Transitional Creep Mechanisms in Textured Low c/a-Ratio Hexagonal Close Packed Metals

技术概要：钛和锆合金除了在生物医学和化学工业中的其他应用之外，还分别在运输和能源技术中找到广泛的应用。特别需要出现在这些纹理合金的基本变形机制的理解，因为遇到较低的应力，在服务条件下的那些。最近对细晶Ti3Al2.5V合金管材的研究揭示了一种新的变形机制，在较低的应力下，应变速率被发现比扩散蠕变机制（如Coble蠕变）预测的应变速率高几个数量级。此外，在位错蠕变占主导地位的测试条件下，已经表明刃型位错的攀移控制低应力下的蠕变，而啮合螺型位错在高应力下做出显著贡献。蠕变后的微观结构需要区分这些机制，因为蠕变参数的测量，如应力指数和活化能不能区分它们。使用不同的加载条件下，对其他钛和锆合金的进一步研究是必要的。这些低c/a比的六方金属表现出导致复杂的双轴各向异性的晶体织构，必须适当地考虑。建议的研究解决这些重要方面，通过研究，不仅涉及标准的单轴蠕变试验，但也各向异性双轴蠕变的表征使用封闭端内部加压薄壁管叠加轴向loading.NON-TECHNICAL摘要：锆和钛合金通常被用作结构材料在核和化学工业作为薄壁管进行复杂的双轴载荷。为了预测由这些材料制成的结构的尺寸变化，需要对变形机制有透彻的了解。当接近与典型操作条件下的应力相等的较低应力时，机制的变化尤其如此。这些随时间变化的变形的转变（即，蠕变）可通过将短期高应力和高温数据盲目外推到这些低水平而导致危险的应变率和寿命的非保守估计。目前提出的研究强调在这些重要的结构合金的过渡蠕变机制。接触这些实验和建模工作的学生将能够欣赏在攻击真实的生活问题时需要解决的复杂现象。该项目将积极参与青年研究者计划，为高中学生在夏季通过参与这些学生的研究。

TECHNICAL SUMMARY: Ti and Zr alloys find extensive applications in transportation and energy technologies respectively in addition to other applications such as in biomedical and chemical industries. Particular needs arise in the basic understanding of the underlying deformation mechanisms in these textured alloys as lower stresses are encountered, comparable to those under service conditions. Recent investigations on fine grained Ti3Al2.5V alloy tubing revealed a new deformation mechanism at lower stresses, under which strain-rates were found to be orders of magnitude higher than those predicted by diffusional creep mechanisms such as Coble creep. In addition, under testing conditions where dislocation creep is predominant, it has been shown that climb of edge dislocations controls creep at lower stresses while jogged screw dislocations make significant contribution at higher stresses. Microstructures following creep are required to differentiate these mechanisms since the measurement of creep parameters such as the stress exponent and activation energy cannot distinguish between them. Further investigations using different loading conditions, on other Ti and Zr alloys, are imperative. These low c/a-ratio hexagonal metals exhibit crystallographic textures that lead to complex biaxial anisotropy that must be properly taken into account. The proposed research addresses these important aspects through studies involving not only standard uniaxial creep tests, but also the characterization of anisotropic biaxial creep using closed-end internally pressurized thin-walled tubing superimposed with axial loading.NON-TECHNICAL SUMMARY: Zr and Ti alloys are commonly used as structural materials in the nuclear and chemical industries as thin-walled tubing subjected to complex biaxial loadings. In order to predict the dimensional changes of the structures made from these materials, a thorough understanding of the deformation mechanisms is needed. This is especially true of the changes in the mechanism(s) as lower stresses, equivalent to those under typical operating conditions, are approached. These transitions in time-dependent deformation (i.e., creep) can lead to dangerous non-conservative estimates of the strain-rates and lifetimes by blind extrapolation of the short-term high-stress and high-temperature data to these low levels. The current proposed study emphasizes the transitional creep mechanisms in these important structural alloys. Students exposed to these experimental and modeling efforts will be able to appreciate the complex phenomena that need to be addressed in attacking real life problems. The project will actively participate in Young Investigator programs for high school students during the summers by involving these students in the research.