Multi-phase equation of state of ultrapure hafnium to 120 GPa.

Hafnium (Hf) is an industrially important material due to its large neutron absorption cross-section and its high corrosion resistance. When subjected to high pressure, Hf phase transforms from its hexagonal close packed α-Hf phase to the hexagonal ω-Hf phase. Upon further compression, ω-Hf phase transforms to the body centered cubic ß-Hf phase. In this study, the high pressure phase transformations of Hf are studied by compressing and decompressing a well-characterized Hf sample in diamond anvil cells up to 120 GPa while collecting x-ray diffraction data. The phase transformations of Hf were compared in both a He pressure transmitting medium (PTM) and no PTM over several experiments. It was found that the α-Hf to ω-Hf phase transition occurs at a higher pressure during compression and lower pressure during decompression with a helium (He) PTM compared to using no PTM. There was little difference in the ω-Hf to ß-Hf phase transition pressure between the He PTM and no PTM. The equation of state was fit for all three phases of Hf and under both PTM and no-PTM.

Grain-size-independent plastic flow at ultrahigh pressures and strain rates.

A basic tenet of material science is that the flow stress of a metal increases as its grain size decreases, an effect described by the Hall-Petch relation. This relation is used extensively in material design to optimize the hardness, durability, survivability, and ductility of structural metals. This Letter reports experimental results in a new regime of high pressures and strain rates that challenge this basic tenet of mechanical metallurgy. We report measurements of the plastic flow of the model body-centered-cubic metal tantalum made under conditions of high pressure (>100 GPa) and strain rate (∼10(7) s(-1)) achieved by using the Omega laser. Under these unique plastic deformation ("flow") conditions, the effect of grain size is found to be negligible for grain sizes >0.25 µm sizes. A multiscale model of the plastic flow suggests that pressure and strain rate hardening dominate over the grain-size effects. Theoretical estimates, based on grain compatibility and geometrically necessary dislocations, corroborate this conclusion.

Why are some Interfaces in Materials Stronger than others?

Grain boundaries (GBs) are often the preferred sites for void nucleation in ductile metals. However, it has been observed that all boundaries do not contribute equally to this process. We present a mechanistic rationale for the role of GBs in damage nucleation in copper, along with a quantitative map for predicting preferred void nucleation at GBs based on molecular dynamics simulations in copper. Simulations show a direct correlation between the void nucleation stress and the ability of a grain boundary to plastically deform by emitting dislocations, during shock compression. Plastic response of a GB, affects the development of stress concentrations believed to be responsible for void nucleation by acting as a dissipation mechanism for the applied stress.

Electrical stimulation of nerve regeneration in the rat: functional evaluation by a twitch tension method.

The effect of D.C. electrical fields on in vivo nerve regeneration following rat sciatic nerve transection was assessed by a toe-twitch tension method for motor recovery. Two groups for baseline comparisons received either a nerve transection and a neutral battery implant or a nerve crush and no implant. Two treatment groups each received a nerve transection and an implant delivering either cathodal (-10 µA) or anodal (+10 µA) current via electrodes positioned on either side of the transection site. The contralateral nerve was exposed but not given a lesion. The functional recovery was assessed at weekly intervals by comparing the amplitude and area of the twitch tension curves on both sides, expressed as a ratio (index of motor recovery = IMR). By 28-35 days postoperative (dpo) the crushed/untreated group had mean IMR values not significantly different than the preoperative values. The transected/neutral group appeared to have the worst recovery throughout the study. Compared to these two groups, both the cathodal and anodal treatment groups with a transected nerve showed intermediate IMR values, but by 63 dpo the group differences were less. These results indicate that following a nerve transection lesion steady D.C. electrical fields may temporarily enhance the early functional recovery of motor nerve regeneration compared to conventional nerve repair alone.

Comparison of nurses' self-directed learning activities.

This study compares the self-directed learning (SDL) practices of baccalaureate (40) and master's (40) nurses. Using a personal interview format, nurses were queried as to the number and nature of their SDL projects. Based on the study data, nurses spend a greater portion of their SDL time on professional area content. Respondents indicated they spend an average of 313 hours per year on independent learning projects with 217 of those hours on professional topics. Master's-prepared nurses spend more time per year (398 hours) and slightly more time on professional content (71%) compared to the time baccalaureate-prepared nurses spend per year (230 hours) on professional content (67%). Implications for nursing education are identified with respect to learner motivation and accessibility of materials.

Report on a survey of program directors regarding selection factors in graduate medical education.

A national random sample of 25 percent of the graduate education program directors in internal medicine, family medicine, surgery, and pediatrics was sent a questionnaire; subjects were asked to judge the importance of 31 variables in the selection of house staff. A rank-ordering of variables for all respondents placed interpersonal skills demonstrated in the interview as number one. When rank order correlations were calculated for all possible pairs of program specializations, strong positive relationships were revealed. A two-way analysis of covariance was also undertaken to assess how selected program characteristics, such as size of program, type of program, and affiliation or nonaffiliation with a medical school, affected the judgment of the importance of the variables. The results have implications for further studies in several areas.