High Throughput Nanomechanical Response of Rolled Homogenous Armor (RHA) Steel

Rolled homogeneous armor (RHA) steels have been used extensively in vehicle armor due to their high strength, high resistance to ballistic impacts, and penetration coupled with ease of fabrication. These steels are initially cast into required sizes and undergo various thermomechanical treatments to achieve desired properties. Subsequently, they are hot rolled to homogenize the grain structure across the plate and reduce thickness. Existing literature on these steels has primarily focused on compositional variations or thermal history adjustments aimed at enhancing strength or toughness. However, a comprehensive understanding of the microstructure and elastic-plastic damage properties of the microscale constituents of these steels has been lacking despite its critical role in the ballistic response and the processing-structure-property-performance relationship.<br/>The martensitic RHA steel microstructure constitutes the hierarchy of the parent prior austenite grain boundary (PAGB), further divided into packets, blocks, and laths. This hierarchy is associated with the variants resulting from the transformation of parent austenite to martensite. During the diffusion-less transformation process, 24 possible martensitic variants can form, each sharing specific orientation relationships (ORs). If the resulting martensite laths share a common Bain orientation and originate from the same habit plane, the misorientation angle between adjacent laths will be low, typically between 2-3°. In contrast, a significantly higher misorientation angle, ranging from 50-60°, occurs if the laths arise from different Bain orientations. Laths within a specific block exhibit the same crystallographic orientation. The presence of only small misorientations between laths indicates that these structures are transparent to dislocation motion and, as a result, do not serve as strong barriers. Since the lath boundaries are the only low-angle boundaries, the blocks can be effectively considered as single grains within martensitic microstructures. The 24 possible orientation relationships (ORs) for the parent austenite allow for the formation of four distinct blocks within a given austenite grain. Blocks are separated by high-angle grain boundaries (HAGBs), which act as significant barriers to dislocation motion. Consequently, the size of these blocks is crucial in determining the overall strengthening of the material. The subsequent building unit in this series is a packet, which is formed by a collection of blocks sharing the same habit plane. To achieve a comprehensive understanding of the RHA steel microstructure, a detailed microstructural characterization using XRD, EBSD, and TEM has been carried out to outline the hierarchical microstructure of this material.<br/>The complex microstructural details in MIL-DTL-12560K RHA steel, where the length scales of interest can span from the nm to µm ranges, require specialized small-scale mechanical tools for testing and characterization. We utilized high throughput nanoindentation techniques coupled with the HR-EBSD technique to probe local nanomechanical response at various strain rates. Our current work provides nanoindentation properties as a function of orientation and strain rate from a single block without the influence of nearby high-angle grain boundaries. The indentation yield (<i>Y_ind</i>) surface map indicates a distinct orientation dependence for RHA steel at a strain rate of 10^-2 s^-1. The linear strain hardening co-efficient, however, showed an inverse correlation to Yind. The hardness and change in hardness with strain rate showed less sensitivity compared to yield to capture the orientation effect. Further analysis demonstrated that the block boundary misorientation, distance, and block size also affected the nanomechanical properties apart from block orientations themselves.