High strain rate behavior, transformation-induced plasticity and fracture toughness characterization of cast and additionally tempered Fe85Cr4Mo8V2C1 alloy manufactured using a rapid solidification technique

Abstract

This study focuses on the characterization of the microstructures of an FeCrMoVC alloy in two states (an as-cast and a heat-treated state) as well as the compressive strain rate-dependent material and fracture toughness behavior. Both microstructures consist of martensite, retained austenite and complex carbides. Tempering results in a transformation of retained austenite into martensite, the precipitation of fine alloy carbides, and diffusion processes. High yield stresses, flow and ultimate compressive strength values at a relatively good deformability were measured. The yield and flow stresses at the onset of deformation are higher for the heat-treated state due to higher martensitic phase fractions and fine precipitations of alloy carbides respectively. Compressive deformation causes a straininduced transformation of retained austenite to a 0 -martensite. Hence, both high-strength alloys are TRIP-assisted steels (TRansformation-Induced Plasticity). However, the martensitic transformation is more pronounced in the as-cast state due to higher phase fractions of retained austenite already in the initial state. Examinations of strained microstructures showed decreased crystallite sizes with increasing deformation. It is assumed that, during plastic deformation, the amount of low angle grain boundaries increases while the incremental formation of a 0 -martensite leads to decreased crystallite size. In general, lower microstrains were determined in the heat-treated state as a consequence of stress relaxation during tempering. In comparison to commercially available tool steels, the determined fracture toughness K Ic of both variants revealed relatively high fracture toughness values. It was found that the lower shelf of K Ic is already reached at room temperature. Higher loading rates _ K resulted in lower dynamic fracture toughness K Id values. Notch fracture toughness K A measurements indicate that the critical notch tip radii of the examined materials are slightly smaller than 0.09 mm.

The dynamic response of β titanium alloys to high strain rates at room and elevated temperatures

“…Hongyi Zhan 116 The University of Queensland Table 3 Parameters for the modified ZA model for Ti6554 with data at 1173K included in fitting process…”

“…Yang et al [114] and Bintu et al [115] reported that the strain hardening rate decreases at high strain rates in TWIP steels as adiabatic heating brought by high strain rate deformations will retard twinning. Russel et al [116] observed that in the Fe85Cr4Mo8V2C1 alloy higher strain rates promoted formation of martensite at the onset of deformation. Whether martensitic transformation will increase the flow stress or not depends on the hardness of martensite in relation to parent phase.…”

“…Li et al [166] also found that the triggering stress for the SIM transformation in the β Ti-V-(Cr,Fe)-Al alloys increased with increasing strain rates as the adiabatic heating induced by higher strain rates can stabilize the β matrix, thereby retarding the initiation of SIM. For TRIP steels: The results of experiment conducted by Russel et al [116] on the Fe85Cr4Mo8V2C1 alloy indicate that although high strain rates can promote the formation of martensite at the onset of deformation, the accompanying adiabatic heating will decrease the transformation rate with further straining. Lee et al [167] summarized the results of several dynamic experiments on steels where a contradiction aroused about the effects of strain rate on the martensitic transformations.…”