Interplay between Diffusive and Displacive Phase Transformations: Time-Temperature-Transformation Diagrams and Microstructures

Abstract

Materials which can undergo extremely fast displacive transformations as well as very slow diffusive transformations are studied using a Ginzburg-Landau framework. This simple model captures the essential physics behind microstructure formation and time-temperature-transformation diagrams in alloys such as steels. It also predicts the formation of mixed microstructures by an interplay between diffusive and displacive mechanisms. The intrinsic volume changes associated with the transformations stabilize mixed microstructures such as martensite-retained austenite (responsible for the existence of a martensite finish temperature) and martensite-pearlite.

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

“…Fig. 1(d), the system exhibits a microstructure that is not observed in bulk systems: 22,23 dots aligned on a square lattice with axes along 01 . For these widths, the transformation is not completely suppressed and thus we observe retained austenite along with the two martensite variants, especially in Fig.…”

“…In some materials systems, there is a significant volume change associated with the transformation (the volume of the martensite unit cell is different from that of austenite), so that the phase transformation generates hydrostatic stress, thus affecting the microstructure. 22,23 Figures 2(a) and 2(c) show that in the presence of a volume change, microstructures change somewhat: dots may become triangular and/or elongated and twins are no longer parallel.…”

Effect of magnetism on kinetics of γ–α transformation and pattern formation in iron

“…Despite this, we are still far from a complete understanding of mechanisms and kinetics of phase transformations in steel (see Refs. [3][4][5][6][7]), the processes playing a crucial role in the formation of the structural state. The reason is the complexity of these nonequilibrium processes involving several spatial scale levels, from microscopic (atomistic) to macroscopic (at the level of the grain size).…”

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

“…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.…”

“…3 shows that too high a value of x a hinders martensite formation in the active layer, which reduces the strain -there is a trade-off between strain generation and martensite formation in the active layer, which favors martensite formation in the passive layers at intermediate volume change. 10,11 D. Nanocomposites To reduce the interaction between a layer and its own periodic images, the simulations for layers thinner than 250 nm have four pairs of layers rather than two. )…”