Samuel M. Allen

Bio: Samuel M. Allen is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Magnetic shape-memory alloy & Crystal twinning. The author has an hindex of 37, co-authored 159 publications receiving 9368 citations. Previous affiliations of Samuel M. Allen include Swiss Federal Laboratories for Materials Science and Technology.

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

Abstract: 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.

Kinetics of materials

Abstract: Preface. Bibliography. Acknowldegments. Notation. Symbol Table-Roman. Symbol Table-Greek. 1. Introduction. PART I: MOTION OF ATOMS AND MOLECULES BY DIFFUSION. 2. Irreversible Thermodynamics: Coupled Forces and Fluxes. 3. Driving Forces and Fluxes for Diffusion. 4. The Diffusion Equation. 5. Solutions to the Diffusion Equation. 6. Diffusion In Multi-Component Systems. 7. Atomic Models for Diffusion. 8. Diffusion in Cerystals. 9. Diffusion Along Crystal Imperfections. 10. Diffusion in Noncrystalline Materials. PART II: MOTION OF DISLOCATIONS AND INTERFACES. 11. Motion of Dislocations. 12. Motion of Crystalline Surfaces. 13. Motion of Crystalline Interfaces. PART III MORPHOLOGICAL EVOLUTION DUE TO CAPILLARY AND APPLIED MECHANICAL FORCES. 14. Surface Evolution due to Capillary Forces. 15. Coarsening due to Capillary Forces. 16. Morphological Evolution, Diffusional Creep, and Sintering. PART IV: PHASE TRANSFORMATIONS. 17. General Features of Phase Transformations. 18. Spinodal and Order-Disorder Transformations. 19. Nucleation. 20. Growth of Phases in Concentration and Thermal Fields. 21. Concurrent Nucleation and Growth. 22. Solidification. 23. Precipitation. 24. Martensitic Transformations. Appendix A: Densities, Fractions, and Atomic Volumes of Components. Appendix B: Structure of Crystalline Interfaces. Appendix C: Capillarity and Mathematics of Space Curves and Interfaces. Illustration Credits. Cited Author Index. Figure Index. Topic Index.

Microstructural development during solidification of stainless steel alloys

Abstract: 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.

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Spin glasses: Experimental facts, theoretical concepts, and open questions

Abstract: This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

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

Abstract: 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.