Effect of grain boundary segregations on martensitic transformation temperatures in NiTi bi-crystals

Abstract

NiTi alloys are very important in a number of applications since they demonstrate shape memory effect, which is due to the martensitic phase transition with the transition temperatures close to the room temperature. Many factors affect transition temperatures of the alloy, including a variation of its chemical composition and thermo-mechanical treatment, which affects grain size, dislocation density, and other crystal structure parameters. It is well-known that the chemical composition of the alloys in grain boundaries can differ significantly from that in the bulk due to segregation of certain elements from the matrix to the grain boundaries. The effect of grain boundary segregations on the martensite transformation temperatures is still poorly understood. In the present molecular dynamics study, the possible effect of segregation of Ti or Ni atoms along Σ25 tilt grain boundary on the forward and reverse martensitic transformations is analyzed. The segregation is simulated by replacing the monoatomic Ti or Ni layer in the grain boundary with Ni or Ti layer, respectively. The results are compared to the case of no segregations. We analyze the initial relaxed and thermalized structures of the bi-crystals in austenite state as well as the temperature dependencies of potential energy per atom and volumetric dilatation. It is found that segregations may significantly decrease the start and finish temperatures of the martensitic transformation, and this effect is more pronounced for segregations of Ni.

Dispersion of the Steel Structure and its Effect on Magnetic and Mechanical Properties of Steel

“…The values of the grain size factor for heat-treated samples of 15KHSND steel are shown in Figure 3. It can be noted that the greatest value in the grain size factor is observed at a tempering temperature of 200 C, which is most likely associated with the incipient process of breaking the coherence of the martensite and cementite lattice [4,5] as a result of the onset of carbon evolution [6,7]. As a result, a region with a carbon-depleted phase is formed, which has low hardness, and a new phase in the form of ferria and cementite.…”

“…Such dropouts of points from the general dependence can be explained from the standpoint of changes in the structural-phase composition. For example, when quenching 15KHSND steel, a structure is formed, where the effect of coherence violation of the martensite and cementite lattices is not observed [4,5]. In this state, both phases have a low density of defects in the crystal lattice structure [6,7], which affects all properties of steel, making it more magnetically hard and solid.…”