The influence of boron microalloying on the microstructural and mechanical properties of Ni-Mn-Sn-Gd shape memory alloy

This study systematically investigates the microstructure, martensitic phase transformation, crystal structure, and mechanical properties of (Ni₄₃Mn₄₇Sn₉Gd₁)_100−xB_x (x = 0, 0.8, 1.5 and 3 at%) shape memory alloys. Experimental results reveal that these alloys consist of a matrix phase and precipitated phases. The introduction of Gd elements leads to the formation of milky-white particles dispersed along grain boundaries, with the composition identified as GdNiSn. When the B element content reaches 1.5 at%, bright-white particles form and are uniformly distributed within the matrix. Their concentration increases with higher levels of B doping, and they are characterized as Mn₂B. Simultaneously, the initially present Gd-rich milky-white particles distributed along grain boundaries, exhibit a diminishing trend with increasing B doping. B doping elevates the alloy’s phase transition temperature, and the compressive strength of the alloy approximately follows a linear trend with increasing B content. At a B doping level of 3%, the annealed alloy demonstrates a compressive strength of up to 1313 MPa with a compressive fracture strain of 11.6%, marking a 110% improvement. For the as-cast alloy, a compressive strength of 1652 MPa is achieved, accompanied by a compressive fracture strain of 12.3%, representing a 130% enhancement. Transmission electron microscopy reveals pronounced twinning features on the alloy surface, resulting in the formation of numerous fine lines in the as-cast state, that are magnified into voids after heat treatment. This phenomenon is detrimental to the alloy’s mechanical performance; hence, the as-cast compressive strength is favored over the annealed state.