Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grainboundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials.

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This letter reveals that unusual elasticity of nanocrystalline diamond is consistently explained by stacking fault inside the diamond grains instead of the graphitic plate inclusion, which was only possible mechanism. Ab initio calculation shows that stacking fault introduced in the diamond structure behaves as graphitic sp2 bonds, and the elastic constants calculated from the strain-energy relationship agree with the acoustic measurements.

Quasi-static and dynamic tensile testing was performed upon machined tensile specimens fabricated from bulk primitives produced by the consolidation of water atomized, 17-4 precipitation hardened stainless steel powder by layer-based, selective laser melting. Such mechanical evaluation was performed by a screw-driven uniaxial tension testing machine and a split-Hopkinson tensile bar apparatus. Strain rates evaluated include 10-3 , 10-1 and 10 3 s-1. Prior to tensile testing, specimens underwent additional thermal processing in accordance with industry standards. Evaluations of the solution heat treatment and peak-age conditions were made alongside similarly prepared, but traditionally processed specimens meeting the same material standard for chemistry (drawn rod). Tensile strength across all strain rates is higher for these selective laser melted (SLM) specimens as a result of microstructure refinement through rapid melt, solidification and cooling during processing. Ultimate tensile and yield strengths increase with increasing strain rate and show no preferential direction relative to the building direction. Elongation anisotropy is observed as a consequence of directional porosity stemming from pores limited to within individual layers. Specimens loaded normal to the SLM building plane commonly rupture at small elongation because of this mechanical fibering from the selective laser melting process.

In this paper, a picosecond ultrasound measurement is conducted to evaluate low-temperature elasticity of thin films and semiconductors. Specimens are cooled with liquid He through a heat exchanger in a cryostat, and an ultrahigh-frequency acoustic pulse is generated using a femtosecond light pulse, which propagates in the film-thickness direction. One of three phenomena, pulse echoes, acoustic phonon resonances, and Brillouin oscillations, is observed by the reflectivity changes of the time-delayed probe light, depending on the material. They give the longitudinal-wave out-of-plane elastic constant. When the stiffness at low temperature is known, the last phenomenon provides the refractive index. We determined the stiffness of Pt thin film and refractive index of Si at 5 K. The methodology developed in this paper is effective to study elastic and optic properties of metallic thin films and transparent materials at low temperatures.

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The effects of P b doping, oxygen doping (6 = 0.1,0.2,0.4 and 0.5) and high pressure (4, 8, 15 and 20GPa) on the electronic structure of Hg,,,Pb,,,Ba,Ca,Cu,O,+, have been examined by the recursion method. Our calculations show that Pb doping only decreases the hole concentration slightly and oxygen doping increases the hole concentration monotonically and significantly as 6 varies from 0 to 0.5, with each excess oxygen atom contributing about 1.7 holes to the CuO, layers. The optimal 6 is estimated to be around 0.4. The hole concentration increases initially with pressure but decreases as P > 8 GPa (i. e., dn/dP does change sign at N 8 GPa). The suppressed T,(P) by Pb substitution has to be described in the modified Neumeier's model.

The effect of impact compression on age hardening behavior was examined for Meso20 and 6061 aluminum alloys using a single stage gun. The hardness of Meso20 and 6061 aluminum alloy applied with an impact compression (about 5.0GPa) after the solution treatment increased with the aging time. The cluster of point defects like stacking fault tetrahedral (SFT) was observed in the 6061 aluminum alloys with the impact compression (5.3GPa) after the solution treatment. Even after the impact compression, distribution of the aging precipitates was clearly identified.