the two-phase region solution treatment is to dissolve most of the α phase into the β phase, and only a certain proportion of strip-shaped α phase is retained.

[0014] Furthermore: In the said step 1, the solution temperature is 815 °C and the heat preservation time is 1 hour.

[0015] Furthermore: In the said step 2, furnace cool to 550 °C to 650 °C, and then take it out of the furnace for air cooling. The furnace cooling and air cooling treatment can cause the remaining strip-shaped α phase to coarsen and grow, and improve the plastic deformation ability of the primary strip-shaped α phase.

[0016] Furthermore: In the said step 3, aging treatment is carried out at 500 °C to 650 °C, preferably 550 °C, and the heat preservation time is 8 hours. The aging treatment can promote the further precipitation of fine and dispersed secondary α phase in the residual metastable β phase, thereby enhancing the strength of the material.

[0017] Furthermore: The molybdenum equivalent (Mo [eq] ) of the metastable β titanium alloy with a basket weave structure is between 10 and 20.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] (1) By subjecting to solution treatment in the two-phase region and then furnace cooling + air cooling, a certain proportion of micron-scale strip-shaped α-phase is formed. Through aging treatment, nano-scale needle-shaped secondary α-phase precipitates in the residual β-phase, and finally a heterogeneous structure tissue in which the micron-scale strip-shaped α-phase surrounds the nano-scale needle-shaped secondary α-phase and the residual β-phase is obtained.

[0020] (2) The coarsened strip-shaped α-phase formed by subjecting to solution treatment in the two-phase region and then furnace cooling + air cooling improves the deformation ability of the α-phase. The fine secondary α-phase precipitated through aging treatment improves the strength of the alloy, and finally enables the alloy to have excellent strength-ductility matching.

[0021] (3) By finely regulating the proportion, morphology and size of the α-phase, the tensile strength of the alloy is 984 - 1034 MPa, the yield strength is 916 - 953 MPa, the elongation at break is 18.9% - 20.8%, and the fracture toughness is 110 - 121 MPa·m 0.5 , and it has excellent comprehensive properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be described in detail below with reference to the accompanying drawings.

[0023] Figure 1 are the micrographs of titanium alloys at different magnifications in Example 1 after solution treatment at 815°C for 1 h, furnace cooling to 650°C and then taking out for air cooling, and then aging treatment at 550°C for 8 h and taking out for air cooling.

[0024] Figure 2 are the micrographs of titanium alloys at different magnifications in Example 2 after solution treatment at 815°C for 1 h, furnace cooling to 600°C and then taking out for air cooling, and then aging treatment at 550°C for 8 h and taking out for air cooling.

[0025] Figure 3 are the micrographs of titanium alloys at different magnifications in Example 3 after solution treatment at 815°C for 1 h, furnace cooling to 550°C and then taking out for air cooling, and then aging treatment at 550°C for 8 h and taking out for air cooling.

[0026] Figure 4 are the micrographs of titanium alloys at different magnifications in Comparative Example 1 after solution treatment at 815°C for 1 h, furnace cooling to 700°C and then taking out for air cooling, and then aging treatment at 550°C for 8 h and taking out for air cooling.

[0027] Figure 5 are the micrographs of titanium alloys at different magnifications in Comparative Example 2 after solution treatment at 870°C for 1 h, furnace cooling to 550°C and then taking out for air cooling, and then aging treatment at 550°C for 8 h and taking out for air cooling.

[0028] Figure 6 are the micrographs of titanium alloys at different magnifications in Comparative Example 3 after solution treatment at 815°C for 1 h and taking out for air cooling.

[0029] Figure 7 These are the micrographs of titanium alloys with different magnification factors after solution heat treatment at 815°C for 1 h, air cooling after removal, then heat treatment at 550°C for 8 h and air cooling after removal in Comparative Example 4.

[0030] Figure 8 These are the tensile stress-strain curves of the titanium alloys in Examples 1 to 3 and Comparative Examples 1 to 4. Detailed Embodiments

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the described content.

[0032] It should be noted that the terms such as "upper", "lower", "left", "right", "middle", etc. cited in this specification are only for the convenience of description and are not used to limit the scope of implementation. The change or adjustment of their relative relationship, without substantial change in the technical content, should also be regarded as the scope of implementation of this application.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs; the term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0034] For those conditions not specified in the examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For those reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0035] As used herein, the term "about" is used to provide flexibility and imprecision associated with a given term, measurement, or value. Those skilled in the art can easily determine the degree of flexibility of a specific variable.

[0036] As used herein, the term "at least one of..." is intended to be synonymous with "one or more of...". For example, "at least one of A, B, and C" clearly includes only A, only B, only C, and their respective combinations.

[0037] The present invention uses a metastable β titanium alloy forging blank with a basket weave structure for heat treatment. The basket weave structure is one of the four common types of structures in titanium alloys and can be obtained through processes such as β forging and quasi-β forging disclosed in existing literature. The alloy used in the implementation cases of the present invention is Ti-4Al-6Mo-3Nb-2Cr-1Zr-1Sn (β transformation point is 845°C), and the molybdenum equivalent is 11.6, but the heat treatment method proposed by the present invention is not limited to this alloy.

[0038] Example 1

[0039] This embodiment includes the following steps:

[0040] Step 1: Place the basket-structured metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0041] Step 2: Cool the alloy obtained in Step 1 directly in the furnace to 650 °C, and then take it out of the furnace and air-cool it.

[0042] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0043] Figure 1 is the SEM microstructure of the alloy after heat treatment in this embodiment. It can be seen that inside the β grains, there are coarse micron-sized strip-shaped α phases and fine nano-sized needle-shaped secondary α phases. The large-sized α phases surround the fine secondary α phases, forming a heterogeneous structure.

[0044] Example 2

[0045] This embodiment includes the following steps:

[0046] Step 1: Place the basket-structured metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0047] Step 2: Cool the alloy obtained in Step 1 directly in the furnace to 600 °C, and then take it out of the furnace and air-cool it.

[0048] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0049] Figure 2 is the SEM microstructure of the alloy after heat treatment in this embodiment. It can be seen that inside the β grains, there are coarse micron-sized strip-shaped α phases and fine nano-sized needle-shaped secondary α phases. The large-sized α phases surround the fine secondary α phases, forming a heterogeneous structure.

[0050] Example 3

[0051] This embodiment includes the following steps:

[0052] Step 1: Place the basket-structured metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0053] Step 2: Cool the alloy obtained in Step 1 directly in the furnace to 550 °C, and then take it out of the furnace and air-cool it.

[0054] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0055] Figure 3It is the SEM microstructure of the alloy after heat treatment in this embodiment. It can be seen that inside the β grains, there are coarse micron-sized strip-shaped α phases and fine nano-sized needle-shaped secondary α phases. The large-sized α phases surround the fine secondary α phases, forming a heterogeneous structure.

[0056] Comparative Example 1

[0057] This comparative example includes the following steps:

[0058] Step 1: Place the basket-weave metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0059] Step 2: Directly cool the alloy obtained in Step 1 in the furnace to 700 °C, and then take it out of the furnace and air-cool it.

[0060] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0061] Figure 4 It is the SEM microstructure of the alloy after heat treatment in this comparative example. It can be seen that the β grains contain more secondary α phases, which greatly hinder the dislocation slip, resulting in a significant decrease in the plasticity of the alloy compared with Examples 1 to 3.

[0062] Comparative Example 2

[0063] This comparative example includes the following steps:

[0064] Step 1: Place the basket-weave metastable β titanium alloy material in a heat treatment furnace and hold it at 870 °C (25 °C above the β phase transformation point) for 1 hour.

[0065] Step 2: Directly cool the alloy obtained in Step 1 in the furnace to 550 °C, and then take it out of the furnace and air-cool it.

[0066] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0067] Figure 5 It is the SEM microstructure of the alloy after heat treatment in this comparative example. Since it is solution-treated in the β phase region, furnace cooling causes a large amount of lath α phases to precipitate, crisscrossing each other, and there are parallel α bundles, which is not beneficial to the strength-ductility matching of the alloy.

[0068] Comparative Example 3

[0069] This comparative example includes the following steps:

[0070] Step 1: Place the basket-weave metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0071] Step 2: Directly take out the alloy obtained in Step 1 and air-cool it.

[0072] Figure 6 This is the SEM microstructure of the alloy after heat treatment in this comparative example. It can be seen that only a small amount of strip-shaped α phase is contained inside the β grains.

[0073] Comparative Example 4

[0074] This comparative example includes the following steps:

[0075] Step 1: Place the basket-weave metastable β titanium alloy material in a heat treatment furnace and hold it at 815 °C for 1 hour.

[0076] Step 2: Directly take out the alloy obtained in Step 1 from the furnace and air-cool it.

[0077] Step 3: Hold the alloy obtained in Step 2 at 550 °C for 8 hours, and then take it out of the furnace and air-cool it.

[0078] Figure 7 This is the SEM microstructure of the alloy after heat treatment in this comparative example. It can be seen that a large number of α phases are precipitated and diffusely distributed in the β grains, presenting a typical basket-weave structure, with relatively high strength but obvious lack of plasticity.

[0079] Performance comparison

[0080] The titanium alloys in Examples 1 to 3 of the present invention exhibit a heterogeneous structure and show excellent strength-ductility matching. The specific performance parameters are as follows:

[0081] The ultimate tensile strength (UTS) is 984 - 1034 MPa, the yield strength (YS) is 916 - 953 MPa, the elongation at break (EL) is 18.9% - 20.8%, and the fracture toughness (K IC ) is 110 - 121 MPa·m 0.5 , and the comprehensive performance is significantly better than that of Comparative Examples 1 to 4. The main performance comparison test results are shown in Table 1 and Figure 8 .

[0082] Table 1: Performance comparison corresponding to the heat treatment method of the present invention and other heat treatment methods

[0083]

[0084] Figure 8 This is the tensile curve graph of the examples and comparative examples of the present invention. It can be seen that the alloy cooled in the two-phase region to 700 °C + aged treatment has acceptable strength, but general plasticity and fracture toughness; the alloy cooled in the β single-phase region to 550 °C + aged treatment also does not have a very high strength-ductility matching; only the solution-treated sample has relatively low strength and general plasticity; the strength of the solution + aged sample has increased, but the plasticity and fracture toughness have decreased simultaneously, while Examples 1 to 3 of the present invention have both high strength, high plasticity and high fracture toughness, overcoming the bottleneck of the strength-ductility matching of the alloy.

Claims

1. A heat treatment method for improving the strength-ductility matching of metastable β titanium alloy, characterized in that The molybdenum equivalent of the metastable β titanium alloy with a basket-weave structure is between 10 and 20, and it includes the following steps: Step 1: Solution treatment in the two-phase region Place the metastable β titanium alloy with a basket-weave structure in a heat treatment furnace and perform solution heat preservation at a two-phase region temperature for 0.5 to 1.5 hours; Step 2: Furnace cooling and air cooling treatment Directly furnace cool the alloy after solution treatment to 500°C to 650°C, and then take it out of the furnace for air cooling; Step 3: Aging treatment Place the alloy after solution treatment, furnace cooling and air cooling in a heat treatment furnace at 500°C to 650°C, perform aging heat preservation for 4 to 8 hours, and then take it out of the furnace for air cooling.

2. The method according to claim 1, characterized in that, Perform solution heat preservation for 0.5 to 1.5 hours at a temperature 25°C to 50°C below the β transformation point.

3. The method according to claim 1, characterized in that, The solution temperature is 815°C and the heat preservation time is 1 hour.

4. The method according to claim 1, wherein In Step 2, furnace cool to 550°C to 650°C, and then take it out of the furnace for air cooling.

5. The method according to claim 1, characterized in that In Step 3, in a heat treatment furnace at 550°C, perform aging heat preservation for 8 hours.

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