SGER: Thermal Modification of Alloy Composition Profile

技术：一组给定体积成分的金属合金可以沿着许多火法冶金途径中的任何一种进行加工。元素组成可能表现出特定于加工途径的位置依赖性；组成元素的表面分离是一个长期存在的例子。输运性质受元素组成结构的敏感影响，因此依赖于加工历史。文献中充满了一个给定性质的数值变化很大的例子，例如光谱发射率和热扩散率。这种状态持续存在有两个基本原因：第一，在介观尺度上没有任何强有力的理论框架来连接微观晶格级分析和宏观热化学模型；二是迄今为止，还没有相应的基于组分剖面表征热物性的实验方法。高风险/高回报和变革性的项目带来了一种新的测量方法和必要的仪器，可以通过激光产生的等离子体（LPP）羽流发射的时间分辨光谱同时测量选定的输运性质和局部元素组成。该项目得到了大量证据的支持，这些证据表明，可以以可重复的方式对给定合金试样的成分剖面进行热修改；组成原子的不同热输运会引起近表面组成谱的显著改变。该项目的目标是通过表征施加的热驱动和通过直接位置分辨测量检查元素组成剖面的运动来证明修改是因果关系。几种不同的热循环时间表将应用于样品，组成剖面将通过LPP方法进行分析。这项研究最初将集中在镍铬二元合金上。项目研究具有高风险，因为上述合金对热循环的反应方式的概化图可能不适用于更大范围的合金系统。研究中使用的主要系统（低熔点四元素伍德合金）作为模型系统可能被证明是一个例外，而不是广泛的合金类别的代表性系统。我们认为，使用更简单的系统，如双元素合金，将是一种有效的方法来检查潜在的不确定性。非技术性：近表面成分异常现象表明，通过有意改变合金的近表面成分，可以以可预测的方式主动改变合金表面性能。它可能是为了提高材料的耐腐蚀性，抗划伤性或材料表面的磁性和电学性能。对相关原子输运机制的严格理解将有助于制定预测模型来实现这些目标。这种能力的影响将为美国的金属工业提供许多新的发展选择，更广泛的影响还将有助于开发特定功能合金材料加工的介观模型。这项研究一旦得到证实，将对合金设计和加工领域产生变革性的影响；这将为定量建模提供依据。调查还将有助于从研究生队伍中培养新一代专家；在为期一年的SGER项目期间，将完成一项硕士学位工作。基于第一性原理的原子输运过程的理解对于新材料的设计和材料利用的成本效益管理至关重要。

TECHNICAL: A run of metal alloy of given bulk composition may be processed along any one of many pyrometallurgical pathways. Elemental composition may exhibit position dependence specific to the processing pathway; surface segregation by constituent elements is a long-standing example. Transport properties are sensitively affected by the structure of the elemental composition profile and therefore become dependent of processing history. The literature is filled with examples of widely varying numerical values for a given property, such as spectral emissivity and thermal diffusivity. This state of affair has persisted for two basic reasons: one, there has not been any robust development of a theoretical framework in mesoscopic scales that bridges the microscopic lattice-level analysis and the macroscopic thermo-chemical models; and two, there had not been any commensurate experimental methodologies for composition profile based characterization of thermophysical properties until now. The high-risk/high payoff and transformative project brings to the table a new measurement methodology and requisite instrumentation, which can provide measures of selected transport properties and local elemental composition simultaneously by time-resolved spectroscopy of laser-produced plasma (LPP) plume emissions. The project is supported by a body of evidence that the composition profiles of a given alloy specimen can be modified thermally in a reproducible manner; disparate thermal transport of constituent atoms can incur significant modifications of near-surface composition profiles. The goal of the project is to demonstrate that the modification is causal by characterizing the imposed thermal drive and examining the movement of the elemental composition profile by direct position-resolved measurement. Several different schedules of thermal cycling will be applied to the specimens and the composition profiles will be analyzed by the LPP methodology. The study will be focused initially on a nickel-chromium binary alloy. The project research has high risk in that the above-generalized picture of the ways an alloy responds to thermal cycling may not bear out over wider ranges of alloy systems. The primary system used in the study (low-melting point four-element Wood's alloy) as a model system could prove to be an exception, rather than a representative system for a wide class of alloys. We reason that use of simpler systems, such as two-element alloys, will be an effective approach to examine the potential uncertainty. NON-TECHNICAL: The phenomenon of near-surface composition anomaly suggests a potential for active modification of the alloy surface properties in a predictable manner by intentionally changing the alloy's near-surface composition. It may be to enhance material's corrosion resistance, scratch resistance or magnetic and electrical properties at material surfaces. Rigorous understanding of the relevant atom transport mechanisms will help formulate the predictive models to achieve these goals. The impact of such capability will provide a number of new development options to metals industries in the U.S. The broader impact will also be to help develop mesoscopic models of materials processing for functionality-specific alloys. The research once proven will have a transformative impact on the alloy design and processing fields; it will provide a basis for quantitative modeling. The investigation will also help produce a new generation of experts from the ranks of graduate students; during the one-year SGER project period a M.S. degree work will be completed. First-principle based understanding of the atom transport processes will be crucial to design of new materials and cost-effective management of materials utilization.