Effect of solution treatment on the martensitic transformation behavior of a Ni_(43)Co_7Mn_(39)Sn_(11) polycrystalline alloy

The effect of solution treatment on the martensitic transformation behavior of a Ni43Co7Mn39Sn11 polycrystalline alloy fabricated by an arc melting method was investigated by scanning electron microscopy（SEM）, energy-dispersive X-ray spectroscopy（EDS）, and differential scanning calorimetry（DSC）. The examination indicates the presence of severe chemical segregation in the dendritic as-cast structure because of solidification. This chemical segregation completely impedes the intrinsic martensitic transformation. Annealing at 1223 K for 24 h is identified as the threshold annealing condition to eliminate the microstructural segregation and begin the martensitic transformation, as indicated by a broad and obscure feature. Annealing at 1273 K for 24–48 h is found to be effective at promoting notably the martensitic transformation, but the martensitic transformation exhibits a multiple-step feature. Complete homogeneity is achieved by annealing at 1273 K for 72 h, which produces a sharp, single-step martensitic transformation. The microstructural evolution and the valence electron concentrations of alloys（e/a ratio） are evaluated, which are reflective of the degree of compositional homogeneity of alloys, confirming that high annealing temperature and long holding time are vital to reveal the intrinsic martensitic behavior of this alloy. The adequately homogenized alloy displays a martensitic transformation at 292 K and an enthalpy of 11.2 J/g.

pH-mediated synthesis of monodisperse gold nanorods with quantitative yield and molecular level insight

Although gold nanorods(GNRs)have been produced with different dimensions and aspect ratios,the current synthesis methods through seed-mediated growth are far from ideal,for instance,the quality(rod yield)and the quantity(gold conversion)cannot be simultaneously satisfied.More critically,there is no molecular level understanding of the growth mechanism.Here,we solved the problem by employing the stoichiometric ratio of reactants and tuning the reactivity of the reductant through adjusting the initial pH value of the growth solution to achieve both good quality and high quantity simultaneously.We also extended our strategy to other enols besides ascorbic acid,such as phenolic compounds,and found that the optimal pH for GNRs synthesis depends on the structure of the individual compound.The mechanistic insight greatly enriches the toolbox of reductants for GNRs growth and makes it possible to synthesize GNRs at both acidic and basic conditions.An interesting phenomenon is that for most of the phenolic compounds we tested,the morphology of the final products follows the same sphere-rod-sphere trend as the initial pH value of the reaction increases,whether it is under acidic or basic conditions,which cannot be explained by any previously proposed mechanism.The effect of pH is mainly attributed to the regulation of the reduction potential of the reductants,and thus the reaction rate.A model has been proposed to explain the dependence of anisotropic growth of GNRs on the concentration gradient of reactants around the seeds,which is decided by both the reaction rate and diffusion rate.

Scattering of steady cross-plane shear waves by surfaces of arbitrary shape in homogeneous and transversely isotropic media

Two-dimensional scalar equation for the displacement of steady cross-plane shear (SH) waves in homogeneous and transversely isotropic media like unidirectional fibrous com-posites is given. Then, thrbugh a simple coordinate system transform, the scalar equation is standardized into a Helmholtz equation. Corresponding integral equations are derived for the scattering problems and boundary element method (BEM) is used to calculate the scattered fields of arbitrarily shaped obstacles with both soft and rigid boudary conditions numerically.A discussion is given on the numerical results which is mainly focused on the influence of the a-nisotropy of the media to the directivity of the scattered fields by circular cylindrical voids.