The δ phase evolution and its effect on the austenite grain growth in the solution treatment of superalloy GH4169 were investigated at the solution temperatures of 1233, 1253, 1273 and 1293 K and the holding time of 30, 45 and 60 min. The δ phase is dissolved and transformed from short bars to fine bars or particles gradually with the increasing solution temperature. The holding time affects the morphology and the amount of δ phase slightly. The austenite grain grows with the increasing solution temperature and holding time. It grows slowly at the solution temperatures ranging from 1233 to 1373 K, but sharply from 1273 to 1293 K. The phenomenon suggests that the δ phase generates a pinning effect on the austenite grain growth. The kinetic analysis about the austenite grain growth was carried out at the solution temperatures ranging from 1233 to 1373 K. The grain growth exponent n value is calculated to be 1·2 which confirms the pinning effect of δ phase on the austenite grain growth, and activation energy Q value is calculated to be 394·1 kJ mol−1. Finally, a model with an average difference 9·22% for the austenite grain growth was proposed.
Abstract. The TC8 titanium alloy was isothermally compressed at 1133 K and 1213 K in the (α+β) two phase region. The microstructural evolution and restoration mechanism in the α and β phases were characterized by optical microscopy and transmission electron microscopy. The results show a significant effect of phase content on the microstructural evolution and restoration mechanism. The grain refinement occurs in the α phase at both temperatures, but in the β phase only at the higher temperature of 1213 K. This difference in microstructural evolution is attributed to the different temperature dependence of restoration mechanisms in the two phases. A significant increase in the volume fraction of β phase makes the restoration mechanism in the β phase change from dynamic recovery (DRV) to dynamic recrystallization (DRX), which subsequently induces the β grain refinement.
In situ EBSD investigations were conducted to polycrystalline NiTi alloy during tensile deformation to study the variation selection of martensitic transformation and its effect on pseudoelasticity. Experiments show that more than one martensite variants are formed in most grains, and the directions of traces for some variants are close to the direction of macroscopic Lüders band. Then the Schmid factors of activated variants in each grain were calculated, the results show that Schmid factors of activated shear systems can be divided into three categories, I: maximum Schmid factor; II: sub-maximum Schmid; III: small Schmid factor. The appearance of the latter two categories is mainly influenced by constraints of neighboring grains and variety of local stress state caused by Lüders band. And the Schmid factors of residual martensites belong to the third category. It is mainly because dislocations are easily introduced at the front of the Lüders band during the transformation process, which decrease the order degree of B2 phase and increase the difficulty of reverse phase transformation, and the sample shows poor pseudoelasticity.
Solution treatment was carried on a post-deformed GH4586 superalloy in the solution temperature range of 1020°C to 1100°C and solution time of 2 h and 4 h. The results showed that the static recrystallization (SRX) occurred due to the residual distortion energy caused by high temperature deformation. With increasing solution temperature from 1020°C to 1100°C, the average grain size of γ matrix firstly decreased and then increased. With increasing solution time from 2 h to 4 h, the average grain size of γ matrix gradually increased from 24.2±3.2 μm to 30.9±3.8 μm. Grain boundaries and twinning boundaries are the main nucleated site for SRX. And when solution treated at 1020°C for 2 h, carbides can also become the preferential nucleation position. In addition, by analysing the relative deviation (v/vm ) distributions of Σ3 boundaries, it could be found that the higher solution temperature would lead to the increasing fraction of coherent Σ3 boundaries and the decreasing fraction of incoherent ones. Besides, the annealing twins nucleated and grew gradually during the migration of recrystallized grain boundaries, and their length fraction increased from 10.0% to 54.9% with increasing solution temperature.
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