Alloy design by tailoring phase stability in commercial Ti alloys

G.-H Zhao,D.V Louzguine-Luzgin,P.E.J Rivera-Díaz-Del-Castillo,M.B Gamża,H Mao,X.Z Liang,X Xu

doi:10.1016/j.msea.2021.141229

Abstract

The mechanical characteristics and the operative deformation mechanisms of a metallic alloy can be optimised by explicitly controlling phase stability. Here an integrated thermoelastic and pseudoelastic model is presented to evaluate the β stability in Ti alloys. The energy landscape of β→α′/α″ martensitic transformation was expressed in terms of the dilatational and transformational strain energy, the Gibbs free energy change, the external mechanical work as well as the internal frictional resistance. To test the model, new alloys were developed by tailoring two base alloys, Ti–6Al–4V and Ti–6Al–7Nb, with the addition of β-stabilising element Mo. The alloys exhibited versatile mechanical behaviours with enhanced plasticity. Martensitic nucleation and growth was fundamentally dominated by the competition between elastic strain energy and chemical driving force, where the latter term tends to lower the transformational energy barrier. The model incorporates thermodynamics and micromechanics to quantitatively investigate the threshold energy for operating transformation-induced plasticity and further guides alloy design.

Highlights

The thermodynamics and micromechanics of phase stability are fundamental to design Ti alloys displaying effects such as twinninginduced plasticity (TWIP), transformation-induced plasticity (TRIP), shape memory and superelasticity [1,2,3]
In order to design alloys combining superior mechanical characteristics, the first priority is to understand the microstructure at ambient temperature
For Ti alloys, the high temperature body-centred cubic β phase transforms martensitically into a hexagonal close packed α′ phase upon quenching, where α′ is crystallographically identical to the equilibrium α phase

Summary

Introduction

The thermodynamics and micromechanics of phase stability are fundamental to design Ti alloys displaying effects such as twinninginduced plasticity (TWIP), transformation-induced plasticity (TRIP), shape memory and superelasticity [1,2,3]. The beta-type Ti alloys are arguably the most versatile in the Ti family [4,5,6], and widely applied in safety-critical aerospace components [7,8]. For Ti alloys, the high temperature body-centred cubic (bcc) β phase transforms martensitically into a hexagonal close packed (hcp) α′ phase upon quenching, where α′ is crystallographically identical to the equilibrium α phase. The solid metastable β phase may transform, under external stress, into a distorted hexagonal struc ture designated α′′ which has an orthorhombic unit cell. The α′′ phase nucleates heterogeneously as thin laths at existing subgrain boundaries with (112)β//(020)α′′ and [110]β//[001]α′′ orientation relationships [15]

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Methods

Conclusion