Special cases of martensite compatibility: A near single-variant habit-plane and the martensite of nanocrystalline NiTi

Manuel Petersmann,Thomas Antretter,Thomas Waitz,N Schryvers,J Van Humbeeck

doi:10.1051/matecconf/20153303015

Manuel Petersmann, Thomas Antretter + Show 3 more

Open Access

https://doi.org/10.1051/matecconf/20153303015

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Abstract

Lattice parameters measured near the high temperature (~1000°C) bcc α to hcp β transformation in an intermetallic Mo-containing γ -TiAl based alloy indicate a middle valued eigenvalue of the corresponding deformation gradient near 1. Habit-planes calculated under the assumption of a simple slip as lattice invariant shear, agree with experimentally determined orientations of the lens like plates recorded via electron backscattering. By contrast, twinning as invariant lattice shear has been investigated in nanocrystalline NiTi. Here the grain size causes the formation mechanism of the martensite to change from a “herring-bone” morphology faciliting a habit-plane between two twinned laminates and the austenite to a single laminate, which in the nonlinear theory formally cannot form a habit-plane with the austenite. Since this might cause high accommodation strains, the effectiveness of stress accommodation of martensite formed in neighboring grains of a polycrystal is investigated. Subsequent numerical microstructural modeling is outlined. The resulting energetically most favorable transformation sequence yields the transformation kinetics.

Highlights

Lattice parameters measured near the high temperature (~1000°C) bcc α to hcp β transformation in an intermetallic Mo-containing γ-TiAl based alloy indicate a middle valued eigenvalue of the corresponding deformation gradient near 1
Habit-planes calculated under the assumption of a simple slip as lattice invariant shear, agree with experimentally determined orientations of the lens like plates recorded via electron backscattering
The grain size causes the formation mechanism of the martensite to change from a “herring-bone” morphology faciliting a habit-plane between two twinned laminates and the austenite to a single laminate, which in the nonlinear theory formally cannot form a habit-plane with the austenite

Summary

Near perfect single-variant habit-plane

The lattice correspondence between the parent β- and the product α-phase in Ti-Al-Mo is given by the Burgers orientation relation (OR) [13]. We investigated a bcc-hcp transformation (see conclusions), where it turns out that there are two slip systems with equal minimal shear-magnitude for each BCV with specific {112}ஒ slip plane and 〈111〉ஒ shear direction [12]. Note that this result agrees with the observation that that the Burgers orientation relation renders the (112)ஒ plane nearly parallel to the (11ത00)஑ plane [13]. Since two shear systems have been found for each Bi requiring the calculated minimum shear there are 12 modified Bain-strains ۰ij = SjBi that can be used for the calculation of invariant habit-planes yielding 24 habit planes because of the alternating sign in the solution of Eq.2 [5,6,15]

Compound twinning in polycrystalline nanocrystalline equiatiomic NiTi

Findings

Conclusions