The Morphology of Strain-Induced Martensite and Thermally Transformed Martensite in Fe–Ni–C Alloys

Abstract

The relation between formation temperature and morphology of strain-induced and thermally transformed martensites in various Fe-Ni-C alloys has been studied by means of optical and transmission electron microscopy. The morphology of the strain-induced martensite varied with the formation temperature (i. e., deformation temperature below the Md) even in the The Ms temperature was remarkably depressed in these alloys with decrease in austenitizing temperature. Therefore, the relation between formation temperature and morphology of thermally transformed martensite was clearly determined in the same alloy by utilizing this phenomenon (thermal stabilization of austenite). The morphology of thermally transformed martensite also varied with the formation temperature even in the same alloy. Three types of the martensite were also observed in the same morphologies, and they were similarly formed in the same temperature ranges as the strain-induced martensites, respectively. The main factor determining the morphology of martensite (both the strain-induced and thermally transformed martensites) in Fe-Ni-C alloys was considered to be the formation temperature.

Computer simulation of martensitic transformations

“…However, if the dilation is moderate to large (IRI > 0.2) this plate grows only a short distance before particles of variant (2) and most common, is the "butterfly" morphology that is shown in Figure 5.3(a). It closely resembles the martensite morphology that has been characterized experimentally by Tamura, Ma.ki and coworkers (see, for example, Reference [19]). The transformation that creates this morphology begins as a twinned plate that grows along the (11) plane.…”

“…(3.3). If a matrix cell at position R is transformed to a martensite particle of variant, p, the elastic part of the Gibbs free energy changes by the amount 19 The total change in the free energy when the cell at R transforms to variant p is the sum of chemical and elastic terms when the external stress is missing and the other resistance (e.g., surface energy) are neglected: The total change in the free energy for the transformation of the element (p, R)…”

“…useful information in the details of single particle growth in the simulation. For example, "butterfly martensite", which is a common morphology in the computer simulation, is also observed in real systems [19], and has been difficult to interpret theoretically.…”

Martensitic Transformation and Shape Memory Effect in Ausaged Fe-Ni-Si Alloys.

“…Figure 9 shows the TEM micrograph and the associated selected area diffraction pattern (SAPD) of the thin-plate martensite which forms in the Fe-24%Ni-7%Si alloy ausaged at 400°C for 3 d. It is seen that the martensite plate contains a high density of the {112} aЈ transformation twins that traverse completely from one plate edge to the other. The microstructural features of this martensite plate in the ausaged specimen are similar to those of the thin-plate martensites observed in the Fe-Ni-Co-Ti, 6) the Fe-Ni-C 16) and the as-quenched Fe-28%Ni-7.5%Si alloys. 12) It can be seen that there are some weak superlattice spots corresponding to the {110} L1 2 reflection between the aЈ martensite fundamental spots as indicated by white arrows in Fig.…”

Effect of ausaging on the morphology of martensite in an Fe-25%Ni-7.5%Si alloy

“…These morphologies have been discussed in terms of the Ms temperature, the stacking fault energy of austenite, the crystal structure of martensite, the relative strengths of martensite and austenite, etc., as reviewed by Maki [1], Among these a' martensites, only the thin-plate martensite, which has been observed in several alloys such as Fe-Ni-C [2,3], Fe-Pt [4,5], Fe-Ni-Co-Ti [6], etc., has the potential of showing the perfect shape memory effect. It is well known that the long-range ordering of austenite to the LI2 structure or the coherent precipitates of y'-(Ni,Fe,Co) 3 Ti with the Ll 2 structure induces the formation of thin-plate martensite in the case of Fe-Pt [5] or Fe-Ni-Co-Ti [6] alloy, respectively.…”