Strain-Rate Dependence of the Martensitic Transformation Behavior in a 10 Pct Ni Multi-phase Steel Under Compression

The deformation-induced transformation of metastable austenite to martensite can contribute to improved performance of many steel alloys in a range of applications. For example, one class of Ni-containing steels that has undergone consecutive heat treatments of quenching (Q), lamellarization (L), and tempering (T) exhibits improved ballistic resistance and low-temperature impact toughness. To better understand the origin of this improvement, we tracked the volume fraction of austenite present in a QLT 10 wt pct Ni steel during compression at low and high strain rates ( ε ˙ = 0.001 s - 1 and ε ˙ ≃ 2500 s - 1 , respectively) using ex situ vibrating sample magnetometry measurements and in situ time-resolved X-ray diffraction measurements. We observe that the austenite-to-martensite transformation occurs more readily during quasi-static loading than during dynamic loading, even at small values of applied strain, which is qualitatively different from the behavior of steels known to undergo a strain-induced martensitic transformation mechanism. We propose that the strain-rate dependence of transformation in the QLT 10 pct Ni steel is dominated by the transformation in small austenite particles, where stress-assisted martensitic transformation is likely to be the dominant mechanism. Indirect evidence for this hypothesis is provided by electron backscatter diffraction measurements of deformed specimens.