Study of the Ni50Mn50 alloy after low-temperature long-term aging

Abstract

The study of alloys based on manganese nickelide is relevant because they implement a thermally and magnetically controlled shape memory effect [1–4]. These alloys can have several technological applications: magnetic solid-state coolants [4], magnetic actuators, sensors and devices for spintronics [5–8]. This work investigates the effect of long-term low-temperature aging on the structure, mechanical properties and temperatures of martensitic transformations of the binary equiatomic alloy Ni50Mn50. Scanning electron microscopy was used to study the change in the structure and fracture appearance, the distribution of the main chemical elements, and the effect of the structural changes on microhardness values was analyzed. Using resistometry and differential scanning calorimetry, the martensitic transition temperatures were found. It was established that this alloy is susceptible to separation as a result of long-term low-temperature aging (about 20 years). It was shown that a redistribution of basic chemical elements occurs. “Nickelized” regions appear in which the lamellar martensitic structure is disrupted and a fine-grained structure is formed, whose crystal lattice apparently does not change and corresponds to type L10. In this case, the microhardness of the martensitic matrix decreases by 20–25%, and the microhardness of the formed regions is comparable to the microhardness of the alloy in the initial state. It was fractographically established that precipitates with a higher nickel content have a more ductile fracture pattern than the surrounding martensitic matrix. Apparently, the separation did not have a significant effect on the transition temperatures.