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

织构低 c/a 比六方密排金属的过渡蠕变机制

• 批准号: 0968825
• 负责人: K. Linga Murty
• 金额: $ 42万
• 依托单位: North Carolina State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-09-15 至 2019-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0968825&HistoricalAwards=false
• 关键词: Transitional; Creep; Mechanisms; Textured; Low

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.

技术综述：除了在生物医学和化学工业等方面的应用外，钛和锆合金还分别在交通技术和能源技术中有广泛的应用。在基本了解这些织构合金的基本变形机制时，会遇到与使用条件下的应力相当的较低应力。最近对细晶Ti3Al2.5V合金管材的研究揭示了一种新的低应力变形机制，在这种机制下，应变速率比Coble蠕变等扩散蠕变机制预测的应变速率高出几个数量级。此外，在位错蠕变占主导地位的试验条件下，在较低应力下，刃位错的攀移控制蠕变，而在较高应力下，折弯螺位错对蠕变有显著的贡献。需要蠕变后的微观结构来区分这些机制，因为蠕变参数的测量，如应力指数和激活能，无法区分它们。在不同加载条件下，对其他钛和锆合金进行进一步的研究势在必行。这些低c/a比的六方金属表现出的晶体织构导致了必须适当考虑的复杂的双轴各向异性。这项研究不仅涉及标准的单轴蠕变试验，而且还涉及封闭式内压薄壁管材叠加轴向载荷的各向异性双轴蠕变特性的研究。非技术概述：在核工业和化学工业中，锆钛合金通常用作承受复杂双轴载荷的薄壁管材的结构材料。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。这尤其适用于机构的变化(S)，因为接近较低的应力，相当于在典型操作条件下的应力。通过盲目地将短期高应力和高温数据外推到这些低水平，依赖时间的变形(即蠕变)中的这些转变可能导致对应变率和寿命的危险的非保守估计。目前提出的研究重点是这些重要结构合金中的过渡蠕变机制。接触到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调查员计划，让这些学生参与研究。