Bottom-up fundamental approach for characterizing plasticity and deformation in BCC and FCC high entropy alloys

非技术摘要：最近，有研究表明，当五种或五种以上的元素以几乎相同的原子浓度结合在一起时，一种新的金属类别就会出现。这些金属通常被称为多组分浓缩固溶体合金，或更常见的高熵合金(HEA)。近年来，由于可用于调节其机械、热、电和磁性能的广阔的组成空间，这些新的复杂合金引起了人们的极大关注。在过去的十年里，几种这样的合金被实验证明具有非常好的断裂韧性行为、低温延展性、高温强度保持性和拉伸延性。与传统金属合金相比，这些优异的性能预示着HEAs在广泛的功能和结构应用中具有巨大的潜力。然而，控制这些特性的基本机制还没有完全确定。因此，该奖项将支持从根本上量化控制这些复杂合金性能的机制的研究，特别关注它们在极端环境应用中的高温机械性能。这将通过利用自下而上的多尺度建模方法(即从原子到连续尺度)来实现。这种建模方法具有很高的科学和工程价值，特别是在材料基因组计划方面。该项目将研究、教育和宣传结合在一起也是一个核心组成部分，将侧重于：(1)提高巴尔的摩以非裔美国人为主的小学的STEM成绩；(2)通过机械和材料研究方面的实习，让当地一所历史上黑人大学的代表不足的学生参与进来；技术概要：这项研究的主要目标是从根本上确定控制面心立方(FCC)和体心立方(BCC)多组分浓缩固溶体合金低温延展性、加工硬化响应和高温强度保持的潜在机制。主要集中在面心立方Cantor类合金，如CoCrNi，CoCrFeNi和CoCrFeMnNi合金，它们具有非常良好的断裂韧性和低温塑性，以及耐火的体心立方HEAs合金，如HfNbTiZr，HfNbTaTiZr和NbTiZr，添加V，Mo，Ta和Al，它们具有显著的高温强度保持和拉伸延性。这将通过一种自下而上的耦合方法来实现，该方法结合了分子动力学(MD)、动力学蒙特卡罗(KMC)和三维(3D)离散位错动力学(DDD)模拟来预测面心立方和体心立方HEA中位错和孪生介导的塑性。这里提出的3D DDD模拟将被MD和KMC模拟所通知，还包括孪晶介导的塑性和由于局部原子浓度的统计变化而引起的螺位错交叉滑移激活能的波动。这些多尺度模拟的结果还将被用来开发一个分析模型来预测体心立方多组分浓缩固溶体合金的高温屈服应力。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:Recently, it has been shown that when five or more elements are combined in nearly equal atomic concentrations, a new class of metals emerges. These metals are typically referred to as multi-component concentrated solid solution alloys, or more commonly high entropy alloys (HEAs). These new complex alloys have drawn substantial attention in recent years due to the vast available composition space for tuning their mechanical, thermal, electrical, and magnetic properties. Over the past decade, several such alloys have been shown experimentally to demonstrate very promising fracture toughness behavior, low temperature ductility, high temperature strength retention, and tensile ductility. These superior properties, as compared to traditional metallic alloys, suggest immense potential for HEAs in a wide range of functional and structural applications. Nevertheless, the fundamental mechanisms that control such properties are not yet fully characterized. Accordingly, this award will support research to fundamentally quantify the mechanisms controlling the properties of these complex alloys, with specific focus on their high temperature mechanical properties for extreme environment applications. This will be accomplished by utilizing a bottom-up multiscale modeling approach (i.e. from atoms to continuum scale). This modeling approach is of high scientific and engineering interest especially with respect to the materials genome initiative. The integration of research, education and outreach in this project is also a central component and will focus on: (1) improve STEM achievement in a predominately African American elementary schools in Baltimore; (2) involve under-represented students from a local historically black college through internships on research in mechanics and materials; and (3) develop an education portfolio that will increase the knowledge of undergraduate and graduate students in fundamentals of state-of-the-art multiscale modeling.TECHNICAL SUMMARY:The primary objectives of this research are to fundamentally identify the underlying mechanisms controlling the ductility at low temperatures, the work-hardening response, and the retention of strength at high temperatures in face-centered cubic (FCC) and body-centered cubic (BCC) multicomponent concentrated solid solution alloys. Primarily, the focus will be on FCC Cantor-like alloys, such as CoCrNi, CoCrFeNi and CoCrFeMnNi alloys, which show very promising fracture toughness behavior and low temperature ductility, as well as refractory BCC HEAs alloys, such as HfNbTiZr, HfNbTaTiZr, and NbTiZr with V, Mo, Ta, and Al additions, which show significant high temperature strength retention and tensile ductility. This will be achieved through a bottom-up coupled approach, which combines molecular dynamics (MD), Kinetic Monte Carlo (KMC), and three-dimensional (3D) discrete dislocation dynamics (DDD) simulations to predict dislocation and twinning mediated plasticity in both FCC and BCC HEAs. The 3D DDD simulations proposed here will be informed by the MD and KMC simulations to also incorporate twinning mediated plasticity and fluctuations in screw dislocation cross-slip activation energy due to statistical variations in local atomic concentrations. The results of these multiscale simulations will also be used to develop an analytical model to predict the high temperature yield stress of BCC multi-component concentrated solid solution alloys.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.