Samuel M. Allen

A microscopic theory for antiphase boundary motion and its application to antiphase domain coarsening

Abstract A microscopic diffusional theory for the motion of a curved antiphase boundary is presented. The interfacial velocity is found to be linearly proportional to the mean curvature of the boundary, but unlike earlier theories the constant of proportionality does not include the specific surface free energy, yet the diffusional dissipation of free energy is shown to be equal to the reduction in total boundary free energy. The theory is incorporated into a model for antiphase domain coarsening. Experimental measurements of domain coarsening kinetics in Fe-Al alloys were made over a temperature range where the specific surface free energy was varied by more than two orders of magnitude. The results are consistent with the theory; in particular, the domain coarsening kinetics do not have the temperature dependence of the specific surface free energy.

6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni–Mn–Ga

Field-induced strains of 6% are reported in ferromagnetic Ni–Mn–Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependence of strain on magnetic field and external stress using as input parameters only measured quantities.

Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression includin...

Mechanisms of phase transformations within the miscibility gap of Fe-rich Fe-Al alloys

Abstract The coherent phase diagram of the Fe-Al system possesses a tricritical point where a line of higher-order transitions ends at a miscibility gap at about 23 at.% Al and 615°C. Rules of general applicability governing phase separation within the miscibility gap of such a system are developed. Application of the rules to the Fe-Al system results in detailed predictions about the mechanisms of decomposition and ordering reactions and their sequences. Electron microscopy is used to study the reactions experimentally and the results are in agreement with theoretical predictions.

Microstructural development during solidification of stainless steel alloys

The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×106°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates.

Empirical mapping of Ni–Mn–Ga properties with composition and valence electron concentration

A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K

Large field induced strain in single crystalline Ni–Mn–Ga ferromagnetic shape memory alloy

A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from √ to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis o...

Coherent and incoherent equilibria in iron-rich iron-aluminum alloys

Abstract The discrepancy between two recent phase diagram determinations of the Fe-Al system is resolved experimentally. Both diagrams are correct, but one displays metastable coherent equilibria. The form of the combined coherent and incoherent phase diagram for these alloys is critically discussed. A theoretical treatment of the effect of coherency stresses on lowering the miscibility gap in this system, given in an appendix, is compared with experimental observations.

Pulsed magnetic field-induced actuation of Ni–Mn–Ga single crystals

The field-induced actuation of Ni–Mn–Ga single crystals through twin-boundary motion has been demonstrated with magnetic fieldpulses of various intensities lasting 620 μs. It is shown that the complete field-induced strain can be obtained in 250 μs, which implies the possibility of full 6% cycling of Ni–Mn–Ga at 2 kHz, for crystals having dimensions in the range of a few millimeters. The final extension increases with the peak driving force, which is not linear with the field and saturates at 7.85 kOe. An increase of the field beyond the saturation level produces no additional strain but reduces the time for field-induced detwinning.

Foil thickness measurements from convergent-beam diffraction patterns

Abstract The analysis of convergent-beam electron diffraction patterns for foil thickness is easily carried out in the two-beam approximation to dynamical diffraction theory. The effects of this approximation on the accuracy of foil thickness determinations are considered, as well as the range of thicknesses which can be measured with the technique. Inaccuracies due to multiple-beam effects are minimized by avoiding diffracting conditions involving low-order reflections. For 100 keV electron energy, errors of less than 2% in thickness determination are possible in Al, Cu and Au, if the diffraction vector g is equal to or larger than 200, 220 or 311 respectively. The minimum thickness measurable for these materials diminishes as |g| increases, but is approximately 20 nm for the lower-order reflections. The maximum thickness measurable is limited by absorption, and is estimated for Al, Cu and Au to be approximately 600, 130 and 40 nm respectively.