Z. Lin

Reaction mechanism in high Nb containing TiAl alloy by elemental powder metallurgy

High Nb containing TiAl alloy was fabricated in argon atmosphere by reactive hot pressing process. Reaction mechanism was investigated by means of microstructural analyses and thermodynamic calculations. The results show that it is feasible to prepare high Nb containing TiAl alloy with fine lamellar colonies by reactive hot pressing process. The reaction between Ti and Al powders is dominant in Ti-Al-Nb system. Nb powders dissolve into the Ti-Al matrix by diffusion. Pore nests are formed in situ after Nb powders diffusion. The hot pressing atmosphere is optimized by thermodynamic calculations. Vacuum or argon protective atmosphere should be adopted.

Microstructure instability of fully lamellar TiAl alloy containing high content of Nb after long-term thermal cycling

Abstract Microstructure instabilities of the fully lamellar Ti–45Al–8.5Nb–(W,B,Y) alloy were investigated by SEM and TEM after long-term thermal cycling (500 and 1000 thermal cycles) at 900 °C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling: 1) The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2) The α 2 lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of a 2 lamellae through phase transformation of α 2 →γ. The γ grains nucleate within the α 2 lamellae or (α 2 +γ) lamellae in a random direction.

Homogenization treatment of high Nb containing TiAl alloys with as-cast and as-forged microstructures

Abstract The effect of heat treatment on the microstructure evolution of a high Nb containing TiAl alloy has been studied. The results indicate that β-segregation, α-segregation and S-segregation in the as-cast and as-forged alloys can be effectively eliminated at the temperature above T α (1350-1400°C) for long holding time (12-24 h) and the full lamellar (FL) microstructure is gained. For the two alloys, the lamellar colony sizes are 120 μm and 2000 μm, respectively after heat treatment at 1400°C for 12 h. Meanwhile, the sizes are 210 μm and 3000 μm, respectively at 1350°C for 24 h. To get a fine homogenous microstructure, the primary as-cast alloy is first subjected to preheat treatment for eliminating the segregations. After the preheat treatment, the alloy is processed by the multi-step canned forging to attain the microstructure with fine grain size.

Dynamic recrystallization during hot compression of α-Fe

Specimens of high purity α-Fe were deformed in the GLEEBLE-1500 at temperatures of 550°C, 700°C, 800°C and 900°C at strain rates ranging from 0.001 to 10 s−1. The microstructural changes, which occur during the hot compression, have been investigated by optical microscopy and related to the true stress-true strain curves. The experimental results show that the dynamic recrystallization is accelerated with increase of deformation temperature and decrease of strain rate. The relation between the dynamic recrystallization and Z-parameter has been investigated. Dynamic recrystallization takes place approximately in a certain range of Z parameter, i.e., 25 < lnZ < 37.