The Influence of Microstructure and Mechanical Resistance on the Shape Memory of Ecae Processed Stainless Fe-Mn-Si-Cr-Ni-Co Steel

Abstract

In the current work, XRD, SEM, EBSD and TEM techniques were used to evaluate the microstructure of stainless Fe-Mn-Si-Cr-Ni-Co shape memory steel processed by ECAE and annealed for one hour at temperatures ranging from 650ºC to 950ºC. The results were then correlated with the mechanical and shape-memory properties of the steel. It was observed that the samples containing large grains and a microstructure free of defects or precipitates presented a high volume fraction of multi-variant thermal martensite and stress-induced martensite, resulting in good shape recovery, owing to the memory effect. The grain refinement and precipitation of second-phase particles decreased the volume fraction and number of martensite variants and considerably increased the mechanical resistance, enhancing the elastic shape recovery. It was shown that shape memory properties were essentially related to the mechanical resistance of the matrix, which in turn was related to the microstructure.

Effect of Cerium Addition on the Microstructure and Shape Memory Properties of Austenitic Fe–Mn–Si–Cr–Ni Alloys

“…As for the Ce‐rich particles, they were obstacles to the growth of ε‐martensite plates within the grain, as shown for SMA‐3 in Figure 10b. It has been proposed [ 7,19,49 ] that the pinning effect induced by precipitated particles over ε‐martensite plates during stress‐induced martensitic transformation creates a local back‐stress effect, where the stress fields facilitate the reverse movement of Shockley partial dislocations, leading to a ε → γ transformation immediately after unloading, resulting in a macroscopic volume change. Thus, the contribution of the ESR to the TSR increases with the number of Ce‐rich particles, which is consistent with the highest ESR observed for SMA‐3.…”

“…The shape recovery ratio was determined by measuring the return angles after heating at 600 °C for 10 min based on three measurements of each alloy. TSR was calculated as the sum of ESR and SMR, given by Equation (2) [19] and (3), [1,27] respectively.…”

“…Recent studies reported by Wang et al [17] and Yong et al [18] have shown that grain refinement increases the constraining effects of grain boundaries and annealing twins on the stress-induced martensitic transformation and supports plastic deformation, thereby reducing SME. In addition, Käfer et al [19,20] showed that the improvement in TSR with austenitic phase strengthening due to grain refinement and precipitate formation primarily comes from ESR instead of SMR. These findings cast doubt on the role of Ce in improving the shape recovery of Fe-Mn-Si-Cr-(Ni) SMAs, and further studies are needed to clarify its influence.…”

“…The shape recovery ratio was determined by measuring the return angles after heating at 600 °C for 10 min based on three measurements of each alloy. TSR was calculated as the sum of ESR and SMR, given by Equation () [ 19 ] and (), [ 1,27 ] respectively.where δ is the amount of PS, t is the wire diameter (in mm), d is the rod diameter (in mm), θ e is the angle of ESR upon unloading (in degrees), and θ m is the angle of SME after austenitic reverse transformation (in degrees) at heating.…”

Microstructure evolution in laser powder bed fusion-built Fe-Mn-Si shape memory alloy

“…Martensite is formed from austenite by either a stress or thermally induced transformation [41,[108][109][110] , which results in the observed inverse relationship between the two phases. The effect of LED on the volume fractions of the phases was associated with the grain size, and that is, fine grains are detrimental to the formation of the ε-martensite phase [111,112] . The increase in ε-martensite volume fraction may also be caused by the decrease in Mn concentration at high LED [Figures 3B and 9] [113] .…”