Abstract. Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magneticshape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams.
Solidification behavior of SiCw/Al-18Si metal matrix composites (MMCs) was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC) in order to reveal the effects of strontium addition and whisker content. The results show that the Si phase does not nucleate on SiC whisker surface. With the increasing of SiC whisker content, solidification onset and peak temperatures of primary Si decrease. Sr addition lowers solidification onset and peak temperatures of primary Si, and reduces its size. Whisker content has larger effects on solidification onset and peak temperatures of primary Si without Sr addition than that of primary Si with Sr addition.But solidification onset and peak temperatures of eutectic are barely affected by whisker content and Sr addition.
It is attractive to use multi-wall carbon nanotubes (MWNTs), which has a high modulus
and strength, to enhance the properties of metal matrix composites by hybrid strategy. In this paper,
novel aluminum borate whisker (ABOw) and MWNTs hybrid composites were prepared by squeeze
cast technique and the properties were investigated. The results show that hybrid preforms may be
made by co-deposition of ABOw and MWNTs in a solution by wet method. MWNTs separate the
ABOw from whisker-to-whisker contact and decrease the compressive deformation of the hybrid
preforms during squeeze cast processing. MWNTs distribute along mainly in the grains of
aluminum matrix, making the mechanical properties of the hybrid composites higher than singularly
reinforced composite.
The effect of the Al 2 O 3 -SiO 2 short fiber on the solidification processing of the -Al and Al-Si eutectic phases in the LD11 alloys was studied by the optical microscope (OM), differential scanning calorimeter (DSC) and scanning electron microscopy (SEM). The results revealed that the -Al phase nucleated in the inter-fiber spacing then grew towards the fiber. However, the eutectic silicon nucleated on the fiber surface and grew towards the inter-fiber spacing. It was further proved by the DSC measurement that growth processing of the -Al phase was restricted by the fiber, but nucleation behavior of the -Al phase was not affected by the fiber. In contrast to the -Al, the nucleation and growth processing of the eutectic silicon was accelerated by the fiber.
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