Microstructural evolution and age hardening behavior of a new metastable beta Ti-2Al-9.2Mo-2Fe alloy

A study on microstructural evolution and age hardening behavior of a new metastable beta titanium alloy Ti-2Al-9.2Mo-2Fe was undertaken by microscopic observation, Vickers hardness and tensile tests in this research. The result showed that the athermal ω phase was formed in the beta matrix after solution treatment and followed by water quench, but it seemed that the athermal ω phase did not result in a considerable hardening (300 HV in hardness and 670 MPa in YS in ST condition). However, the isothermal ω phases with 10-40 nm and nano-scaled α platelets with 30-100 nm were observed in the alloy aged at lower temperatures (300-450 °C). The nano-scaled ω and α phase led to an attractive hardening effect (400-500 HV in hardness and above 1500 MPa in YS). However, micro-scaled α phase with 0.5-3 μm obtained in the samples aged at temperatures (500-600 °C) showed a moderate hardening (350-450 HV in hardness and 1100-1500 MPa in YS). The hardening went through an under-aging, peak-aging and over-aging due to the continuation of nucleation and growth of the α phases and subsequent coarsening. The coarsen α phases (3-5 μm) and grain boundary α layers (0.1-0.5 μm in thickness) obtained at high temperatures aging (650-750 °C) showed a poor hardening or even a softening (around 300 HV in hardness and below 1000 MPa in YS). The Ti-2Al-9.2Mo-2Fe alloy had a fast aging response, and can be age hardened to high hardness above 400 HV within 30 min at a broad aging temperature region. The age hardening curves can also provide a basic criterion for selecting aging treatment for a given stress level.