S.A. Dregia

Grain growth in anisotropic systems: comparison of effects of energy and mobility

Grain growth in systems of anisotropic grain boundary energy and mobility is investigated by computer simulations in a two-dimensional textured polycrystalline system. The energy and mobility are allowed to depend on both grain boundary inclination and misorientation. Mobility anisotropy alone does not significantly change the growth kinetics or statistical distributions of misorientation, grain size and number of grain edges, even though grain shapes evolve in a self-dissimilar fashion where the aspect ratio of grains and inclination distribution of grain boundaries are time dependent. Energy anisotropy, however, causes significant deviation of grain growth kinetics, misorientation and edge-distributions from the ones observed in isotropic systems. Moreover, misorientation distribution is skewed towards low energy (special) boundaries. Size distributions are similar in all cases. Mobility anisotropy influences grain growth kinetics only when energy is also anisotropic. Variation of misorientation distribution with time plays the key role in determining the grain growth behavior.

Phase stability of bcc Zr in Nb/Zr thin film multilayers

A change in phase stability from hcp Zr to bcc Zr occurs in Nb/Zr multilayers when the bilayer thickness is reduced to the nanometer scale. This phase stability in these multilayers has been described using a model based on classical thermodynamics. Using a previously reported experimental observation of hcp to bcc transformation in these multilayers, a phase stability diagram (referred to as the biphase diagram) has been proposed. Subsequently, a range of multilayers with varying volume fractions and bilayer thicknesses have been sputter deposited. The crystal structures in these multilayers have been determined using X-ray and electron diffraction. The hcp and bcc Zr phases within the Nb/Zr multilayers are in agreement with the predictions afforded by the proposed biphase diagram. The sequence of the Zr bcc phase stability was accomplished by forming its β-Zr (high temperature bcc phase) prior to forming a bcc coherent interface with Nb. First approximations of the structural and chemical contributions to the interfacial energy accompanying the changes in hcp to bcc phase stability for Zr have been evaluated.

Phase stability in Al/Ti multilayers

Structural stabilities in thin-film Al/Ti multilayers have been studied using transmission electron microscopy. It is shown that models based on interface-induced modifications to bulk stacking fault energies, and/or coherency-strain energy may not be used to explain the various phase transitions observed. Rather, phase stability may be rationalized by reference to a new model based on classical thermodynamics, involving the competition between bulk and interfacial energies. Thus, these phase transitions are shown to be driven by reductions in the overall interfacial energies. A biphase diagram (reciprocal of bilayer thickness vs composition) has been developed for the multilayers produced by magnetron sputtering. The influence of impurities possibly introduced during thin-foil preparation is considered and rationalized by reference to the biphase diagram for this system.

Stability of f.c.c. titanium in titanium/aluminum multilayers

Crystal structures of as-sputtered Ti/Al multilayers are studied with special emphasis on the formation of f.c.c.-Ti and the effect of coherency strains on phase stability. The results are rationalized on the basis of a recently proposed thermodynamic model for phase stability in multilayers. In this paper, the thermodynamic model is modified to account for the effect of coherency strains on the phase stability. A new phase stability diagram is proposed for Ti/Al multilayers.

Solute segregation transition and drag force on grain boundaries

Abstract We investigate solute segregation and transition at grain boundaries and the corresponding drag effect on grain boundary migration. A continuum model of grain boundary segregation based on gradient thermodynamics and its discrete counterpart (discrete lattice model) are formulated. The model differs from much previous work because it takes into account several physically distinctive terms, including concentration gradient, spatial variation of gradient-energy coefficient and concentration dependence of solute–grain boundary interactions. Their effects on the equilibrium and steady-state solute concentration profiles across the grain boundary, the segregation transition temperature and the corresponding drag forces are characterized for a prototype planar grain boundary in a regular solution. It is found that omission of these terms could result in a significant overestimate or underestimate (depending on the boundary velocity) of the enhancement of solute segregation and drag force for systems of a positive mixing energy. Without considering these terms, much higher transition temperatures are predicted and the critical point is displaced towards much higher bulk solute concentration and temperature. The model predicts a sharp transition of grain boundary mobility as a function of temperature, which is related to the sharp transition of solute concentration of grain boundary as a function of temperature. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in both the concentration–temperature plot and the mobility–temperature plot. These predictions agree well with experimental observations.

On the theory of grain growth in systems with anisotropic boundary mobility

Abstract We analyze grain growth kinetics in systems with anisotropic grain boundary mobility. In contrast to most previous studies of grain growth dynamics, we relax self-similarity assumptions that strongly constrain the dynamics and statistics during microstructural evolution in polycrystalline materials. We derive analytical expressions for the average growth rate within each topological class of n -sided grains as well as for the growth rate of the average grain area; we explain the results using underlying symmetries. Although anisotropic grain growth may in general be non-linear in time, we show, even in the absence of the self-similarity constraint, that the evolution kinetics obeys the von Neumann–Mullins relationship in the two limiting cases of textured and fully random microstructure with a time dependence solely determined by changes in the misorientation distribution. Our analytical results agree well with recent computer simulations using a generalized phase field approach.

Growth of 1-D TiO 2 nanowires on Ti and Ti alloys by oxidation

The growth of titania nanowires by a simple metal oxidation process was investigated for both commercially pure α-Ti and Ti alloys including Ti64 and β-Ti under a limited supply of oxygen. The effects of processing variables including heat treatment temperature, gas flow rate, and process duration on the growth of nanowires were explored. Similarities and differences in the growth of nanowires on pure Ti versus Ti alloys were observed. While the growth window in terms of temperature and flow rate is narrow in pure Ti, the window is much wider in the alloys. However, the trend towards high temperature is similar in all the samples promoting faceted oxide crystal growth rather than nanowires.

Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition

In a study of the influence of ZrB2 additions on the irreversibility field, μ0Hirr and the upper critical field Bc2, bulk samples with 7.5at% ZrB2 additions were made by a powder milling and compaction technique. These samples were then heated to 700–900°C for 0.5h. Resistive transitions were measured at 4.2K and μ0Hirr and Bc2 values were determined. An increase in Bc2 from 20.5Tto28.6T and enhancement of μ0Hirr from 16Tto24T were observed in the ZrB2 doped sample as compared to the binary sample at 4.2K. Critical field increases similar to those found with SiC doping were seen at 4.2K. At higher temperatures, increases in μ0Hirr were also determined by M-H loop extrapolation and closure. Values of μ0Hirr which were enhanced with ZrB2 doping (as compared to the binary) were seen at temperatures up to 34K, with μ0Hirr values larger than those for SiC doped samples at higher temperatures. The transition temperature, Tc, was then measured using dc susceptibility and a 2.5K drop of the midpoint of Tc was obs...

Crystallography of surface nucleation and epitaxial growth of lithium triniobate on congruent lithium niobate

The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and growth of LiNb 3 O 8 precipitates which exhibit an epitaxial orientation relationship. The same orientation relationship is observed on x-, y-, and z -cut LN substrates. The epitaxy arises from similarities between the two crystal structures and provides for an essentially continuous oxygen-ion framework from parent to product. On y -cut LN, the precipitates have a well-defined habit plane, and the interfacial misfit between the two structures is accommodated by a rectangular grid of misfit dislocations. The density, geometry, and imaging behavior of the misfit dislocations suggest that their Burgers vectors serve to accommodate the disregistry in the oxygen-ion framework. Based on these observations, it is concluded that the mechanism of formation of LiNb 3 O 8 consists of preferential nucleation on the surface and subsequent growth, possibly by counterdiffusion of cations in a closed system where the oxygen-ion framework remains essentially fixed in space.

Homogeneous carbon doping of magnesium diboride by high-temperature, high-pressure synthesis

We have used high-pressure, high-temperature synthesis at 1500–1700 °C and 10 MPa to create homogeneously C-substituted MgB2 from a B4C + Mg mixture. X-ray diffraction analysis showed large peak-shifts consistent with a decrease in the a lattice parameter for the B4C-derived MgB2 as compared to an undoped sample (0.033–0.037 A, depending on the sample). Microstructural investigation showed a three-phase mixture in the B4C-derived ingots: MgB2−xCx (with 0.178 < x < 0.195), MgB2C2, and Mg. The carbon concentration determined from the lattice parameter shift (5.95 at. %) matched well with the calorimetrically derived concentration of 5.3–5.8 at. % C. Furthermore, the carbon content measured by electron probe micro-analysis was shown to be 6.2 ± 1.3 at. %. Finally, we performed bulk specific heat measurements to determine the homogeneity of C-doping in the MgB2. The width of the Tc distribution for the C-doped MgB2 was only 3–6 K with a full-width half maximum (FWHM) of 1.4 K, compared to a width of 2.5 K and...