Heat treatment of nickel-containing cast titanium alloy
Many titanium-aluminum alloy aerospace and automotive engine parts are formed by precision casting process, and heat treatment is one of the key technologies to improve the microstructure of cast titanium-aluminum alloy [1].
The original as-cast microstructure of the cast titanium aluminum alloy is generally a γ-TiAl/α2-Ti3Al layer structure. The layer of the layer is coarse, and the size and orientation distribution of the layer are not uniform [2]. High temperature heat treatment in the alpha single-phase region can achieve homogenization of its structure [3]. However, since the original microstructure is coarse and the α phase grain growth is difficult to control at high temperatures, the obtained cast titanium aluminum alloy full lamellar-FL usually still has a large layer of lamellar [4]. In order to improve the room temperature tensile ductility of cast titanium-aluminum alloy, a fine full-layer structure of cast titanium-aluminum alloy was successfully obtained through a multiple heat treatment process [5,6]. The process includes 1α single-phase zone homogenization, 2900-1150 °C thermal cycle, 31150 °C isothermal treatment and 4 reheating to a slightly higher temperature than Τα temperature for a short time isothermal treatment. However, this heat treatment process is complicated and has a long processing cycle, which is not conducive to engineering applications.
In this paper, the heat treatment process of cast Ti-46.5Al-2.5V-1.0Cr (atomic percent, the same below) alloy with nickel microalloying was studied, and the nickel microalloying simplified the homogenization and refinement of the cast titanium alloy. The mechanism of the heat treatment process and the formation mechanism of the fine full-thickness microstructure of the titanium-aluminum alloy containing nickel were analyzed and discussed.
1 The test material is a cast titanium-aluminum alloy Ti-46.5Al-2.5V-1.0Cr (%) containing nickel (0.2-0.5%% (atomic percent, the same below), smelting in a cold head vacuum induction suspension furnace, remelting 3 After that, it was poured into a copper mold to obtain an ingot of φ40 mm. A 30° fan-shaped heat-treated sample was taken from the ingot by a wire cutting method.
The heat treatment test was carried out under a vacuum of 0.133 Pa. Reference to cast Ti-46.5Al-2.5V-1.0Cr alloy to obtain the heat treatment system of the equiaxed near gamma-NG and fine fully lamellar-FFL [5,6], test temperature The time and time were taken as 1150 ° C × (48 ~ 168) h and 1370 ° C × (5 ~ 10) min.
Tissue observation was performed under ordinary optics and an optical microscope with polarized light. The metallographic sample was eroded with (% by volume) 1% HF + 10% HNO3 + 89% H2O solution.
2 It was observed that the original as-cast state of (99.8～99.5)%(Ti-46.5Al-2.5V-1.0Cr)+(0.2-0.5)%Ni alloy is a full-layer structure with a certain preferred orientation (Fig. 1). The pellet size is about 500 to 1500 μm. After 1150°C×72h, the alloy structure showed obvious continuous coarsening of the layer (Fig. 2a). After 144h isothermal treatment, the original coarse and uneven as-cast microstructure was transformed into fine and basically uniform axis. The NG structure (Fig. 2b) has an average grain size of about 30 μm.
Figure 1 (99.8 ~ 99.5) (Ti-46.5Al-2.5V-1.0Cr)
-(0.2~0.5) Ni (%) alloy original as-cast microstructure
Figure 2 Cast (99.8 ~ 99.5) (Ti-46.5Al-2.5V-1.0Cr)
-(0.2 to 0.5) Ni (%)
The structure obtained by isothermal treatment of the alloy at 1150 °C (a) is isothermal for 72 h, and the layer is continuously roughened by layering (b) 144 h isothermal, comprehensively equiaxed.
Studies have found that nickel has the effect of expanding the γ single-phase region of titanium-aluminum alloy [7], and adding nickel with an atomic fraction of 0.5% or more can transform Ti-48Al alloy into single-phase γ structure. In the 100-fold polarized metallographic microscope, rotating the stage 360°, it was observed that a small amount of equiaxed α2 grains in the NG structure obtained in this study showed obvious four-time bright extinction phenomenon [8]. Compared with the nickel-free titanium alloy NG structure obtained in the literature [5], it is qualitatively observed that the number of α2 phases in the NG structure of the nickel-containing alloy is significantly less. Therefore, it can be qualitatively stated that the addition of 0.2% to 0.5% of nickel can increase the driving force of α2 (or α)→γ phase change of titanium aluminum alloy at 1150 ° C, and the enhancement of energy fluctuation in the layer structure is increased. It promotes the occurrence of discontinuity of the layer shape in the layer structure [9]. These relatively large number of intra-layer end points produced in a relatively large amount of time effectively promote the segmental continuous roughening of the plies, thereby enabling the nickel-containing titanium-aluminum alloy to achieve homogenization of the structure in a relatively simple heat treatment process. Refine.
The experiment found that the fine full-thickness microstructure of titanium-aluminum alloy has the best comprehensive mechanical properties [10]. Therefore, the obtained NG structure is reheated to 1370 ° C for 5 to 10 minutes, and then cooled to obtain a fine-layered equiaxed full-layer sheet structure (Fig. 3). The average size of the layer group is about 50 μm, which is slightly smaller than that of cast Ti. -46.5Al-2.5V-1.0Cr alloy FFL tissue layer pellet obtained by the same process [6]. According to the fact that the layer group is equiaxed and slightly larger than the γ grain size of the NG structure of the substrate, it is considered that the formation mechanism of the nickel-cast titanium alloy FFL structure is different from that of the titanium-free aluminum alloy FFL structure [6]. However, the high-temperature α-equal axis is formed on the γ-phase matrix and grows slightly. After the cooling process, it is produced according to the general full-layer structure mechanism [11], that is, after cooling to the α+γ two-phase region, the γ phase is in α. The flaky precipitation in the crystal grains constitutes a γ/α layer structure, and then is converted into a γ/α2 layer structure during cooling to room temperature.
Figure 3 Cast (99.8 ~ 99.5) (Ti-46.5Al-2.5V-1.0Cr)
- (0.2 ~ 0.5) Ni (%) alloy fine full-layer sheet microstructure.
 in conclusion
(1) For the cast Ti-46.5Al-2.5V-1.0Cr alloy containing 0.2% to 0.5% (atomic fraction) of nickel, the as-cast coarse and uneven lamellar structure can be obtained by a relatively simple 1150 ° C × 144 h isothermal treatment. Transformed into a fine uniform equiaxed near-gamma structure.
(2) The obtained near-γ structure is reheated to 1370 ° C × (5 to 10) min, and then cooled to obtain a fine-layered equiaxed full-layer sheet structure.
Zhang Ji: Ph.D., senior engineer, is engaged in the study of high temperature structural intermetallic compound materials. The research direction includes the relationship between composition, structure and properties of TiAl, Mo5Si3 and Fe3Si based alloys. The manuscript is the National Natural Science Foundation of China, 59881004.
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