Synthesis And Mechanical Properties Of Large Bulk Nanocrystalline Metal Silver And Nanocrystalline Metal Tungsten

Synthesis, microvoids and mechanical properties ( including microhardness, tensile and compressive strength) for large bulk nanocrystalline metal silver (nano-Ag) were studies in this thesis. The main results are summarized as follows:The large disk-shaped bulk specimens of nanocrystalline metals silver (nano-Ag),with a diameter of 80mm, thickness of 7.8mm,as well as an average grain sizes of 50nm, respectively, were successfully synthesized by an inert gas condensation and in-situ warm compacting technique (IGC) in our home made apparatus. The densities of nano-Ag is 97% of the theoretical density of silver. The radius of the micro-pores in the nano-Ag is in the range of 16.5 to 18nm but the amount of the micro-pores is quite small. The microhardness of the nano-Ag is 0.9GPa which is about 2 times of that for coarse-grained polycrystalline metal silver. The tensile yield stress and fracture stress of the nano-Ag are 80Mpa and 169MPa, respectively, which are about 1.51 and 1.3 times of that for coarse-grained polycrystalline metal silver. The tensile elongation of the nano-Ag is about 3.58%. The strain hardening curve of the nano-Ag can be divided into three stages in a lg-lg plot except the yielding stage and the softening stage which appears when the true-strain is larger than 3.35%.Thus,there are 3 strain hardening exponents n₁,n₂ and n₃ which are 0.257,0.199 and 0.110,respectively. The compressive yield stress of the nano-Ag is in the range of 200-280MPa, which is 4 to 5 times of that for cast silver. The compressive strength of the nano-Ag enhances with increasing strain rate, while it is not sensitive to the height-diameter ratio of the compressive specimens. In compression process the nano-Ag did not fail even under thecompressive ratio of ⁶8%.Synthesis, microvoids and microhardness of nanocrystalline metal tungsten (nano-W) were also studied in this thesis. The mam results are summarized as follows:The bulk nanocrystalline samples of the refractory metal tungsten (nano-W) , with a a -W phase and an average grain size of 34nm, were synthesized by a gas-solid reaction process and subsequent a vacuum hot pressing technique under the pressure of lGPa and temperature of 570 °C in our home made apparatus. The dimension, density and microhardness of nano-W are 4>10Xlmm, 88.8% of the theoretical density and 4.8GPa, respectively. The density and microhardness of nano-W enhance quickly with increasing hot-pressing pressure, temperature, and pressing time.