EFFECT OF SELF-ACCOMMODATION FOR TWINNED MARTENSITE AND INVARIANT HABIT PLANE

The shear components of strain tensors for matrix and twin crystal of winned martensite were roughly opposite after calculation,and the strain energy would be reduced.The growth of martensitic plates along the normal direction of the twin plane may depend upon the effect of self-accommodation between the matrix and twin crystals.Based upon the analysis on the displacement vectors of the normals for different habit planes,the{10,3,15}_f plane could form the macroscopic invariant habit plane;but the(55)_f,(22)_f,and(11)_f planes are difficult to become the invariant planes.

A molecular dynamics study on formation of the self-accommodation microstructure during phase transformation

Self-accommodation microstructure,a typical crystallographic texture developed from phase transformation,is often observed in various alloys.In this work,a molecular dynamics simulation was conducted to reveal the fine details of self-accommodation microstructure evolution during the phase transformation from austenite to ferrite in pure iron.The growth and shrinkage of ferrite grains with different orientation relationships(ORs)are interpreted based on the analysis combining the elastic interaction energy and the interfacial energy.It was found that the strain energy determines the priority of potential ORs,while the interfacial energy selects the specific preferred ORs to form.The present atomistic process and energetic interpretation of the self-accommodation microstructure provide helpful insight into phase transformation textures observed in various alloys.

Effect of Incomplete Thermal Cycle on Transformation Behavior of Deformed TiNi Thin Film

Compared with the undeformed TiNi film, the martensite-austenite transformation (M-A) of the deformed one is elevated to a higher temperature on the first heating, but it nearly returns back to the original temperature on the second heating. An incomplete M-A transformation of the deformed TiNi film on the first heating divides the total martensite population into the self-accommodating martensite M2 and the oriented martensite M1. Thus, two transformations corresponding to M1-A and M2-A transition occur on the second heating. However, the forward transformation is not affected by the incomplete thermal cycle.

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys(BCC-HEAs).In this study,we report a novel dynamic strengthening mechanism,martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA(TiZrHf)Ta(at.%)that can profoundly enhance the work hardening capability,leading to a large uniform ductility and high strength simultaneously.Different from conventional transformation induced plasticity(TRIP)and twinning induced plasticity(TWIP)strengthening mechanisms,the martensitic twinning transformation strengthening mechanism combines the best characteristics of both TRIP and TWIP strengthening mechanisms,which greatly alleviates the strengthductility trade-off that ubiquitously observed in BCC structural alloys.Microstructure characterization,carried out using X-ray diffraction(XRD)and electron back-scatter diffraction(EBSD)shows that,upon straining,α”(orthorhombic)martensite transformation,self-accommodation(SA)α”twinning and mechanicalα”twinning were activated sequentially.Transmission electron microscopy(TEM)analyses reveal that continuous twinning activation is inherited from nucleating mechanical{351}type I twins within SA“{351}”<■11>typeⅡtwinnedα”variants on{351}twinning plane by twinning transformation through simple shear,thereby accommodating the excessive plastic strain through the twinning shear while concurrently refining the grain structure.Consequently,consistent high work hardening rates of 2–12.5 GPa were achieved during the entire plastic deformation,leading to a high tensile strength of 1.3 GPa and uniform elongation of 24%.Alloy development guidelines for activating such martensitic twinning transformation strengthening mechanism were proposed,which could be important in developing new BCC-HEAs with optimal mechanical performance.