Tracking phase-level properties in heat-treated high-chromium cast irons using mechanical microscopy

Determining individual microstructural components in multicomponent and multiscale materials can be a challenging task. High chromium cast irons (HCCI), particularly in heat-treated states, exhibit complex microstructures that include austenite or martensite on a meso-scale, as well as hard and micro-sized eutectic carbides, and secondary carbides on a sub-micron or nanometric length scale. Assessing the mechanical properties and quantities of these phases requires small scale characterization over relatively large areas. In the current work, the mechanical properties of each individual constitutive phase were determined by using high-speed nanoindentation maps as a function of four different heat treatments. An area of 400 × 300 μm containing 30 000 indentations on each sample was analyzed. Statistical deconvolution and machine-learning clustering methods were used to determine the hardness and elastic modulus of the different microstructural components. The results illustrate how the microstructure and mechanical properties of the various phases evolve through heat treatment, supporting the notion that secondary carbides reinforce and strength the HCCI matrix, rather than acting as an individual component.