Crystallographic Theory Of Martensitic Transformations Research Articles

On Deformation of A-M Interface in Single Crystal Shape Memory Alloys and Some Related Issues

Recent experimental results on the deformation field of single crystal CuNiAl shape memory alloys (SMA) by using Moire´ interference technique are reported and two kinds of austenite-martensite (A-M) interfaces with different deformation features are identified. The experimental discovery questioned the invariant plane hypothesis used in crystallographic theory of martensitic transformation. Some fundamental issues on the property of A-M interface and the related micro- and macro-deformation features are discussed.

Modification and verification of the “twinning shear” method according to phenomenological crystallographic theory of martensitic transformation

The phenomenological crystallographical theory of martensitic transformation has been successfully applied to many alloys, which has two mathematical methods accomplished independently by different researchers i.e. the famous WLR and BM methods. These two methods can produce the same results, however, the authors think that the original WLR method may reflect more clearly the physical meaning of the transformation process. Therefore, in this paper proper modification and simplification in mathematical treatment is advanced simultaneously, and the modified method is verified by being applied to martensitic transformations from bcc to bct in FeNiC alloy and that from tetragonal to monoclinic in ZrO{sub 2} ceramics.

Infinitesimal deformation approach of the phenomenological crystallographic theory of martensitic transformations

The infinitesimal-deformation approach has been used to reformulate the phenomenological crystallographic theory of martensitic transformations. Simple analytical solutions for the habit plane orientation, the direction and magnitude of the lattice invariant shear, the orientation relationship between parent and product phases, etc. were derived for the cubic to tetragonal transformation. The derived results were numerically compared with those obtained from the original phenomenological theory by taking the cubic to tetragonal transformation in a zirconia alloy as an example. The expected magnitude of the differences in solutions between the present and phenomenological crystallographic analyses was also discussed.

Group theory and crystallography of the martensitic transformation in a Cu-26.71Zn-4.15Al shape memory alloy

A study of the crystallography of the martensitic transformation in a Cu-26.71Zn-4.15Al shape memory alloy by means of group theory is presented. The habit plane of the martensite calculated from the phenomenological crystallographic theory of martensitic transformations proposed by W-L-R is found to be (−1, 7.55, 7.83) assuming a model of B2 (parent) to modified 9R monoclinic structure (martensite) which is quite consistent with that of experimental results (−1,7,8). From group theory we have obtained the irreducible representation and the 24 martensite variants associated with the martensitic transformation in this alloy system. A self-accommodating group is found to be D 2 k and the symmetry in one and the set of self-accommodating groups is analyzed. The results show that group theory may be an important tool for the study of martensitic transformations.

The crystallography of hydride formation in zirconium: I. The δ → γ transformation

The phenomenological crystallographic theory of martensitic transformations has been applied to the transformation from δ (fcc) to e (fct) zirconium hydride, using published lattice parameters. The habit plane, orientation relationship, lattice invariant shear, and interface characteristics were determined by transmission electron microscopy and diffraction. The shape strain was observed by interference microscopy. Good agreement between the predictions of the theory and the measured crystallography was obtained. The predicted and observed lattice invariant shear was twinning on 101. These twins which are found within alternating bands of hydride variants produce a herringbone morphology, and the bands produce a roof gable type of surface relief. For a given plate, the measured habit plane, twin plane, unique Bain contraction axis, and orientation relationship were mutually consistent with the respective predictions for a single variant. The magnitude of the lattice invariant shear was in excellent agreement with the predicted value. The interfaces separating the e hydride bands were found to be of two types, which alternated, often filling an entire grain. One of these, termed a spear interface, was found to be a twin plane, across which the twinned regions of the two bands “matched-up”. The other, termed an impingement interface, was found to have twin regions which did not “match-up”. This morphology can be explained as a pair of e-hydride plates which share a spear interface. When two growing spears impinge, the resulting impingement interface is of the second type.