Thermomechanical training effects of multifunctional modules processed by high-speed high pressure torsion

Specimens with truncated cone shell shapes (modules) are processed by high-speed high pressure torsion (HS-HPT), from circular crowns cut from shape memory alloy (SMA) ingots with nominal chemical composition Fe-28Mn-6Si-5Cr (mass%). Mechanical tests are performed comprising hardness measurements and loading-unloading compression cycles applied between dry or lubricated flat surfaces. A hardness gradient of 22 HV/mm is obtained along truncated cone generator, increasing from inner to outer areas, due to different deformation degrees, in these zones. Each loading and unloading stage was accompanied by steady and reproducible force plateaus, attributed to gradual elastic flattening of the modules, due to hardness gradient, suggesting a "superelastic-like" behaviour which is never reported in Fe-Mn-Si based SMAs. During cycling, the modules experienced a training effect consisting in the increase of relative shape recovery degree, up to 97%, with the number of cycles. Trained modules displayed multifunctional character, being able to develop constrained recovery forces when heated in compressed state, above a certain critical temperature. Constrained recovery forces are higher than those reached during isothermal compression. The modules are characterized, in different training stages, by optical and scanning electron microscopy, as well as by X-ray diffraction (XRD). As an effect of HS-HPT processing the formation of highly distorted martensite plate variants is revealed on the XRD patterns, which also emphasized the stress-induced formation of crystalline reoriented [varepsilon] hexagonal close-packed martensite after compression cycles.