This article presents room-temperature deformation mechanisms in polycrystalline Mg alloys. Dislocation slip of basal 〈a〉 and prismatic 〈a〉 types are shown to occur nearly at the same ease when the basal planes are tilted in such a way that the Schmid-factor ratio (equivalent to the critically resolved shear stress (CRSS) ratio) of prismatic 〈a〉 to basal 〈a〉 slip is larger than a value ranging from 1.5 to 2.0, depending on the initial texture distribution and grain size. Grain-boundary sliding (GBS) also occurs at room temperature up to 8 pct of total strain, enhanced by plastic anisotropy as well as by the increasing number of grain-boundary dislocations. Twinning plays an important role in both flow and fracture behaviors. Twins are induced mostly by stress concentrations caused by the anisotropic nature of dislocation slip. Twins can be classified into two types based on their shape: a wide lenticular type and a narrow banded type. The wide twins are 
$$\{ 10\bar 12\} $$

 twins appearing in the early stage of deformation and accompany little change of surface height. The narrow twins are 
$$\{ 10\bar 11\} $$

 or 
$$\{ 30\bar 32\} $$

 appearing in the late stage of deformation and accompany a substantial change in surface height. The formation of the narrow twins seems to give rise to highly localized shear deformation within the twin, leading to strain incompatibility and to final failure.

Enhanced deformation mechanisms by anisotropic plasticity in polycrystalline Mg alloys at room temperature

Influence of texture and grain size on work hardening and ductility in magnesium-based alloys processed by ECAP and rolling

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

/pdf/new-materials-for-micro-scale-sensors-and-actuators-an-21qnaonn0l.pdf

New materials for micro-scale sensors and actuators An engineering review

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

https://iopscience.iop.org/article/10.1088/0022-3727/39/5/S13/pdf

Modelling the phase diagram of magnetic shape memory Heusler alloys

Geometrical criterion for the activation of prismatic slip in AZ61 Mg alloy sheets deformed at room temperature

Rolled sheets of AZ31 Mg alloys were subjected to tensile testing at temperatures ranging from room temperature to 523K The occurrence of grain-boundary sliding (GBS) at room temperature was demonstrated by the displacement of scribed lines across grain boundaries of deformed samples Surface relief of deformed samples was measured by use of a scanning laser microscope GBS strain was calculated from the measured surface step height, and its temperature dependence was analyzed by a Dorn-type constitutive equation GBS above 423K was found to be pure GBS that was activated by resolved applied shear stress acting on grain boundaries The activation energy for GBS was found to be 80 kJ/mol, which is in agreement with the activation energy for grain boundary diffusion Meanwhile, GBS below 373K was found to be slipinduced GBS, and its extent was found to be significantly greater than that expected from extrapolation of high-temperature values The slipinduced GBS is considered to occur by plastic compatibility conditions in the presence of plastic strain anisotropy and by absorption and dissociation of lattice dislocations at grain boundaries

Grain-Boundary Sliding in AZ31 Magnesium Alloys at Room Temperature to 523 K

Low temperature crystal structure of Ni-Mn-Ga alloys

The intermetallic compound Ni2MnGa has a shape memory effect and a ferromagnetic property. Films of this compound may be applied for an actuator of micro-machines. The Ni-rich Ni2MnGa films with a nearly 5 μm thickness were deposited on an Al 2 O 3 substrate by a radio-frequency magnetron sputtering apparatus with a Ni 52 Mn 24 Ga 24 target. The sputtering power was 400 W and the substrate temperature was kept at 323 K by cooling water. The composition of the deposited films was Ni-23.4 mol%Mn-23.0 mol%Ga. They were annealed at 1073 K for 36 ks for homogenization and ordering, and slowly cooled down in the furnace. Martensitic transformation temperatures of the films were measured by a differential scanning calorimeter, and the results were M s = 315 K, M f = 311 K, As = 316 K and A f = 321 K. To induce a shape memory effect, the annealed films were aged at 573-773 K for 0.9-3.6 ks in a constrained condition. It was found that they showed the two-way shape memory effect by thermal cycling.

Shape memory effect of sputtered Ni-rich Ni2MnGa alloy films aged in constraint condition

Ni2MnGa alloy is an intelligent material with ferromagnetic and shape memory properties. The application of the alloy films to micro-actuators has been proposed. The Ni-rich Ni2MnGa alloy films with a thickness of nearly 5 \\micron were deposited on Al2O3 substrates by a radio-frequency magnetron sputtering apparatus using a Ni52Mn24Ga24 target. They were heat-treated at 1073 K for 36 ks for homogenization and ordering. The martensitic transformation temperatures of the heat-treated films were higher than room temperature. To investigate the effect of aging time on shape memory properties, the heat-treated films were aged at 673 K for various times between 0.9 and 57.6 ks in a constrained condition. The constraint-aged films showed the two-way shape memory effect by thermal cycling. Fine precipitates with a crystal structure of L12 were observed in the constraint-films aged for a long period. As for their two-way shape memory properties, a range of transformation temperature narrowed and the amount of macroscopic shape change increased with increasing aging time.

Effect of Aging Time on Shape Memory Properties of Sputtered Ni-rich Ni2MnGa Alloy Films

Transmission electron microscope (TEM) observation at room temperature of the low temperature phase of Ni2.14Mn0.86Ga shows stress-induced 5 periodicity modulation along a pseudo-[111] cubic zone axis. On cooling to 173 K, the periodicity changes to 7. In stress-free areas, a [001] zone axis with new lattice parameters for the tetragonal phase was observed. The new lattice parameters are smaller by a factor of √2 than the expected lattice parameters. A new model for the tetragonal compound Ni2MnGa and the non-stoichiometric phases was developed with the help of TEM data, X-ray powder diffraction measurements, and a computational simulation based on the X-ray powder diffraction patterns.

Motohiko Suzuki

Papers

Grain-Boundary Sliding in AZ31 Magnesium Alloys at Room Temperature to 523 K

Low temperature crystal structure of Ni-Mn-Ga alloys

Shape memory effect of sputtered Ni-rich Ni2MnGa alloy films aged in constraint condition

Effect of Aging Time on Shape Memory Properties of Sputtered Ni-rich Ni2MnGa Alloy Films

Low Temperature Crystal Structure of Ni—Mn—Ga Alloys.