The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

Abstract

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, d-ferrite, and the (Fe,Mn) 3 Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe 5 Ni 3 Si 2 type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal martensite. The athermal aЈ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the ␥--␣Ј transformation sequence.

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

“…As could be noticed, in the alloy containing 10 pct Si, the c phase is totally absent within the microstructure. The formation of d-ferrite and Fe 5 Ni 3 Si 2 type intermetallic phase was reported earlier by Maji et al [23] in the Fe-Mn-Si-Cr-Ni alloy. However, the Fe 5 Ni 3 Si 2 type intermetallic phase only precipitates out after aging in the temperature range of 973 K to 1173 K (700°C to 900°C) at the tri junctions of grain boundaries.…”

“…Generally, the formation of d-ferrite phase has been noticed in these alloys after heat treatment at higher temperature. [23] The presence of Fe 5 Ni 3 Si 2 type intermetallic phase has been noticed in alloys containing more than 7 pct Si. Therefore, it looks like the stability of the c phase decreases when the microstructure contains more than 6 pct Si.…”

Hardness-gradient reversion in FeMnSiCr shape memory alloy modules produced by high-speed high pressure torsion

“…Since the modules obviously experienced constrained recovery SME, the stress-induced formation of ε-hcp martensite and its thermally induced reversion to γ-fcc austenite would be expectable, after plastic deformation and heating, as it is the case of common FeMnSiCr SMAs [21]. For crystallographic calculations, the parameter of fcc unit cell of γ austenite was considered as a γ = 0.36 nm, while the parameters of hexagonal unit cell of ε martensite have been a ε = 0.254 nm and c ε = 0.4 nm [22]. Due to the elevated levels of micro-strains, caused by HS-HPT processing, α' martensite was observed, its bcc unit cell having the parameter a α' = 0.287 nm [23].…”

Factors influencing martensite transitions in Fe-based shape memory alloys

“…After pre-straining, the amount of ε-hcp martensite tends to increase. A representative example of phase evolution tendencies, is illustrated in Fig.5, corresponding to the specimens which were solution treated at 1273 K. The diffraction maxima of XRD patterns were identified based on the crystallographic parameters of the unit cells of: (i) ε-hcp, a ε = 0.254 nm and c ε = 0.4 nm and (ii) α'-bcc martensites, a α' = 0.287 nm, as well as (iii) γ-fcc austenite, a γ = 0.36 nm [35,36]. Using the ratios between the intensities of non-overlapping peaks, (110) α' , (101) ε , (200) γ , (200) α' (103) ε and (222) γ , semi-quantitative analysis [37] was done, as summarized in Table 1.…”