基于背靠背相变的橡胶金属钛合金优异性能的微观机理

Recently, the Toyota research center developed one group of multifunctional high-strength Ti alloy, termed “Gum Metal”. These alloys exhibit a series of unique properties, such as high strength, nonlinear superelasticity, low elastic modulus, Invar and Elinvar behavior after severe cold deformation. They also claimed that all these superior properties originated only from one new plastic deformation mechanism, i.e. giant fault. However, later research with solid experimental facts proved that the plasticity of gum metal involved dislocation, deformation twinning and phase transformations. The underlying mechanism of the unique properties and deformation behaviors in gum metal has become a hot topic of strong dispute and also a key question to be solved in order to further advancing the alloy design of multifunctional high-strength Ti alloy.. .Our on-going research work has found a new phase transformation phenomenon – a nanotwinning-assisted β-->α”/a-ω backpack transformation in the gum Ti system: upon quenching, gum Ti exhibits the backpack formation of {-220}α” compound twins and athermal-ω (a-ω) accommodation phases. It results in a microstructure of nano-layer α”/a-ω composite. Furthermore, by first-principle simulations, gum metal sits in a tri-critical region of Ti-Nb phase diagram, where the β, α and ω have similar thermodynamic instability. It provides a physical reasoning for such a unique transformation mode to proceed, and also indicates that the phase transformations in gum metal are very sensitive to the composition altering, which might be the origin of the strong dispute in this community...Therefore, all the superior properties of gum metal Ti-alloy should originate from this unique backpack transformation, rather than the so call “giant fault”...The projects in the current proposal are designed to reveal the underlying microscopic mechanisms based on the newly found backpack transformation. Firstly, the reversibility of the transformation will be studied during heating/cooling and tensiling, in order to reveal the microscopic mechanism of the superelasticity in the as-quenched gum metal; secondly, among the steps of cold-rolling, the microscopic phase morphology will be monitored by post-mortem TEM to examine how the nano-layer α”/a-ω composite evolves into the final as-rolled structure, and it will be further studied to reveal the mechanism of the Invar and Elinvar properties by insitu heating/cooling TEM observations. At last, the reversibility of the final as-rolled structure as well as its interaction with the dislocation and deformation twin will be studied down to atomic scale, based on which the microscopic mechanisms of the low modulus, nonlinear superelasticity and high strength in the cold-worked gum Ti-alloy will be revealed. Our study may serve as a guideline for the alloy designing of gum-metal-like Ti alloy with more advanced mechanical properties.

橡胶金属钛合金的Invar、伪弹性以及低模量高强度等优异性能均被认为源于大平面缺陷新型塑性变形机制，但亦有研究却表明其可能源于孪晶或位错或应力诱发相变。申请人前期工作在淬火态橡胶金属中发现了新型β-->α”/a-ω背靠背相变并形成了纳米层状α”/a-ω复合相；结合理论计算发现的三相点亦证明了该相变的合理性并指出该领域的争议可能源于橡胶金属性能的成分敏感性。因此，橡胶金属的优异性能可能完全来自上述背靠背相变，而非大平面缺陷。据此，本项目拟首先研究淬火态合金微观组织的可回复性能，完善对该相变模式的理解；进而研究冷加工过程中合金相结构演化规律及其微观机制，揭示橡胶金属Invar等特性的微观起源；最后通过研究冷加工后合金拉伸过程中显微相组织演变和可回复性能规律及其与位错和变形孪晶相互作用规律，揭示其低模量和超大伪弹性的物理起源以及相变增强微观机制，为设计性能更加优异的类橡胶金属钛合金提供理论基础。

橡胶金属钛合金的Invar、超弹性以及低模量高强度等优异性能均被认为源于大平面缺陷新型塑性变形机制。然而，申请人前期工作在淬火态橡胶金属中发现了新型β-->α”/a-ω背靠背相变并形成了纳米层状α”/a-ω复合相；结合理论计算发现的三相点亦证明了该相变的合理性并指出该领域的争议可能源于橡胶金属性能的成分敏感性。因此，橡胶金属的优异性能可能完全来自上述背靠背相变，而非大平面缺陷。.基于这一发现，本项目研究并证明了淬火态合金微观组织的可回复性能，完善对该相变模式的理解；进而研究冷加工过程中典型热弹马氏体相变及新型β-->α”/a-ω背靠背相变合金中微观结构演化规律。在冷变形后的典型马氏体相变Ti-Ni基合金中发现了新型位错致应变玻璃相变并进而发现具有新型短程有序（B19’）的位错致应变玻璃相变，阐明位错致应变玻璃相变中的大温区超弹性的微观机制。基于此，揭示了冷变形态橡胶金属低模量、超大伪弹性和Invar特性的微观起源。本项目的研究成果为设计性能更加优异的类橡胶金属钛合金提供理论基础。.