No abstract

The grain-size effect on deformation twinning in nanocrystalline copper is studied. It has been reported that deformation twinning in coarse-grained copper occurs only under high strain rate and/or low-temperature conditions. Furthermore, reducing grain sizes has been shown to suppress deformation twinning. Here, we show that twinning becomes a major deformation mechanism in nanocrystalline copper during high-pressure torsion under a very slow strain rate and at room temperature. High-resolution transmission electron microscopy investigation of the twinning morphology suggests that many twins and stacking faults in nanocrystalline copper were formed through partial dislocation emissions from grain boundaries. This mechanism differs from the pole mechanism operating in coarse-grained copper.Nanocrystalline ͑nc͒ materials have been reported to have superior mechanical properties such as high strength, which can coexist with very good ductility. 1-3 These superior mechanical properties are attributed to their unique deformation mechanisms, which are different from those in their coarse-grained ͑CG͒ counterparts. 4 -8 For example, molecular dynamics simulations, which used extremely high strain rates in the order of 10 6 to 10 8 s Ϫ1 , predict that NC Al deforms via partial dislocation emission from grain boundaries, which consequently produces deformation twins. 7 These predictions have recently been verified experimentally in nc Al powder processed by ball milling at liquid nitrogen temperature 9 and in nc Al film produced by physical vapor deposition. 8 These observations are very surprising because deformation twinning has never been observed in CG Al.High strain rate, low temperature, and nanometer grain size are major contributing factors for deformation twinning in the ball-milled Al powder. 9,10 In fact, both high strain rate and low temperature are known to promote deformation twinning. 11,12 For example, CG copper does not deform by twinning 13,14 except at very high strain rate 15,16 and/or low temperature. 17 However, the grain-size effect is not so clear. It has been suggested that both the critical slip stress and twinning stress follow the Hall-Petch ͑HP͒ relationship, with the HP slope for twinning (k T ) significantly larger than that for slip (k S ) for many CG metals and alloys. 18 For copper, the k T is about 0.7 MN/m 3/2 , while k S is about 0.35 MN/m 3/2 . 19 Consequently, dislocation slip rather than deformation twinning is expected to become the preferred deformation mode when the grain is smaller than a certain size. Indeed, Meyers et al. 20 reported that shock compression at 35 GPa produced abundant deformation twins in copper samples with grain sizes of 117 and 315 m, but virtually no twinning in a copper sample with a grain size of 9 m. On the other hand, it has been well known that the HP relationship fails in nc materials. 4,5 These literature observations raise some fundamental questions on the grain-size effect on deformation twinning. Does the trend that smaller grains are harder to ...

Proton exchange membrane (PEM) fuel cells 2 showing promise of a high level of performance have, up to the present time, used electrodes containing a high platinum loading (4 mg/cm2). We report improvements in performance of PEM fuel cells utilizing electrodes with only one-tenth of this platinum loading by (i) extension of the three-dimensional reaction zone by incorporation of a proton conductor (Nation) into the electrode structure; (ii) optimization of the amount of Nation impregnated into the electrode structure; (iii) hot-pressing the impregnated electrodes to the Nation membrane at 120~ and 50 atm; (iv) optimal humidification of reactant gases at a temperature above that of the cell (5~ for 02 or air and 10~176 for H2); and (v) operation at elevated temperatures and pressures. The performance of the cells was analyzed from measurements of cell potential vs. current density and of cell potential at constant current density vs. time. Cyclic voltammerry proved to be a useful tool to ascertain the electrochemically active area of the electrodes.

We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

No abstract