Comparative study of the structures of Fe-Mn-Si-Cr-Ni shape memory alloys obtained by classical and by powder metallurgy, respectively

Hot rolled specimens of low-manganese Fe-Mn-Si-Cr-Ni shape memory alloys, produced by classical and by powder metallurgy (CM and PM) with mechanical alloying, respectively, were analysed by tensile loadingunloading tests (TENS), by dilatometry (DIL), by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Solution annealed specimens had two-phase structure, comprising γ-austenite and thermally induced α-martensite. The formation of γ’-stress-induced martensite during TENS was ascertained by SEM and XRD being accompanied by rounded loading portions on stress-strain curves, characteristic to transformation induced plasticity, which preceded long stress plateaus with low tilt. Even if loading behaviour changed from transformation induced plasticity, on first loading, to slip induced plasticity, during subsequent ones, the specimens maintained their pseudoelastic behaviour on each unloading. DIL responses of the elongated CM and PM specimens emphasised a thermallyinduced reversion, noticeable only during first heating, which was associated with thermally induced reversion of γstress-induced martensite

Structural changes caused by high-temperature holding of powder shape memory alloy 66% Fe - 14% Mn - 6% Si - 9% Cr - 5% Ni

Shape memory alloy Fe – 14Mn – 6Si – 9Cr – 5Ni (wt.%) obtained by cold pressing and sintering of a mixture of powder elements or of mechanically alloyed powder is studied. To compact the alloy additionally the specimens are subjected to hot rolling. Solution treatment of different duration (0.6 – 4.8 ksec) at 1473 K in low vacuum, nitrogen or argon is performed. The structure of the alloy after such treatments is studied by the methods of x-ray diffraction and optical and scanning electron microscopy.status: publishe

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

367-375Specimens 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 ε hexagonal close-packed martensite after compression cycles