Micromechanical behavior and thermal stability of a dual-phase α+α’ titanium alloy produced by additive manufacturing

Abstract

In order to improve the tensile properties of additively manufactured Ti 6Al 4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920°C followed by water quenching generates a dual-phase α+α′ microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α′ material as well as the thermal stability of the metastable α' martensite. To that end, annealing of the α+α′ microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α′ microstructure, the α′ phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α′ phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.

Towards work-hardenability of Ti-6Al-4V through a quenching and partitioning approach

“…The progressive decomposition of the martensite during the partitioning of the two as-quenched microstructures lead to different mechanical properties. As reported by several authors [5]…”

“…Then, fast cooling leads to the martensitic transformation of the stable high-temperature β phase into a V-enriched α' phase. When the quenching temperature is increased, the α' gets more enriched in aluminium and more depleted in vanadium [5]. For this purpose, with the rise of the quenching temperature, the chemical composition of the martensite is closer to the αroom temperature chemical composition and further from that of β. Consequently, α' martensite has an out-of-equilibrium composition that changes according to the solution treatment temperatures.…”

Tribological Behavior of Titanium Alloy Treated by Nitriding and Surface Texturing Composite Technology

“…Titanium and titanium alloys possess good comprehensive properties, such as low density, high specific strength, high temperature resistance, corrosion resistance, magnetic resistance, impact resistance, and good biocompatibility, having extensive application prospects in the aviation field [1,2]. However, the low hardness and poor tribological performance limit the applications of titanium alloys, such as the application of titanium alloy fasteners in aerospace [3,4].…”

Влияние охлаждения подложки на микроструктуру и фазовый состав изделий из титанового сплава Ti-6Al-4V, полученных методами аддитивных технологий

“…Многочисленные исследования микроструктуры титанового сплава Ti−6Al−4V, полученного методами аддитивных технологий либо подвергнутого электроннопучковой обработке [14,[21][22][23], показали, что при высокой скорости охлаждения титанового сплава Ti−6Al−4V атомы ванадия не успевают диффундировать к границам зерен или границам пластин α ′ -мартенсита, оставаясь в твердом растворе на основе α-Ti, и тем самым уменьшая вероятность образования β-фазы. При этом наличие ванадия в твердом растворе должно приводить к уменьшению параметров решетки α-Ti, а также к увеличению полных среднеквадратичных смещений атомов.…”