Texture evolution, phase transformation mechanism and nanohardness of selective laser melted Ti-45Al-2Cr-5Nb alloy during multi-step heat treatment process

摘要

This paper for the first time systematic study the effect of heat treatment on the grains characteristics, texture evolution, phases transformation mechanism and nanohardness of Ti-45Al-2Cr-5Nb (at.%) alloy processed by selective laser melting (SLM). With increasing the annealing temperature, the grain size generally increases while the crystallographic orientations basically remain unchanged. The grains of all the samples are dominated by high-angle grain boundaries, with its volume fraction increases with increasing the annealing temperature. The content of B-2 phase decreases, while the contents of alpha(2) and gamma phases increase with the increase of annealing temperature. The phases evolution mechanism during the heat treatment is described as: (200)beta -> (11 (2) over bar0)alpha(2) + (111)gamma. The structure change from beta(beta(2)) phase to alpha(alpha(2)) phase is most likely achieved by the dispelling of Nb and Cr atoms, namely through transforming from the "... Ti-Nb/Cr-Al-Ti-Nb/Cr-Al ..." stacking sequence in (110) plane of the bcc structure to the "... Ti-Al-Ti-Ti-Ti-Al-Ti-Ti ..." stacking sequence in (0001) plane of the D0(19) (hcp) structure. The structure change from alpha(alpha(2)) to gamma is most likely achieved by the rearrangement of Ti and Al atoms through transforming from the "... Ti-Al-Ti-Ti-Ti-Al-Ti-Ti ..." stacking sequence in (0001) plane of the D019 (hcp) structure to the "... Ti-Al-Ti-Al ..." stacking sequence in (110) plane of the L1(0) (bcc) structure. The nanohardness ranging from 8.36 +/- 0.42 to 9.75 +/- 0.49 GPa with increasing the annealing temperature, which is much higher than the roll bonding fabricated TiB2 reinforced TiAl alloy. (C) 2017 Elsevier Ltd. All rights reserved.