Influence of Boron on the Microstructural and Mechanical Properties of Ni53.5Mn26.0Ga20.5 Shape Memory Alloy

Abstract

Boron addition to Ni53.5Mn26.0Ga20.5 alloy is found to modify the microstructure and mechanical properties substantially. Studies on (Ni53.5Mn26.0Ga20.5)Bx alloys reveal that boron addition causes grain refinement which led to an increase in compressive strength in x = 0.5 alloy which also retained multimode twinning. Maximum compression strengths of 124 MPa, 198 MPa, and 111 MPa and corresponding compression strains of 5.2%, 4.3%, and 5.5% respectively have been recorded for B-0, B-0.5, and B-1.0 alloys. Shape memory strain of 1.5% was recorded even after addition of 0.5 at.% boron which reduced to 0.4% at 1 at.% boron and showed a correlation to the extent of multimode twinning. Substantial second-phase segregation rich in Ni was seen at grain boundaries, the extent of which increased with boron content. This led to a compositional shift in the matrix phase which resulted in a reduction in the martensitic transformation temperature and which in turn caused an easy deformation at low stresses and suppression of multimode twinning in x = 1.0 alloy. Microhardness shows an increase with boron content. B-0.5 alloy with larger compression strength, substantial shape memory effect, and higher hardness has a potential for shape memory applications at room temperature.