Papers by Author: Samuel M. Allen

Abstract: Segregation of solute atoms and vacancies to migrating D03–type antiphase boundaries (APBs) in Fe3Al of the stoichirometric and an Al-rich compositions has been investigated in detail by a phase-field method developed in the previous study [Koizumi et al. Acta Mater 2009;57:3039] focusing on the quantitative relationship among the segregation, APB energy, driving-force, drag-force and APB mobility. It has been revealed that the drag-force by solute segregation depends considerably on the alloy composition and is responsible for the significant composition dependence of the migration kinetics of D03-APB whereas the composition dependences of driving-force and mobility are negligibly small.

Abstract: Composites of Ni–Mn–Ga particles in a polyurethane matrix can be made by mixing the particles with the polymer, and allowing them to cure under a magnetic field to texture the composites. These composites show large hysteresis and mechanical losses, when subjected to a cyclic stress, that were far larger than the matrix polymer ones. The additional losses are attributed to the motion of twin boundaries in the filler particles and provide a way for obtaining mechanical energy absorption in a wide frequency range. By means of X-ray and neutron diffraction we present evidence that confirms that twins are present in the particles and that they do move when mechanically loading the composite

Abstract: Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have emerged as a promising class of active materials capable of producing a large (up to 10%) magnetic-field-induced strain (MFIS). This large strain is not the familiar anisotropic magnetostriction; it results from field-induced twin-boundary motion and has appropriately been referred to as magnetoplasticity. FSMAs still have several characteristic shortcomings that may limit their potential applications. A threshold field of 150 to 300 kA/m must be overcome to initiate twin-boundary motion and a larger field is required to achieve full strain. The operating window of the stress output from FSMA actuators is limited to the range between 1 and 1.5 MPa. Outside this operating range, the strain output diminishes significantly. This paper addresses these performance limitations and describes an acoustic-assist technique that has been shown to decrease the required threshold field and increase the stress and strain output of FSMA actuation. The application of an acoustic assistance from a 33-mode piezoelectric stack is shown to improve MFIS of Ni–Mn–Ga single crystals by reducing the required threshold field and twinning-yield stress. Threshold field reductions of up to 80 kA/m are observed, and the twinning-yield stress can be reduced by up to 0.5 MPa. The effect of acoustic assistance on FSMA actuation can be understood as a form of time varying stress waves that facilitate twin-boundary motion. A stress wave analysis is shown to give a quantitative understanding of the measured reduction in the twinning-yield stress. For FSMA cyclic actuation, both operating stress and strain outputs of the FSMA actuation are significantly enhanced by acoustic assistance. Without the acoustic assistance, the maximum reversible strain of the sample used here is 3% and appears only in the limited external stress range between 0.7 and 1 MPa. With the acoustic assistance, the maximum reversible strain increases to 4.5% and appears in a broader range of stress output between 0.4 and 1.2 MPa. The reduction in the twinning-yield stress due to the acoustic assistance significantly improves the FSMA cyclic actuation performance; magnetic energy not used to drive twin-boundary motion can be utilized to work against a larger external load.

Abstract: Ni-Mn-Ga ferromagnetic shape memory alloys (FSMA) are a potential new class of actuator materials able to respond at higher frequencies (at least 300 Hz) with comparable strains (up to 6 %) in a moderate field (below 1 T)[1]. Magnitude of the strain depends on the values of several critical material parameters, most importantly the martensitic transformation temperature (TMart), Curie temperature (TC) and saturation magnetization (MS)[2]. It is well known that these parameters are strongly dependent on the composition of the alloy. Composition dependence of TMart, TC and MS have been experimentaly explored [3,4]. Therefore, it is possible to compile a more complete, and hence more useful composition map for designing Ni-Mn-Ga FSMAs. Ageing behavior is important in these newly developed FSMAs because ageing can affect the reliability of devices using the alloys. Ni-Mn-Ga FSMAs and Au-Cd[5] alloys have several important common characteristics, including off-stoichiometry alloy composition (designed for operation at ambient temperature) and easy twin boundary motion in the martensite state, thus similar ageing behavior is expected in Ni-Mn-Ga alloys. Ni-Mn-Ga alloys have also demonstrated strong damping due to the motion of twin boundaries[6]. Low-frequency mechanical properties are typically measured using the technique of dynamical mechanical analysis (DMA)[7]. In this paper, we present studies of composition design, subtle structure changes associated with ageing, and the temperature dependence of the low-frequency mechanical properties of several Ni-Mn-Ga single crystal alloys.