Weakening aging-induced embrittlement via deformation-assisted regulation of isothermal ω precipitation in metastable Ti−15Mo alloy

Abstract

In order to overcome the embrittlement of metastable titanium alloys caused by the precipitation of ωiso phase during aging, regulation of isothermal ω precipitation was investigated in Ti−15Mo alloy. The results show that the sample is brittle when direct aging (A) is applied at 350 °C for 1 h after solution treatment (ST). If pre-deformation (D) is performed on the ST sample to induce {332} twins and secondary α′′ phase, subsequent aging at 350 °C (STDA350) improves the strength to 931 MPa with a good ductility of about 20% maintained. However, when aging is performed at 400 °C or 450 °C (STDA400/450), the strength can be further improved, but the ductility is dramatically reduced. Atomic-scale characterizations show that the partial collapse of ω phase in the STDA350 sample effectively eliminates aging-induced embrittlement, but complete collapse leads to poor ductility in the STDA400/450 sample.

1. Introduction

Metastable β type titanium alloys have been used in a wide range of aerospace, biomedicine and energy industries due to their high specific strength, low elastic modulus, and excellent corrosion resistance [1−3]. The tensile properties of the β titanium alloys have been shown to be significantly dependent on the deformation mode as a function of the stability of the β phase, which is controlled by the content and type of alloying elements [4,5]. With the increase of β phase stability, the deformation is alternately dominated by stress-induced martensite and ω phase [6,7], {332}〈113〉 twinning [8−11], {112}〈111〉 twinning [8,12], and dislocation gliding [8,13]. Transformation-induced plasticity (TRIP) and/or twinning-induced plasticity (TWIP) effects are introduced to achieve an excellent uniform elongation of above 20%, but usually result in a low yield strength generally below 600 MPa [13−15].

In order to increase the yield strength and maintain high work hardening rate for expanding the application of metastable titanium alloys, considerable efforts have been recently devoted to adjusting alloy composition for purposely activating one or multiple specific deformation modes. For example, minor alloying of Fe element into Ti−15Mo has successfully increased the yield strength to 837 MPa, but deteriorates the uniform elongation seriously [16]. A novel Ti−7Mo−3Cr alloy is designed to couple two twinning systems in the early deformation, achieving a yield strength of 695 MPa and a substantial uniform elongation of 33.3% [17]. Solution treatment (>Tβ) followed by water quench is usually applied in metastable β-Ti alloys in order to obtain complete β-grains with the presence of dense athermal ω phase (ωath) formed via a displacive phase transformation [18]. During aging at a low temperature range of 150−475 °C, ωath can further evolve into the isothermal ω phase (ωiso) by a diffusion-controlled process, resulting in uniformly dispersed ω precipitates in β matrix [19,20], but the ductility is deteriorated in the transition from ωath precipitates to ωiso ones [21]. Since first discovered in Ti−8Cr (wt.%) alloy [22], ω phase has attracted extensive attention due to its significant strengthening effect.