A study on embrittlement and fatigue behavior of ultra-fine grain crystals under corrosive environment

研制了一种获得专利的新型等通道转角挤压（ECAP）模具，并利用该模具制备了超细晶铜。多晶铜材料在第一道次ECAP过程中，在粗晶内形成亚微米级位错胞。在1 kmol/m^3的NaNO 2水溶液中进行低应变速率应力腐蚀试验（SSRT）。从结果来看，超细晶铜的应力腐蚀敏感性远低于商业多晶铜。应力腐蚀开裂优先在晶界处发生，并沿晶界沿着适当地形成门控。另一方面，裂纹在普通晶粒尺寸铜的内部形核。在UFG铜萌生和扩展的裂纹尖端，观察到晶粒长大，并向一定方向生长。在上述水溶液中进行了恒定塑性应变幅的拉压疲劳试验。UFG铜的腐蚀疲劳寿命上级优于商业多晶铜，且没有观察到粗晶铜中常见的持久滑移带。通过ECAP工艺，在合金中形成了{111}、{001}和{112}三种取向的剪切织构，织构宽度约为200 nm<112><110><110>

New dividable die for equal channel angular pressing (ECAP) which was gotten a patent was produced, and using this die ultra fine grain (UFG) copper was manufactured. Dislocation cell that has submicron size was nucleated in the course grain of polycrystalline copper materials by first pass of ECAP process. By each pass under the processes, grain size became small, and after 8 passes, UFG copper having about 200nm in average diameter was obtained.Low strain rate stress corrosion test (SSRT) was performed under 1 kmol/m^3 NaNO_2 aqueous solution. From the results, the SCC susceptibility of UFG copper was extremely lower than that of commercial polycrystalline copper. SCC was preferentially initiated at the grain boundary and proper gated along the grain boundary in UFG copper. On the other hand, the crack was nucleated at the interior of the ordinary grain size copper. At the crack tips initiated and propergated in UFG copper, grain growth, which grows to a certain direction, was observed.Tension-compression fatigue test with constant plastic strain amplitude was performed under the above aqueous solution. Corrosion fatigue life of UFG copper was superior than that of commercial polycrystalline copper, and no persistent slip bands that is usually observed in course grain copper was observed.Grain refining process during ECAP process was discussed, using single crystals having 7 kinds of axis orientation. By ECAP process, 3 kinds of shearing texture which was about 200nm in width and were {111}<112>, {001}<110> and {112}<110> orientation, were formed