Systematic Identification of Constitutive Parameters for Crystal Plasticity Models of Non-Cubic Metal Alloys

非技术摘要构成我们日常生活基础设施（交通、发电等）的大部分材料主要是多晶和金属，例如钢、铝、镁或铜。多晶性意味着材料中的所有原子排列在规则的网格上，并且网格取向在相邻体积（其被称为“晶粒”或“微晶”）中不同。为了掌握这些结构重要材料的制造步骤并预测其使用性能，工程师们利用模型来描述这些材料的机械行为。这种模型有相当多的可调参数，这些参数对每种材料都是不同的，需要对每种材料进行量化。对于钛、镁、锡等原子排列复杂的金属来说，参数识别非常复杂。在本研究项目中，PI通过用特殊形状的针进行微小压痕的新方法，取代了通过观察难以获得的样品的变形来进行参数识别的传统方法。 具有这种成本效益的方法来建立材料模型将导致整个制造链的时间，能源和材料成本的大幅节省，并改善最终材料性能的预测。该计划还将通过让学生在多个层次接触科学和工程，包括研究生和本科生的教育和培训，为国家的知识基础设施做出贡献，从而带来广泛的好处。技术摘要由于可能缩短开发时间，节省成本和增强可靠性，计算建模与过程开发和设计的集成将继续加速。在基本水平上，结构金属的机械行为中的控制因素是位错对滑移的阻力，即位错运动的临界分解剪切应力，以及同时发生的结构演变（例如加工硬化）。因此，为了准确地描述构成结构部件的多晶阵列的变形、可能的损伤成核和断裂行为，有必要具有涉及物理变形过程的合理模型和进入这种模型的可调参数的准确值。虽然这些本构参数可以很容易地确定为许多立方金属使用成熟的单晶方法，他们更难以确定在许多非立方金属。这是因为即使在可以获得合适的单晶的情况下，不同滑移系类型的激活应力的大差异也可能使得在单晶的标准单轴测试中选择性地激活一些（组）滑移系成为不可能。多相材料带来了进一步的挑战，在历史上，在不受相邻相影响的情况下对单个相进行分析是非常困难的。只要选定的本构模型的可调参数是可信的，就可以进行任意变形路径的全场或平均场晶体塑性模拟。（例如，在金属成形），然后可以用来理解和预测各向异性变形和损伤成核在非-所提出的研究应用了一种新开发的方法，以相对快速和成本-可有效地应用于许多不同的单相和双相合金。在这项技术中，球锥形纳米压痕是用来连续询问足够多的不同的晶体取向在多晶样品的表面。然后，原子力显微镜被用来测量这些凹痕周围的地形，这是一个强大的功能的晶体取向和特定的本地活动的不同滑移系统。晶体塑性有限元（CPFE）模拟的压痕过程中，然后进行不同的本构参数，直到实现最佳匹配之间的测量和模拟的形貌在几个不同的压痕晶体上具有不同的取向/形貌。这种方法是有效的，因为轴对称球锥压痕几何形状导致许多不同的滑移系统以不同的速率和沿着不同的应变路径操作，这取决于材料的位置下的压痕。在本研究中，这种方法将被用来确定在一系列的钛基合金含有六方和体心立方相，以及在tetraxanthin的纯度为99%的本构参数确定的商业上重要的合金将有直接的影响能力，开发集成的计算材料工程（ICME）的数据和模型在各种长度尺度。这将大大节省时间、能源和材料成本。总体而言，能够可靠地预测非均匀变形对材料加工非常重要，这是在基于物理的置信度进行性能或可靠性预测之前所需的。

Non-Technical SummaryMost of the materials that make up our daily-life infrastructure (transportation, power generation, etc.) are predominantly polycrystalline and metallic, such as steel, aluminum, magnesium, or copper. Polycrystallinity means that all atoms in the material are arranged on a regular grid and the grid orientation is different in neighboring volumes (which are termed "grains" or "crystallites"). To master the manufacturing steps and predict the in-service performance of such structurally important materials, engineers make use of models that describe the mechanical behavior of these materials. Such models have a fair number of adjustable parameters, which are different for each material and need to be quantified for each of them. This parameter identification is quite involved, particularly for metals that have complicated atomic arrangements, such as, for instance, titanium, magnesium, or tin.In this research project, the PIs replace the traditional way of parameter identification done by observing the deformation of difficult to obtain samples, by a new method that puts small impressions with a specially shaped needle. Having such a cost-effective means to establish the material model will lead to substantial savings in time, energy, and material cost across the manufacturing chain and improved prediction of final material properties. This program will also have broad benefits by exposing students at multiple levels to science and engineering, including the education and training of both graduate and undergraduate students to contribute to the nation's intellectual infrastructure.Technical SummaryThe integration of computational modeling into process development and design continues to accelerate due to the potential shortened development times, cost savings, and enhanced reliability. At the fundamental level, the controlling factors in the mechanical behavior of structural metals are the resistance of dislocations to slip, i.e. the critical resolved shear stress for the motion of dislocations, and the concurrent structural evolution (e.g. work hardening). Thus, in order to accurately describe the deformation, possible damage nucleation, and fracture behavior of the polycrystalline arrays that make up structural components, it is necessary to have a sound model with physical deformation processes involved and accurate values for the adjustable parameters that enter such models. While these constitutive parameters can be readily determined for many cubic metals using well established single crystal methods, they are much more difficult to ascertain in many non-cubic metals. This is because even in cases when suitable single crystals can be obtained, the large differences in activation stress for different slip system types can make it impossible to selectively activate some (set of) slip systems in standard uniaxial tests of single crystals. Multiphase materials pose a further challenge, where historically it has been very difficult to carry out analysis on the individual phases without the influence of neighboring phases. Provided that adjustable parameters of a selected constitutive model are available with confidence, full or mean field crystal plasticity simulations of arbitrary deformation paths (occurring, for instance, in metal forming) can then be used to understand and predict the anisotropic deformation and damage nucleation in non-cubic metals.The proposed research applies a newly developed approach to determine parameters of the constitutive description in a relatively rapid and cost-effective manner to a number of different single and dual phase alloys. In this technique, sphero-conical nano-indentation is used to serially interrogate a sufficiently large number of different crystal orientations at the surface of polycrystalline samples. Atomic force microscopy is then used to measure the topography around these indents, which is a strong function of the crystal orientation and the specific local activity of different slip systems. Crystal plasticity finite element (CPFE) simulation of the indentation process is then carried out with varying constitutive parameters until an optimal match is achieved between the measured and simulated topographies in several different indents on crystals with different orientations/topographies. This method is effective because the axisymmetric sphero-conical indentation geometry causes many different slip systems to operate at different rates and along different strain paths depending on the material location beneath the indent. In the present study this approach will be used to determine the constitutive parameters in a range of titanium-based alloys containing hexagonal and body-centered cubic phases as well as in tetragonal tin of 99% purity.The constitutive parameters identified for the commercially important alloys will have a direct effect on the ability to develop integrated computational materials engineering (ICME) data and models across a variety of length scales. This will lead to substantial savings in time, energy, and material cost. Overall it is very important for materials processing to be able to reliably predict heterogeneous deformation, which is required before prediction of performance or reliability can be made with physically-based confidence.