Influence of Thermal and Mechanical/Powder Processing on Microstructure and Dynamic Stiffness of Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The influence of thermal processing and mechanical alloying of powder particles on the dynamic behavior and microstructural changes occurring in an Fel4Mn6Si9Cr5Ni (mass%) alloy is investigated. Five batches of 66Fe-14Mn-6Si-9Cr-5Ni (mass%) powder mixtures were produced via powder metallurgy. After powder mixing, different volume fractions (from 0 to 40 vol%) were mechanically alloyed, remixed with the rest of as-blended particles, compressed, sintered and subsequently hot rolled to a thickness of 1 mm. They were then subjected to a thermal treatment (annealing) at different temperatures and subsequently quenched into water. Thus, we had 25 different specimens for experimentation. The specimens were then machined and loaded in tension and subjected to dynamic mechanical analysis (DMA) by varying the temperature and strain amplitude. In the DMA tests, the samples were cycled between the ambient temperature and 673 K keeping the amplitude constant, while varying the temperature. These results were then correlated to the thermomagnetic properties that were measured in the same range of temperatures. Based on these data, the internal friction (tan δ ) maxima, correlated with storage modulus ( E ′) discontinuities, were associated with the multistage reverse transformations of martensite ( ε , hcp) to austenite, superimposed over the antiferromagnetic–paramagnetic transitions and accompanied by magnetization peaks (Neel temperature). The temperature ranges pertaining to tan δ maxima, E ′ discontinuities and magnetization peaks were determined and discussed in terms of thermal processing temperature and volume fraction of mechanically alloyed powder particles. The variation of storage modulus with strain for three different cycles (with strain amplitude varying from zero to maximum) was traced for three different temperatures: (i) T = RT; (ii) T T tan δ (max) . The role of magnitude of thermal treatment temperature and volume fraction of mechanically alloyed powder particles on the storage modulus plateaus were given prominence for these temperatures (RT, below tan δ max and above tan δ max ) as they influence the evolution of the microstructure.