Effects of external stress aging on morphology and precipitation behavior of θ'' phase in Al-Cu alloy

The exposure of Al-5Cu alloy to an external stress with normal aging was carried out. The effects of external stress-aging on the morphology and precipitation behavior of θ＂ phase were investigated by transmission electron microscopy （TEM）, differential scanning calorimetry （DSC） and first principle calculation. The size of the θ＂ phase precipitated plates in stress-aging （453 K, 6 h, 50 MPa） is 19.83 nm, which is smaller than that of those present （28.79 nm） in stress-flee aging （453 K, 6 h）. The precipitation process of θ＂ phase is accelerated by loading external stress aging according to the analysis of DSC results. The apparent activation energy for the external stress-aging is 10% lower than the stress-free one. The first principle calculation results show that the external stress makes a decrease of 6% in the interface energy. The effects of the stress on aging process of the alloy are discussed on the basis of the classical theory. The external stress changes the morphology and precipitation behavior of θ＂ phase because the critical nucleation energy is decreased by 19% under stress aging.

铅蓄电池负极有害气体的探讨

牵引车用铅蓄电池和特殊用途蓄电池由于使用了含锑等元素的板栅,导致充电末期水的分解,析出氢气、锑化氢和砷化氢等有毒有害的气体。本文重点对锑化氢的析出原理、析出动力学进行探讨,分析产生的原因,并提出相应的措施,以提高铅蓄电池的使用性能及使用者的健康及安全。

In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary(GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain(UFG) interiors of 7075 Al alloy by equal-channel-angular(ECAP) processing at 250 ℃ for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075 Al alloy exhibits superior mechanical properties(yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050 Al counterpart(yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075 Al and 1050 Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075 Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.