Characterization and Modeling of the Influence of Microstructure on Deformaton-Induced Martensite Formation in Steels
Abstract
Some structural steels of practical importance to the Navy contain austenite, which is a form of iron with a face-centered cubic crystal structure. In many alloys, austenite is metastable and can transform to martensite (with a body-centered cubic or tetragonal structure) in response to mechanical deformation. This deformation-induced martensitic transformation is useful to alloy designers because it provides additional flexibility in meeting multiple design criteria. For example, by delaying the transformation to large strains the stability of the alloy during plastic flow is improved, resulting in dramatically enhanced resistance to the effects of blast and penetration by ballistic fragments. Although certain principles of alloy design to activate these effects are known, we lack a detailed understanding of precisely how the transformation is influenced by the microstructure of the alloy. This project addresses this need by a combination of advanced, three-dimensional and in situ microstructural characterization and sophisticated crystal plasticity modeling. The structure of alloy steel samples is determined by a combination of high-energy x-ray diffraction microscopy (HEDM) and three-dimensional electron backscatter diffraction (3D-EBSD) using a femtosecond laser for serial sectioning. Together, these techniques provide a complete, 3D view of the structure, including the location, size, shape, and crystallographic orientation of every austenite grain. The 3D microstructural data are used as input to detailed crystal plasticity modeling to determine how the local microstructure influences the martensitic transformation, and how the transformation affects the surrounding material. To validate the models and obtain new insights into the course of the transformation and how it interacts with microstructure, we conduct HEDM in situ during quasi-static loading, and ordinary x-ray diffraction during dynamic loading. In addition to revealing details of the transformation, these techniques allow us to determine how the strain hardening of the steel — which directly affects the flow stability and thus the ballistic performance of the alloy — is influenced by the martensitic transformation. The improved understanding of the relationship between microstructure and deformationinduced formation of martensite is expected to enable design of new steel alloys and heat treatments with significantly improved performance, particularly in applications that require resistance to blast or penetration by ballistic fragments. In addition, our development of new experimental techniques (HEDM and 3D-EBSD) and computational models based on realistic microstructures will enable new studies of a wide range of metallic materials of broad interest to the Navy.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812604
Entities
People
- Todd C Hufnagel
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy