Improvement for the recovery of Fe-Ni Shape Memory Alloy with Hot Dip Aluminum Coating

Abstract

The shape memory effect of Fe-Ni alloy would be improved with hot dip aluminum coating. There were five types of the specimens, namely #1; just after shape memory treatment (SMT), #2; Al-coated after SMT, #3; Al-coated after SMT and then annealed at 400 degree C, #4; the same but 800 degree C, and #5; annealed at 800 degree C after SMT w/o Al-coated. The shape recovery ratio of the Al-coated specimens (#2) was as much as 20% better than those of w/o Al-coated ones (#1) in V-shaped bending test. All the Al-coated specimens had the intermediate layer at the boundary of Fe-Ni and Al layers. The layer consists of solid solutions of Al dissolved by Fe and Ni with 10 – 20 μm thickness evaluated by microscopic observations. The Vickers hardness was higher than the specimens without Al coating and the specimens with Al coating might have higher yielding stress. The solid-solution layer may store the elastic strain energy during the bending test and then release it for assisting inverse martensite transformation during recovery treatment, thus the recovery may have been strengthened.