Abstract
New theory is presented to describe the occurrence of plasticity-induced transitions in titanium alloys. The approach is able to predict the composition dependence of transformation induced plasticity (TRIP), superelasticity, as well as martensite formation upon quenching. Martensite formation in the absence of stress is considered as the result of a competition between elastic strain energy and chemical driving force. Assuming that the formation of martensite is the result of a thermally activated nucleation process followed by athermal growth, a nucleation parameter is postulated to describe the conditions under which martensite is formed upon quenching; the parameter accounts for the ratio between the available thermal energy and an energy barrier for nucleation, suggesting that ω phase is not the main factor controlling martensite inhibition. This nucleation parameter is able to describe, for the first time, martensite occurrence in 130 alloys from the literature, quantifying the martensite start temperature (Ms) reported for 49 alloys with great precision. An empirical parameter ([Fe]eq) is proposed and, when combined with the Ms prediction, it allows to define regions within which TRIP and superelasticity occur. By defining threshold values for the Ms, the [Fe]eq and the nucleation parameter, candidate alloys likely to display TRIP, superelasticity or martensitic transformation upon quenching can be identified. As a result, this method can be adopted to design alloys with tailored plasticity behaviour.
Highlights
Titanium alloys are used for various industrial applications including aircraft, medical devices and marine structures [1]
In the case of transformation induced plasticity (TRIP) alloys, it may seem paradoxical that the calculated martensite start temperature (Ms)∗ lies above room temperature when the experimental evidence shows that no martensite forms upon quenching
In the case of Ti–12Mo, the Ms∗ for a standard grain size of 100 μm is 437 ∘C and stable plates of martensite are expected to form upon deformation
Summary
Titanium alloys are used for various industrial applications including aircraft, medical devices and marine structures [1]. The most famous design tool for TRIP and TWIP alloys is the method initially proposed by Morinaga et al [10], sometimes called “Bond Order method” It consists in mapping alloys as a function of two composition-dependent electronic parameters: the bond order between atoms (Bo) and the metal d-orbital level (Md). A criterion combining Ms∗ and a new empirical parameter ([Fe]eq) is proposed to identify the composition dependence of TRIP, superelasticity or slip/twinning. This is the first approach capable to predict the occurrence and composition dependence of such wide range of transformation and deformation behaviours
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