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

技术摘要：钛合金和锆合金除了在生物医学和化学工业等其他领域的应用外，还分别在交通运输和能源技术领域有着广泛的应用。当遇到与使用条件下相比较低的应力时，特别需要对这些织构合金的潜在变形机制有基本的了解。最近对细晶粒 Ti3Al2.5V 合金管的研究揭示了一种在较低应力下的新变形机制，在该机制下，应变率被发现比 Coble 蠕变等扩散蠕变机制预测的应变率高出几个数量级。此外，在位错蠕变占主导地位的测试条件下，研究表明，刃状位错的爬升在较低应力下控制蠕变，而锯齿螺旋位错在较高应力下做出显着贡献。需要蠕变后的微观结构来区分这些机制，因为应力指数和活化能等蠕变参数的测量无法区分它们。必须对其他钛和锆合金使用不同的负载条件进行进一步研究。这些低 c/a 比的六方金属表现出晶体结构，导致复杂的双轴各向异性，必须适当考虑。拟议的研究不仅涉及标准单轴蠕变测试，还通过叠加轴向载荷的封闭端内部加压薄壁管进行各向异性双轴蠕变的表征，解决了这些重要方面的问题。非技术摘要：Zr和Ti合金通常用作核工业和化学工业中的结构材料，作为承受复杂双轴载荷的薄壁管。为了预测由这些材料制成的结构的尺寸变化，需要彻底了解变形机制。当机构的变化接近较低的应力（相当于典型操作条件下的应力）时尤其如此。这些与时间相关的变形（即蠕变）的转变可能会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对应变率和寿命的危险的非保守估计。目前提出的研究强调这些重要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理解在解决现实生活问题时需要解决的复杂现象。该项目将在暑假期间积极参与针对高中生的青年研究员计划，让这些学生参与研究。

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.