Microstructure and mechanical properties of constrained shape-memory alloy nanograins and nanowires

Abstract

We use the phase-field method to study the martensitic transformation at the nanoscale. For nanosystems such as nanowires and nanograins embedded in a stiff matrix, the geometric constraints and boundary conditions have an impact on martensite formation, leading to new microstructuressuch as dots aligned on a square lattice with axes along 01 -or preventing martensite formation altogether. We also perform tension tests on the nanowires. The stress-strain curves are very different from bulk results. Moreover, they are weakly affected by microstructures -the mechanical response of nanowires with different microstructures may be similar, while nanowires with the same microstructure may have a different mechanical behavior. We also observe that at the transition temperature, or slightly below it, the narrowest wires behave pseudoelastically whereas wider wires are in the memory-shape regime. Moreover the yield stress does not change monotonically with width: it has a minimum value at intermediate width.

Phase field simulations of coupled phase transformations in ferroelastic-ferroelastic nanocomposites

“…These trends are similar to what was observed in Ref. 9. In the presence of a volume change martensite formation generates stress, so that for a high enough value of x a a purely martensitic system is higher in energy than a martensite-austenite mixture.…”

“…In the presence of a volume change martensite formation generates stress, so that for a high enough value of x a a purely martensitic system is higher in energy than a martensite-austenite mixture. 9,10,11 One can see a range of values of x a around 0.2 for which there are four regimes for increasing layer thickness, including two with a mixture of martensite and austenite.…”

“…the variant with a unit cell elongated along x, and blue to rectangles elongated along y, e 2 < 0-, as in Ref. 9.) We will therefore focus on intermediate temperatures, for which martensite is thermodynamically stable in only half the system.…”

“…Moreover, martensite has been studied mostly in the bulk form, with limited work at the nanoscale. 9 …”

Size effects in NiTi from density functional theory calculations

“…Several mechanisms have been postulated to contribute to such size effects including surface and interfacial energies, mechanical constraints, and the resulting changes in the martensite microstructure. 4,16,17 Surfaces are also known to drive phase transformations in nanoscale FCCwires. 18,19,20 MD simulations showedthat surface stresses cause<100> nano-wires to spontaneously change their orientation to<110> in Ni, Ag and Cu;in the case ofAu nanowireswith diameters less than 2 nm, a transformationto a body centered tetragonal structure has been observed.…”