J Mater Sci Technol ›› 2012, Vol. 28 ›› Issue (5): 453-460.

• High Temperature Structural Materials • Previous Articles     Next Articles

Microstructure and Phase Transformation of a Niobium-rich TiAl-based Alloy Containing Boron and Carbon

Ziyong Chen, Xianglin Su, Zhilei Xiang, Zuoren Nie   

  1. College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
  • Received:2011-06-23 Revised:2011-12-13 Online:2012-05-30 Published:2012-05-29
  • Contact: Ziyong Chen
  • Supported by:

    the highrank talents program of Beijing University of Technology (No. J500900120081) and the high-rank talents program of Beijing Municipal Education Commission (No. 00900054R8002)

Abstract: Microstructure and phase transformation of Ti46Al8Nb0.5B0.2C alloy have been investigated. X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that as-cast and hot isostatic pressing (HIP) alloy mainly composed of γ and α2 phase have fully lamellar microstructure with point-like or ribbon-like TiB2 distributing in lamellar colony or at grain boundary. The mean size of lamellar colony is about 150 and 450 μm for as-cast and HIP alloy, respectively. The lamellar spacing is about 550 and 600 nm for as-cast and HIP alloy, respectively. It has been found that cooling rates and quenching temperatures have significant effect on phase transformation of Ti46Al8Nb0.5B0.2C alloy. When the alloy is treated at 1380 °C for 1 h and cooled from α domain, water cooling leads to complete α→α2 transformation, oil cooling leads to predominant α →α2 and part α→γm transformation, air cooling leads to α →α + γp2 →L(α + γ) →L(α2 + γ) transformation, and furnace cooling leads to α→α + γp3→L(α +γ) →L(α2 + γ) transformation. However, when the alloy is treated at 1400 °C for 1 h and cooled from α domain, water cooling leads to predominant α →α2 and part α →α +γp4 →γm transformation, oil cooling leads to α→α +γp5 →γm transformation, air cooling leads to α→ α +γp6→L(α + γ) →L(α2 + γ) transformation, and furnace cooling leads to α→α + γp7 →L(α +γ) →L(α2 +γ) transformation. Microstructural evolution of the alloy during various heat treatments has been examined and the phase transformation mechanisms have been elucidated. Based on the experimental observation, schematic CCT diagrams for the alloy have been given.

Key words: TiAl-based alloys, Heat treatment, Microstructure, Phase transformation