Phase morphology, variants and crystallography of alloy microstructures in cold dwell fatigue

• Published in: International journal of fatigue Vol. 113; pp. 324 - 334
• Main Authors: Zhang, Zhen, Dunne, Fionn P.E.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.08.2018; Elsevier BV
• Subjects: Basketweave; Burgers orientation relationship; Cold dwell fatigue; Creep; Creep (materials); Crystallography; Dependence; Dislocations; Kinematics; Load shedding; Low temperature resistance; Materials creep; Materials fatigue; Metal fatigue; Microscale kinematic confinement; Microstructure; Morphology; Slip; Stress relaxation; Titanium alloy; Titanium alloys; Titanium base alloys; Variants; Titanium alloy; Variants; Creep; Basketweave; Microscale kinematic confinement; Cold dwell fatigue; Burgers orientation relationship
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2018.03.030

•The Stroh model was revisited for the anisotropic rate dependence in hexagonal metals.•In rogue grain combination, basal slip activation gives higher cold creep.•Of the Ti alloy microstructures, multi-variant basketweave inhibits cold creep most.•Multi-variant basketweave inhibits dwell fatigue by microscale kinematic confinement. This paper examines microscale crystal slip accumulation, cold creep, and stress redistribution (load shedding) related to dwell fatigue in a range of α–β Ti alloy microstructures. The role of basal slip and prism slip is evaluated in load shedding in a rogue grain combination. The results enrich the Stroh dislocation pile up interpretation of dwell by accounting for the anisotropic rate dependence of differing slip systems together with morphology. Microstructural morphology has been found to play an essential role in cold creep and load shedding in dwell fatigue. Basketweave structures with multiple α variants have been shown to give the lowest load shedding for which the mechanistic explanation is that the β lath structures provide multiple, small-scale α variants which inhibit creep and hence stress relaxation, thus producing more uniform, diffuse stress distributions across the microstructure through microscale kinematic confinement, imposed by multi (α)-to-single (β) BOR relations (i.e. multiple α variants sharing the same parent β grain). The critical consequence of this is that alloys typically having multi-variant basketweave structure (e.g. Ti-6246), remain free of dwell fatigue debit whereas those alloys associated with globular colony structures (e.g. Ti-6242) suffer significant dwell debit. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.