Titanium with gradient nano-to-micron scale grains from surface to matrix was fabricated by surface mechanical grinding treatment(SMGT) at room temperature.The SMGT-treated titanium shows higher strength than that of as-received one,but moderate ductility between those of ultra-fine grained(UFG) and coarse-grained titanium.Tensile stress-strain curves of SMGT-treated titanium show double strain hardening regimes.The strain hardening rate(dσ/dε) decreases with increasing strain in tensile deformation.The high strain hardening rate at initial yielding is attributed to nano-to-micron-grained surface layer.The low strain hardening rate at large plastic strain regime primarily results from coarse-grained matrix.The SMGT-treated titanium shows a ductile fracture mode with a large number of dimples.The small size of dimples in the treated surface layer is due to the combination of the high strength and strain hardening exponent.The difference between dimple size in nano-to-micron-grained surface layer and coarse-grained matrix is discussed in terms of plastic zone size at the tip of crack in the SMGT-treated titanium.
Mechanical tests on small-volume materials show that in addition to the usual attributes of strength and ductility, the controlla- bility of deformation would be crucial for the purpose of precise plastic shaping. In our present work, a ＂mechanical controlla- bility index＂ （MCI） has been proposed to assess the controllability of mechanical deformation quantitatively. The index allows quantitative evaluation of the relative fraction of the controllable plastic strain out of the total strain. MCI=0 means completely uncontrollable plastic deformation, MCI=∞ means perfectly controllable plastic shaping. The application of the index is demonstrated here by comparing two example cases： 0.273 to 0.429 for single crystal A1 nanopillars that exhibit obvious strain bursts, versus 3.17 to 4.2 for polycrystalline A1 nanopillars of similar size for which the stress-strain curve is smoother.
Using nanoscale electrical-discharge-induced rapid Joule heating, we developed a method for ultrafast shape change and joining of small-volume materials. Shape change is dominated by surface-tension-driven convection in the transient liquid melt, giving an extremely high strain rate of N106 s-1. In addition, the heat can be dissipated in small volumes within a few microseconds through thermal conduction, quenching the melt back to the solid state with cooling rates up to 108 K.s-1. We demonstrate that this approach can be utilized for the ultrafast welding of small-volume crystalline Mo （a refractory metal） and amorphous Cu49Zr51 without introducing obvious microstructural changes, distinguishing the process from bulk welding.
Cheng-Cai WangQing-Jie LiLiang ChenYong-Hong ChengJun SunZhioWei ShanJu LiEvan Ma
Prof.Shan Zhiwei’s group at Xi’an Jiaotong University discovered a novel mechanism for room temperature plastic deformation in magnesium(Nature Communications,2014,5:3297).Magnesium and its alloys have been intensively studied for several decades due to their potential applications in automobile and aerospace industry.Deformation twinning and dislocation slip are known to be
