We studied the role of nitrogen content on the stress-strain response of Hadfield steel (HS) single crystals under compressive loading. Two different nitrogen concentrations were examined for each orientation (0.05 wt pct and 1.06 wt pct) with drastic increase in critical resolved shear stresses (CRSSs) and strain-hardening coefficients compared to HS without nitrogen. The stress-strain response was strongly dependent on both the crystallographic orientation and the nitrogen concentration. Transmission electron microscopy (TEM) results revealed that, for the HS with 1.06 wt pct nitrogen, the hardening is influenced by the coexisting deformation twins and precipitates, which both act as strong obstacles against dislocation motion. A visco-plastic self-consistent (VPSC) model was modified to account for precipitation and twinning length scales in HS with 1.06 wt pct nitrogen for selected crystallographic orientations. Incoherent precipitates in the hardening formulation were treated as factors affecting the mean free path of dislocations. The model also accounts for plastic relaxation of precipitates with increasing strain and accurately predicts the stress-strain response.

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The present paper reports on the cyclic stress–strain response of body-centered cubic ultrafine-grained (UFG) interstitial-free (IF) steel severely plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Low-cycle fatigue tests were conducted with various strain amplitudes and strain rates on samples obtained through different ECAE routes and number of ECAE passes in order to determine the optimum processing route(s) for improved fatigue response of this material. UFG IF steel is superior to its coarse grained counterpart under both monotonic and cyclic loading in terms of properties, such as stress ranges tolerated, strength levels attained, and the corresponding fatigue behavior. All UFG steels subjected to more than 4 ECAE passes exhibit stable cyclic stress–strain response. Moreover, it was shown that dynamic grain coarsening, which usually leads to cyclic softening in UFG materials, is not prevalent in the ECAE processed UFG IF steel. For representing the fatigue life of UFG IF steel, the parameter after Smith, Watson and Topper, which is an indication of energy dissipation per cycle, proved adequate while comparing materials obtained through different ECAE routes.

This work focuses on the stress-induced transformation in solutionized and overaged single-crystal NiTi alloys. The potential role of detwinning on the recoverable strains was investigated both theoretically and also with temperature-cycling experiments. The detwinning is the growth of one variant within a martensite in expense of the other. It is shown that the experimental recoverable strains in tension (near 8.01 pct in the [123], 9.34 pct in the [111], and 7.8 pct in the [011] orientations) exceed the theoretical martensite (correspondent-variant pair (CVP) formation strains (6.49 pct in [123], 5.9 pct in [111], and 5.41 pct in [011]), lending further support that partial detwinning of martensite has occurred in both the solutionized and overaged specimens. In compression, the experimental recoverable strains are lower than the theoretical martensite (CVP) formation strain. In the compression cases, the detwinning strain contribution is calculated to be negligible in most orientations. The transformation strains observed in overaged NiTi are similar to the solutionalized NiTi, suggesting that incoherent precipitates do not restrict the detwinning of the martensite. For the [123] orientation, it is demonstrated that the thermal hysteresis is higher in solutionized NiTi compared to the overaged NiTi. The higher thermal hysteresis can be exploited in applications involving damping and shape stability, while the lower hysteresis is suited for actuators. I. BACKGROUNDning strains in compression [1] for selected orientations. The CVP is defined as the martensite plate with two correspon-SHAPE-MEMORY alloys have unique properties such dent variants. In this study, the focus is on determining both as recoverability of deformation upon heating and pseudothe CVP formation and detwinning strains in tension for all elastic stress-strain behavior. The materials-science backorientations. Transformation-strain contours are provided for ground has been documented for these alloys over the years both tension and compression within the stereographic trianin a variety of materials journals, but a number of issues gle demonstrating orientations favorable (and also unfavorare still under investigation, including detwinning of the able) to detwinning. In the current work, five different crystal martensite and the magnitude of recoverable strains. The orientations have been selected, with [123], [111] and [011] detwinning is the growth of one variant within a martensite exhibiting high degrees of detwinning in tension, [012] in expense of the other. The detwinning produces additional exhibiting moderate detwinning in tension, and [001] exhibrecoverable strain, especially in tension. It has been well iting no detwinning in tension. known that transformation strains in NiTi alloys in tensionThere have been previous attempts to demonstrate the are significantly larger than under compression. This prorole of detwinning in the NiTi alloys, which focused on duces differences in stress-strain response in tension vs comdeformation in the...

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