Variant selection in surface martensite

Baur, Annick P.; Cayron, Cyril; Logé, Roland E.

doi:10.1107/s160057671701398x

Cited by 11 publications

(14 citation statements)

References 24 publications

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“…Besides the work presented in this study, we have acquired some EBSD maps on U-bent deformed NiTi alloys. The results confirm the phenomena of variant selection under strain (variant re-orientation), and we will show that they can be interpreted similarly to our previous works on iron alloys [64,65] and AuCu alloys [66]. We have already obtained some EBSD maps showing martensite with equiaxed morphology, which is in good agreement with the hypothesis that the re-orientation between two variants α i → α j is not a "detwinning" operation but a combination of martensite transformations α i → γ → α j .…”

Section: Future Worksupporting

confidence: 90%

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

Cayron

2020

Crystals

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The complex and intricate microstructure of B19′ martensite in shape memory nickel titanium alloys is generally explained with the Phenomenological Theory of Martensitic Crystallography (PTMC). Over the last decade, we have developed an alternative approach that supposes the existence of a “natural” parent–daughter orientation relationship (OR). As the previous TEM studies could not capture the global crystallographic characteristics of the B2→B19′ transformation required to discriminate the models, we used Electron BackScatter Diffraction (EBSD) and Transmission Kikuchi Diffraction (TKD) to investigate a polycrystalline NiTi alloy composed of B19′ martensite. The EBSD maps show the large martensite plates and reveal the coexistence of different ORs. The TKD maps permit us to image the “twins” and confirm the continuum of orientations suspected from EBSD. The results are interpreted with the alternative approach. The predominant OR in EBSD is the “natural” OR for which the dense directions and dense planes of B2 and B19′ phases are parallel—i.e., (010)B19′//(110)B2 and [101]B19′//[ 1 ¯ 11]B2. The natural OR was used to automatically reconstruct the prior parent B2 grains in the EBSD and TKD maps. From the distortion matrix associated with this OR, we calculated that the habit plane could be (1 1 ¯ 2)B2//(10 1 ¯ )B19′. The traces of these planes are in good agreement with the EBSD maps. We interpret the other ORs as “closing-gap” ORs derived from the natural OR to allow the compatibility between the distortion variants. Each of them restores a parent symmetry element between the variants that was lost by distortion but preserved by correspondence.

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Section: Future Worksupporting

confidence: 90%

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

Cayron

2020

Crystals

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“…Different alloys (stainless steels, iron-nickel and ironnickel-mangenese alloys) exhibit this particular form of martensite [2e4]. Recently, the present authors showed that a natural variant selection takes place in the martensite formed at the surface of Fe-30 wt%Ni alloys [5]. The phenomenon is caused by the presence of the free surface which favours the nucleation of some particular martensite orientations.…”

Section: Introductionmentioning

confidence: 83%

On the chevron morphology of surface martensite

2019

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“…The angular power criterion can be understood as a triggering criterion that only takes into account the steric effect of the atoms at the first instants of the distortion process. Baur et al (2017b) showed that this criterion could explain the variant selection of martensite formed at the surface of Fe-Ni alloys during electropolishing. More experimental work on perfectly oriented single crystals is required to compare the predictions based on simple shear (and Schmid's law) with those based on the maximum volume change or on the derivative of the angular distortion.…”

Section: Possible Applications Of the Continuous Distortionmentioning

confidence: 98%

“…The {225} habit plane of martensite in steels could be deduced from the KS OR by relaxing the IPS assumption of the PTMC and by assuming that the interface plane is only untilted (Jaswon & Wheeler, 1948;Cayron, 2015). If required, the {225} unrotated plane can be rendered invariant (IPS) by coupling two KS variants in a twin OR (Baur et al, 2017a). The exact KS OR imposes a strict invariant line along a dense direction that is common to the f.c.c.…”

Section: A2 Some Compatibility Criteria Used In the Ptmcmentioning

confidence: 99%

The transformation matrices (distortion, orientation, correspondence), their continuous forms and their variants

Cayron

2019

Acta Cryst Sect A

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The crystallography of displacive/martensitic phase transformations can be described with three types of matrix: the lattice distortion matrix, the orientation relationship matrix and the correspondence matrix. Given here are some formulae to express them in crystallographic, orthonormal and reciprocal bases, and an explanation is offered of how to deduce the matrices of inverse transformation. In the case of the hard-sphere assumption, a continuous form of distortion matrix can be determined, and its derivative is identified to the velocity gradient used in continuum mechanics. The distortion, orientation and correspondence variants are determined by coset decomposition with intersection groups that depend on the point groups of the phases and on the type of transformation matrix. The stretch variants required in the phenomenological theory of martensitic transformation should be distinguished from the correspondence variants. The orientation and correspondence variants are also different; they are defined from the geometric symmetries and algebraic symmetries, respectively. The concept of orientation (ir)reversibility during thermal cycling is briefly and partially treated by generalizing the orientation variants with n-cosets and graphs. Some simple examples are given to show that there is no general relation between the numbers of distortion, orientation and correspondence variants, and to illustrate the concept of orientation variants formed by thermal cycling.

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Variant selection in surface martensite

Cited by 11 publications

References 24 publications

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

On the chevron morphology of surface martensite

The transformation matrices (distortion, orientation, correspondence), their continuous forms and their variants

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