Hydrogen-Induced Transformation Superplasticity of Titanium and Ti-6Al-4V

本文从实验和理论上研究了钛合金中最近发现的相变失配塑性和可逆化学循环诱发的超塑性现象。作为一般的变形机制，已知由于在同素异形转变期间产生的内部失配应力的外部应力偏置而发生转变失配塑性和超塑性。这些机制在金属中是众所周知的，金属通过在其同素异形体温度附近的温度循环而经历相变。最近，它表明，相同的机制可以通过在恒定温度下循环的化学组成，在重复添加和去除钛中的氢。这种新的变形现象研究在纯钛和钛合金中进行单向蠕变实验，其中施加的应力和氢循环特性系统地变化。基于控制失配应变发展的微观力学和氢在金属中的扩散的基本机制，连续力学，封闭形式的模型和有限元，数值方法的开发允许定量，预测描述的现象。实验旨在支持这些模型，这些模型将描述变形过程中的瞬时和平均应变率作为实验参数的函数。这是第一个系统的调查的新现象的氢诱导相变失配塑性在一个简单的金属（钛）和它的合金（Ti-6Al-4V）作为所有相关的化学和机械参数的函数。该计划将证明超塑性可以在化学循环下诱导（即，在重复循环时超过100%的应变累积以及平均应变速率和施加的应力之间的线性比例）。将开发模型，提供这种新的变形机制的理论和预测性的理解，内部应力产生和偏置松弛的力学基础上。这些知识将推进非平衡条件下塑性和超塑性的基本科学理解，并最终允许基于氢诱导相变失配超塑性的超塑性工业过程的发展。***

9987593DunandThe recently discovered phenomena of transformation-mismatch plasticity and superplasticity induced by reversible chemical cycling are investigated experimentally and theoretically in titanium. As a general deformation mechanism, transformation-mismatch plasticity and superplasticity are known to occur as a result of biasing by an external stress of internal mismatch stresses produced during an allotropic transformation. These mechanisms are well known in metals subjected to a phase change by temperature cycling around their allotropic temperature. Recently, it was shown that the same mechanism could be induced by cycling the chemical composition at constant temperature, upon repeated addition and removal of hydrogen in titanium. This novel deformation phenomenon is examined in pure titanium and in a titanium alloy by performing unidirectional creep experiments, where the applied stress and the hydrogen cycling characteristics are systematically varied. Based on the fundamental mechanisms controlling the micromechanics of mismatch strain development and the diffusion of hydrogen in metals, continuum-mechanics, closed-form models and finite-element, numerical methods are developed to allow a quantitative, predictive description of the phenomenon. Experiments are targeted to support these models, which will describe the instantaneous and average strain-rate during deformation as a function of experimental parameters.%%%This is the first systematic investigation of the novel phenomenon of hydrogen-induced transformation-mismatch plasticity in a simple metal (titanium) and one of its alloys (Ti-6Al-4V) as a function of all relevant chemical and mechanical parameters. The program will demonstrate that superplasticity can be induced under chemical cycling (i.e., accumulation of strains in excess of 100% upon repeated cycling and linear proportionality between average strain rate and applied stress). Models will be developed that provide a theoretical and predictive understanding of this new deformation mechanism, based on the mechanics of internal stress creation and biased relaxation. This knowledge will advance the basic scientific understanding of plasticity and superplasticity under non-equilibrium conditions, and will eventually allow the development of superplastic industrial processes based on hydrogen-induced transformation-mismatch superplasticity. ***