Powder metallurgy and mechanical alloying effects on the formation of thermally induced martensite in an FeMnSiCrNi SMA

Abstract

Abstract. By ingot metallurgy (IM, melting, alloying and casting), powder metallurgy (PM, using as-blended elemental powders) and mechanical alloying (MA of 50 % of particle volume), three types of FeMnSiCrNi shape memory alloy (SMA) specimens were fabricated, respectively. After specimen thickness reduction by hot rolling, solution treatments were applied, at 973 and 1273 K, to thermally induce martensite. The resulting specimens were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), in order to reveal the presence of ε (hexagonal close-packed, hcp) and α' (body centred cubic, bcc) thermally induced martensites. The reversion of thermally induced martensites, to γ (face centred cubic, fcc) austenite, during heating, was confirmed by dynamic mechanical analysis (DMA), which emphasized marked increases of storage modulus and obvious internal friction maxima on DMA thermograms. The results proved that the increase of porosity degree, after PM processing, increased internal friction, while MA enhanced crystallinity degree.

Powder Metallurgy: An Alternative for FeMnSiCrNi Shape Memory Alloys Processing

“…These XRD patterns of 0_MA specimens confirm the absence of martensite plates at hot rolled specimen and their presence in heat treated states. The DMA thermograms from Figure 6A illustrate a continuous decrease of storage modulus (E ) during heating at all specimens, since martensite is stiffer than austenite and two internal friction (tanδ) maxima, associated with the successive reversions to γ (fcc) austenite of α -bcc and ε-hcp martensites, respectively (Pricop et al, 2015a). More recent studies corroborated the variations of storage modulus and magnetization, during heating.…”

“…FIGURE 5 | SEM micrographs illustrating the evolution of initial hot rolled structure of P/M specimens sintered from pressed as blended powders (0_MA), 50% as blended powders substituted with MA'ed powders (50_MA) and ingot metallurgy specimens (14 Mn), which were subjected to solution treatments at 700 and 1,100 • C (adapted after Pricop et al, 2015a, © owned by the authors).…”

“…The greatest part of experiments were performed on specimens with chemical composition Fe-14Mn-6Si-9Cr-5Ni. Three specimens in each state, 0_MA, 50_MA and IM, respectively, were hot rolled and then one specimen was heat treated at 700 • C and the other at 1,000 • C. The nine resulting representative microstructures, recorded by SEM, are summarized in Figure 5 (Pricop et al, 2015a).…”

“…Increasing heat treatment temperature to 1,000 • C produced finer preferentially oriented martensite plates, in 0_MA and 50_MA specimens but caused the increase of martensite plates widths at IM specimen, since a slight grain coalescence occurred, causing average grain size increase to 40.3 µm. The identification of phase structure was performed by XRD and the dynamic behavior of the nine specimens under study was evaluated by dynamic mechanical analysis (DMA), using three-point-bending specimen holder, as summarized in Figure 6 (Pricop et al, 2015a).…”

“…However, the storage modulus of hot rolled (initial) 14 Mn specimen experienced an increase of E = 12 GPa, in spite of its general low values. It was noticed that heat treatment caused an increase of maximum storage modulus values, and a decrease of maximum internal friction, from 0_MA, to 50_MA and finally to 14 Mn (Pricop et al, 2015a).…”