"Optimization of Fe-15Mn-5Si-9Cr-5Ni Shape Memory Alloy for Pipes and Shafts Couplings"

The shape memory effect is a particular thermo-mechanical behavior that involves the recovery of apparently plastic deformation. For ferrous alloys, it is due to the stress induced transformation of the austenite γ phase (FCC) to HCP ε martensite, and the subsequent ε → γ reverse transformation induced by heating to a temperature above the A_f.

The most important factors influencing the shape memory properties are the stacking fault energy of the alloy and the texture and microstructure of the austenitic matrix. When the material is deformed, the resolved shear stress on the {111} <112> systems must be the highest, to induce the martensitic transformation before plastic deformation occurs in the matrix. With respect to the microstructure, stacking faults are ε nuclei, and the dislocation density affects the movement of the partial dislocations on the slip systems. These aspects depend on the thermo-mechanical treatment given to the material.
We investigated the application of Fe-Mn-Si shape memory alloys to the manufacture of machine parts and devices, in particular pipes and shafts couplings. After having evaluated the optimal parameters affecting shape memory properties, i.e., the chemical composition, the amount of processing deformation, the annealing temperature, and the correlation between the reversibility of the martensitic transformation and the conditions of the microstructure (particularly the role of crystal defects), we found that maximum recovery in a Fe-15Mn-5Si-9Cr-5Ni alloy is achieved by rolling at 800 °C followed by annealing at 650 °C. Furthermore, we investigated certain properties that establish technological feasibility limits to industrial production, including alloy weldability and mechanical response. Using specimens subjected to bending and tensile, we evaluated the expected behavior in parts joined by welding. Finally we developed an original process to produce the couplings.