A simple variant selection in stress-driven martensitic transformation

The study of orientation variant selection helps to reveal the mechanism and dynamic process of martensitic transformations driven by temperature or pressure/stress. This is challenging due to the multiple variants which may coexist. While effects of temperature and microstructure in many martensitic transformations have been studied in detail, effects of stress and pressure are much less understood. Here, an in situ variant selection study of Mn2O3 across the cubic-to-orthorhombic martensitic transformation explores orientation variants at pressures up to 51.5 GPa and stresses up to 5.5 GPa, using diamond anvil cells in radial geometry with synchrotron X-ray diffraction. The diamonds not only exert pressure but also impose stress and cause plastic deformation and texture development. The crystal orientation changes were followed in situ and a {110}c 〈001〉c // (100)o 〈010〉o relationship was observed. Only the {110}c plane perpendicular to the stress direction was selected to become (100)o, resulting in a very strong texture of the orthorhombic phase. Contrary to most other martensitic transformations, this study reveals a clear and simple variant selection that is attributed to structural distortions under pressure and stress.

Crystal structure and crystallographic characteristics of martensite in Ni50Mn38Sb12 alloys

For Ni–Mn–Sb ferromagnetic multifunctional alloys, the crystal structures of martensite variants and the orientation relationships between them are decisive factors for their magnetic field-induced behaviours and are hence of importance. Such information has rarely been reported in the literature. In the present work, the crystal structure, microstructure and orientation relationships of Ni–Mn–Sb martensite were thoroughly investigated by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Through XRD analyses, the crystal structure of the martensite, including the crystal system, the space group, the lattice parameters and the atomic coordinates, was fully resolved. The structure is orthorhombic and can be represented with a 4O superstructure. EBSD analyses show that the Ni–Mn–Sb martensite has a lamellar form. One martensite lamella corresponds to one orientation variant. The lamellae are organized in long plate-shaped colonies. Within each colony, four distinct orientation variants (A, B, C and D) appear repeatedly and extend in roughly the same direction. The four variants are twin related to one another, with variants A and C (or variants B and D) forming a type I twin, variants A and B (or C and D) a type II twin, and variants A and D (or B and C) a compound twin. The complete twinning elements for each twin relation were thus fully determined. The interfaces between the variants were identified to be their corresponding twinning planes. All these results provide fundamental information for Ni–Mn–Sb alloys that is useful for interpreting their magnetic and mechanical characteristics.

Rainures et marques de choc oblongues sur la plate-forme intertidale à Cap-Santé, haut estuaire du Saint-Laurent, Québec

On signale ici pour la première fois des formes d'érosion linéaires observées sur une couche de calcaire de la plate-forme intertidale schisteuse, à Cap-Santé. Il s'agit d'une part de rainures ayant jusqu'à 15 m de longueur et 2-3 cm de profondeur et d'autre part de petites dépressions allongées ayant en moyenne 60 à 80 cm de longueur, 8 à 12 cm de largeur et 2 à 3 cm de profondeur. On décrit aussi des stries courtes d'orientation multiple et des éraflures diverses. Les rainures ont une orientation variant de 75° à 115° avec un système principal à 90°, alors que les dépressions linéaires sont orientées de 75° à 85°, soit à peu près dans la même direction (75°-80°) qu'un système de fissures (diaclases) dans les schistes argileux. Ces diverses formes d'érosion du substrat rocheux sont attribuées à l'activité glacielle dans un contexte lithostructural particulier.

Hrem Observation of the Relaxation of Grain Boundaries in Au At Their Intersection with Free Surfaces

Grain boundaries in fee metals with low stacking fault energy are known to undergo extended relaxations that can at times lead to a thin layer of a different structure. in Cu, for example, it has been found that Σ3{112}} boundaries relax into a 9R phase or even a bec phase. in this work, we have used high resolution electron microscopy to investigate the atomic structure of Σ99 grain boundaries in mazed bicrystal films of Au. On {001} Ge single crystal substrates, Au bicrystals can be grown in two equivalent {110} orientation variants, related to each other by a 90° rotation about the surface normal. There is equal probability to nucleate either orientation variant in early stages of film formation and since there is no reason for preferential growth, the morphology of the bicrystal film consists of many grains in only two orientations. All grain boundaries are of the type Σ99 and display tilt character.Thin films 10-200nm thick were grown by physical vapor deposition on {001} Ge substrate at 280°C and followed by annealing at 200°C to relieve stresses.