Preparation and properties of powder metallurgy Ti-23Al-17Nb

Between Ti ₃ A1 group metal compound having a low density, high temperature performance characteristics, and thus over the material can replace nickel-base superalloy is below 650 ℃ as in space. However, the Ti ₃ A1-based intermetallic compound has a large deformation resistance at room temperature and is difficult to process. After alloying and thermal mechanical processing, it has a certain plasticity, which limits its popularization and application.

Powder metallurgy processes can be used to prepare conventional methods for difficult to machine materials, such as Ti ₃ A1 based alloys. The powder metallurgy process produces materials that have a fine, uniform structure and that can be used to form the required parts. With pre-alloyed powders, powder metallurgy processes can produce parts with complex shapes at a lower cost than traditional forging processes.

In this paper, Ti-23A1-17Nb (%, atomic fraction) was used as the object, and the preparation of powder metallurgy Ti-23A1-17Nb was carried out by using its prealloyed powder.

    First, the experiment

(1) Preparation of Ti-23A1-17Nb prealloyed powder

The Ti-23A1-17Nb prealloyed powder used in this experiment was prepared by gas atomization, and its chemical composition is shown in Table 1. The chemical composition of the powder is close to the nominal composition of the material, and the content of impurity elements such as 0, N is better controlled.

Table 1 Chemical composition of prealloyed powder

The morphology of the powder determines the bulk density and tap density of the powder, which in turn affects the deformation of the alloy material during the compact sintering process. Therefore, the powder morphology is one of the key parameters in the powder metallurgy process. A lot of research has been done to control the morphology of the powder. The morphology and surface state of the Ti-23A1-17Nb prealloyed powder were observed by scanning electron microscopy. The results are shown in Figures 1 and 2.

Fig.1 Electron microscopic scanning photograph of Ti-23Al-17Nb prealloyed powder prepared by gas atomization

Fig. 2 Microstructure of Ti-23Al-17Nb prealloyed powder prepared by gas atomization

From the scanning electron micrograph of Ti-23A1-17Nb alloy powder, the basic shape of the powder is spherical, and some surfaces have tiny planetary spheres. The powder has good fluidity, high bulk density and tap density, and is suitable for direct hot isostatic pressing and densification molding.

(2) Hot isostatic pressing of Ti-23Al-17Nb prealloyed powder

The sheath material used for the hot isostatic pressing of Ti-23A1-17Nb prealloyed powder was stainless steel. The Ti-23A1-17Nb prealloyed powder is placed in the sheath, vibrated, and then vacuumed at a certain temperature to remove the gas. When the required degree of vacuum is reached for a certain period of time, forging is performed. The forged seal is subjected to hot isostatic pressing (Fig. 3). The hot isostatic pressing process of Ti-23A1-17Nb prealloyed powder is as follows: the holding temperature is 900-1100 ° C, the pressure is more than 100 MPa, and the holding time is 3 h. Figure 3 is a jacketed sample containing Ti-23A1-17Nb prealloyed powder before and after hot isostatic pressing.

Figure 3 Ti-23A1-17Nb prealloyed powder hot isostatic pressing jacket sample

(3) Heat treatment of powder metallurgy (abbreviated as PM) Ti-23A1-17Nb alloy after hot isostatic pressing

In order to obtain PM Ti-23A1-17Nb materials with different properties, different heat treatment systems were used to treat the alloy after hot isostatic pressing, and then the properties were tested and the microstructure was observed.

   Second, the results and discussion

(1) Properties of Ti-23A1-17Nb prealloyed powder

After testing, the bulk density of the Ti-23A1-17Nb prealloyed powder was 3.097 g · cm ^{- 3,} a tap density of 3.643 g · cm ^{- 3,} about 75% of its theoretical density. The results show that the deformation of Ti-23A1-17Nb prealloyed powder during hot isostatic pressing is not large and easy to control, which makes it easy to achieve near net shape of parts.

The distribution range of powder particle size has a certain influence on the microstructure of the material after hot isostatic pressing. The wider the powder strength distribution, the more uneven the structure of the material. In the experiment, the particle size distribution of the Ti-23A1-17Nb prealloyed powder was tested using a standard sieve, and the results are shown in Table 2. The particle size of the Ti-23A1-17Nb prealloyed powder is mostly in the range of 25 to 180 μm, which accounts for about 74.5% of the total amount of the powder.

Table 2 Particle size distribution of Ti-23A1-17Nb prealloyed powder

(II) Properties and microstructure of PM Ti-23A1-17Nb after hot isostatic pressing

The hot isostatic pressing temperature of the Ti ₃ A1 based alloy is generally in the É‘2+p two-phase region (Fig. 4). The Ti-23A1-17Nb prealloyed powder was treated by two different hot isostatic pressing processes, and the results showed little difference, as shown in Table 3.

Figure 4 pseudo binary phase diagram of Ti-23A1-17Nb and Nb

Table 3 HIP process and performance of PM-Ti-23A1-17Nb

The microstructure of the hot isostatically pressed PM Ti-23Al-17Nb was observed, and the microstructure was found to be relatively small. The O phase precipitated around the α ₂ phase and showed a network distribution (Fig. 5).

Figure 5 Microstructure of PM Ti-23Al-17Nb after hot isostatic pressing

(III) Heat treatment process of PM Ti-23Al-17Nb

As an intermetallic compound, Ti ₃ Al-based alloys can obtain different microstructures under different heat treatment conditions, and thus obtain different properties.

The strength and elongation of PM Ti-23A1-17Nb after hot isostatic pressing are relatively low, and it is necessary to carry out solid solution + aging treatment in the two-phase region of α ₂ + βB/+O. Four process system tests were carried out in the experiment, and the properties of the obtained materials are shown in Table 4.

Table 4 PM Ti-23A1-17Nb heat treatment process and performance

AC: Air Cooling; FC: Furnace Cooling

PM Ti-23A1-17Nb has higher strength after solution treatment, but the solid solution PM Ti-23A1-17Nb is a metastable state (as in heat treatment systems 1 and 2 in Table 4), and thus requires aging treatment. Figure 6 is a microstructure of FC at 960 ° C, 1 h, AC + 860 ° C, 8 h, FC and 930 ° C, 1 h, AC + 860 ° C, 8 h.

Fig. 6 Microstructure of PM Ti-23A1-17Nb after solid solution aging at 960 ° C, 1 h, AC + 860 ° C, 8 h, after 930 ° C, 1 h, AC + 860 ° C, 8 h, FC solid solution after PM Ti- Microstructure of 23A1-17Nb (b)

The strength of PM Ti-23A1-17Nb decreased after solution aging, but the elongation increased. After analysis, the content of β phase in PM Ti-23A1-17N6 is higher than that in hot isostatic state after solution aging. Secondly, after solution aging, the α ₂ phase is more evenly distributed, so that the alloy has good strength and plasticity.

(IV) Fracture analysis of PM 71-23A1-17Nb hot isostatically pressed test bars

The fracture of the tensile specimen of PM Ti-23A1-17Nb hot isostatically pressed test bar was analyzed by scanning electron microscopy. As can be seen from Fig. 7(a), the fracture of the test bar is relatively flat, similar to brittle fracture. Then the fracture source (Fig. 7(b) and Fig. 7(d)) ​​and the extension zone (Fig. 7(c)) were observed. It was found that the fracture of the test bar started from the boundary of the grain, and the extended zone appeared. The river-like texture proves that the fracture of the PM Ti-23A1-17Nb tensile test bar is a cleavage fracture. In the analysis, a simple chemical composition test was performed on the fracture source in Fig. 7(d) using an electron probe, and it was found that the content of the alloying elements here was much higher than that in other regions. At the grain boundary, PM Ti-23A1-17Nb produces relatively obvious segregation and inclusion aggregation, which also makes it a source of fracture for tensile bars.

Figure 7 Scanning electron micrograph of PM Ti-23A1-17Nb fracture

   Third, the conclusion

(1) The PM Ti-23A1-17Nb alloy with uniform microstructure can be obtained by hot isostatic pressing of the prealloyed powder, but its elongation is relatively low.

(2) The elongation of PM Ti-23A1-17Nb alloy is improved after solution aging. However, the difference in the solution aging process will bring about a large change in the performance of the PM Ti-23A1-17Nb alloy.