Defect and Diffusion Forum Vol. 279

Abstract: The Gibbs Free Energy Difference between the solid and liquid phases (DG) is related to nucleation frequency and has played an important role in predicting the glass forming ability (GFA) of multicomponent metallic alloys. This is due to the fact that the maximum energy for nucleus formation i.e. the activation barrier for nucleation has an inverse square relation with DG. The Gibbs Free Energy Difference of three multi-component bulk metallic glasses namely Mg65Cu25Y10, Zr57Cu15.4Ni12.6Al10Nb5 and Zr52.5Cu17.9Ni14.6Al10Ti5 have been evaluated using two new expressions. The results show that the DG values calculated assuming DCp to be constant lie closer to the experimental values for the Mg based system while in the case of two Zr based systems, DG computed using the hyperbolic variation of DCp show improved agreement with the experimental data.

Abstract: Al–8.6Zn–2.6Mg-2.4Cu–xCe(x = 0–0.4 wt. %) alloys were prepared by metal mould casting method, the effects of Ce on the microstructure and mechanical properties of the alloys were investigated. The results showed that the dendrite as well as grain size were refined by the addition of Ce, and the best refinement was obtained in 0.25 wt % Ce containing alloy. The main phases in the as-cast alloys were α-Al, Mg-Zn32, Mg32 (Al, Zn)49, and Al4Ce phase was found in the alloys containing more than 0.1%Ce. The addition of Ce improved the mechanical properties of the alloys. The strengthening mechanism was attributed to grain refinement and compound reinforced

Abstract: Hydrogen in excess of solid solubility precipitates as hydride phase of plate shaped morphology in hcp α-Zr with the broad face of the hydride plate coinciding with certain crystallographic plane of α-Zr crystal called habit plane. The objective of the present investigation is to predict the habit plane of δ-hydride precipitating in α-Zr at 298 K using strain energy minimization technique. The δ-hydride phase is modeled to undergo isotropic elasto-plastic deformation. The α-Zr phase was modeled to undergo transverse isotropic elastic deformation but isotropic plastic deformation. Accommodation strain energy of δ-hydride forming in α-Zr crystal was computed using initial strain method as a function of hydride nuclei orientation. Hydride was modeled as disk with round edge. Contrary to several habit planes reported in literature for δ- hydrides precipitating in α-Zr crystal, the total accommodation energy minima at 298 K suggests only basal plane i.e. (0001) as the habit plane.

Abstract: The kinetics of dissolution of ordered phase with Pt2Mo structure has been studied in two nickel chromium alloys – one without molybdenum and another with molybdenum - using differential scanning calorimetry. The activation energy for dissolution, determined using three nonisothermal approaches was found to be ~ 418 kJ /mole for both the alloys. This value agreed very well with the activation energy for coarsening of g″ precipitates in Ni-Cr-Mo matrix and is close to activation energy for mobility of chromium and molybdenum in complex nickel alloy matrix.

Abstract: High temperature beta-phase in titanium and zirconium alloy systems decomposes through an eutectoid reaction into a Ti- and Zr-rich a-solid solution and an intermetallic compound. The present paper reports the layer growth kinetics of the b-solid solution phase in elemental diffusion couples of titanium and zirconium. The growth kinetics obeys a parabolic growth law. However, the temperature dependence of the growth rate constant shows a bimodal behavior. The Arrhenius plot of the growth rate constant, which is linear at the start, becomes curved at lower temperature ranges. The deviation from the Arrhenius plot of the growth rate constant is related to the curvature in the solvus line of the b-solid solution. A theoretical model for the reaction diffusion responsible for the growth of b-solid solution is presented. The growth rate of b-phase is described by the equation 2 2 . . W k D C t b = = b D x , where k is a growth rate constant and Wb is the thickness of the b-phase formed over a period of time t, Db is the interdiffusion coefficient for the b-phase, DC is concentration range of b-phase and x is a parameter which is a function of the miscibility gaps in the phase diagram on the either side of the b-phase. The above equation provides a satisfactory description of the various aspect of the phenomenon of the growth of b-phase in Ti-and Zr-alloy systems.

Abstract: Microstructure and magnetic properties of a nanocrystalline soft magnetic material having composition Co64.5 Fe3.5 Si16.5 B13 Ni1.5 Cu1 has been studied. Amorphous ribbon could be produced by melt spinning unit. DSC analysis showed four distinct crystallization events. Heat treated samples were characterized using XRD and TEM techniques. Co2B, and CoB phases were found to crystallize before magnetic phase a−Co. Addition of copper was proved to have adverse effect on soft magnetic properties.

Abstract: In the present study, specimens of Zr were subjected to shock compression of 11.6 GPa. TEM examination of the recovered samples revealed that during shock compression the a phase has transformed into the w phase. The orientation relationships (ORs) between the a and w phase have been determined using both the stereographic projection method and the correspondence matrix method. Our ORs have been found to belong to the Variant I OR given by Usikov and Zilbershtein (UZ) for statically compressed Zr samples. Our ORs are the same as the one reported by Song and Gray (SG) on dynamically compressed samples. In the present paper it has been shown that the OR of SG is a subset of the OR of UZ and is not apart from it. The mechanism of a®w transition with respect to occurrence of an intermediate b (bcc) structure, during the transition has also been explored. We also show in this study that the amount of the transformed w phase decreases with increasing oxygen content in the samples that were shock loaded to the same peak pressure, as is revealed by both the TEM and XRD results.

Abstract: The morphology and substructure of martensite is considered to arise from the lattice invariant shear (LIS) associated with the transformation and this may be slip, twinning or both. Out of the several possible slip shears and twin modes only a few satisfy invariant plane strain criteria of the phenomenological theory of martensite (PTMC). On the basis of crystallographic and energetic criteria, a simple model has been proposed for determining the factors which influence the selection of the preferred LIS mode. In the present work, it is found that for b ® a' martensitic transformation in Zr-2.5 wt%Nb alloy, the preferred slip system is {1101}a'<2113>a' and the preferred twin system is {1101}a'<415 3>a'.

Abstract: Recent studies indicate that the existing criteria for amorphous phase formation are not precise. This study emphasizes that the extent of deviation from the linear relationship of the chemical contribution to enthalpy of the solid solution with the enthalpy difference between the amorphous phase and the constituent elements is a measure of tendency to bypass amorphous phase formation. Mechanical alloying experiments on a number of compositions in Al-Ni-Ti system showed that the compositions, not following the aforesaid relation, yield partial or no amorphization.