SGER: Thermal Modification of Alloy Composition Profile

技术：给定体积成分的一系列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路径的位置相关性；组成元素的表面偏析是一个长期存在的例子。传输特性敏感地受元素组成分布的结构的影响，因此变得依赖于加工历史。文献中充斥着对给定属性的数值变化很大的例子，例如光谱发射率和热扩散率。这种情况之所以持续存在，有两个基本原因：第一，没有在介观尺度上建立起连接微观晶格水平分析和宏观热化学模型的理论框架；第二，到目前为止，还没有任何基于组成分布的热物理性质表征的相应实验方法。高风险/高回报和变革性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)羽流的时间分辨光谱同时测量选定的传输特性和当地元素组成。该项目得到了大量证据的支持，这些证据表明，特定合金样品的成分分布可以以可重复的方式进行热修改；成分原子的不同热传输可能导致近表面成分分布的重大修改。该项目的目标是通过描述施加的热驱动和通过直接位置分辨测量检查元素成分分布的移动来证明修改是因果的。将对样品应用几种不同的热循环程序，并将用LPP方法分析成分分布。这项研究最初将集中在一种镍铬二元合金上。该项目研究具有很高的风险，因为上面概括的合金对热循环的响应方式可能不适用于更广泛的合金系统。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，而不是一个广泛类别的合金的代表性系统。我们的理由是，使用更简单的系统，如双元素合金，将是检查潜在不确定性的有效方法。非技术性：近表面成分异常的现象表明，有可能通过有意改变合金的近表面成分，以可预测的方式主动改变合金的表面性质。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性。对相关原子传输机制的严格理解将有助于制定实现这些目标的预测模型。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为特定功能合金开发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革命性的影响，为定量建模提供基础。调查还将有助于从研究生队伍中培养出新一代专家；在为期一年的SGER项目期间，将完成一项硕士学位工作。以第一原则为基础的对原子传输过程的理解将对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.