Microstructural Evolution in an Fe-10Ni-0.1C Steel During Heat Treatment and High Strain-Rate Deformation

The fine structure in an Fe-10Ni-1.0Mo-0.6Mn-0.6Cr-0.08V-0.1C (wt pct) steel in a multi-step heat-treated condition called 'QLT' is understood by tracking its evolution in each step using transmission electron microscopy (TEM) and atom probe tomography; further, the consequence of high strain-rate deformation on the site-specific change in the QLT microstructure is examined by microdiffraction and energy dispersive spectroscopy in the TEM. The QLT treatment results in two generations of austenite (arising from the L- and the T-step) with differing Ni content. The primary focus of this study is on the thermal and mechanical stability of the precipitated austenite and the influence of austenite chemistry and size on the stability. Kolsky bar compression experiments were conducted to isolate a situation where an adiabatic shear band propagated partway through the specimen. Examination of the microstructure in such a specimen showed both generations of austenite to be present far from the shear band, ahead of the shear band and adjacent to the shear band. The austenite immediately ahead of the shear band was fragmented. Within the shear band, evidence was found for deformation-induced transformation to martensite. These findings are discussed within the context of improved damage tolerance and ballistic resistance observed earlier in field tests.