The thermodynamic calculation of phase equilibria in the Cu-Ni-Si alloy system was carried out using the CALPHAD method. The calculations show that there are three two-phase areas and two three-phase areas in the Cu-r...The thermodynamic calculation of phase equilibria in the Cu-Ni-Si alloy system was carried out using the CALPHAD method. The calculations show that there are three two-phase areas and two three-phase areas in the Cu-rich parts of the isothermal section of the phase diagram at 300-600 ℃,and the three two-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2,FCC-A1(Cu-rich)+δ-Ni2Si and FCC-A1(Cu-rich)+ε-Ni3Si2,two three-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2+δ-Ni2Si and FCC-A1(Cu-rich)+δ-Ni2Si+ε-Ni3Si2. For this reason,an alloy located in the Cu-rich portion may precipitate γ-Ni5Si2,δ-Ni2Si or ε-Ni3Si2;the proportion of each phase depends on the alloy composition and aging temperature. The transmission electron microscope analysis of the Cu-3.2Ni-0.75Si alloy indicates that the precipitates are mainly δ-Ni2Si with only a few γ-Ni5Si2 phase particles,which agrees well with the thermodynamic calculations of phase equilibria.展开更多
The significant positive green environment influence of magnesium alloy usage in transport could be compromised by catastrophic fast fracture caused by stress corrosion cracking(SCC). Transgranular stress corrosion cr...The significant positive green environment influence of magnesium alloy usage in transport could be compromised by catastrophic fast fracture caused by stress corrosion cracking(SCC). Transgranular stress corrosion cracking(TGSCC) of AZ91 was evaluated using the linearly increasing stress test(LIST) and the constant extension rate test(CERT). The TGSCC threshold stress was 55-75 MPa in distilled water and in 5 g/L NaCl. The TGSCC velocity was 7×10-10-5×10-9 m/s. A delayed hydride-cracking(DHC) model for TGSCC was implemented using a finite element script in MATLAB and the model predictions were compared with experiment. A key outcome is that,during steady state TGSCC propagation,a high dynamic hydrogen concentration is expected to build up behind the crack tip. A number of recommendations are given for preventing SCC of Mg alloys in service. One of the most important recommendations might be that the total stress in service(i.e. the stress from the service loading + the fabrication stress + the residual stress) should be below a threshold level,which,in the absence of other data,could be(conservatively) estimated to be about 50% of the tensile yield strength.展开更多
基金Project(2006DFB53050) supported by the International Science and Technology Cooperation Project of the Science and Technology Ministry of China
文摘The thermodynamic calculation of phase equilibria in the Cu-Ni-Si alloy system was carried out using the CALPHAD method. The calculations show that there are three two-phase areas and two three-phase areas in the Cu-rich parts of the isothermal section of the phase diagram at 300-600 ℃,and the three two-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2,FCC-A1(Cu-rich)+δ-Ni2Si and FCC-A1(Cu-rich)+ε-Ni3Si2,two three-phase areas are FCC-A1(Cu-rich)+γ-Ni5Si2+δ-Ni2Si and FCC-A1(Cu-rich)+δ-Ni2Si+ε-Ni3Si2. For this reason,an alloy located in the Cu-rich portion may precipitate γ-Ni5Si2,δ-Ni2Si or ε-Ni3Si2;the proportion of each phase depends on the alloy composition and aging temperature. The transmission electron microscope analysis of the Cu-3.2Ni-0.75Si alloy indicates that the precipitates are mainly δ-Ni2Si with only a few γ-Ni5Si2 phase particles,which agrees well with the thermodynamic calculations of phase equilibria.
文摘The significant positive green environment influence of magnesium alloy usage in transport could be compromised by catastrophic fast fracture caused by stress corrosion cracking(SCC). Transgranular stress corrosion cracking(TGSCC) of AZ91 was evaluated using the linearly increasing stress test(LIST) and the constant extension rate test(CERT). The TGSCC threshold stress was 55-75 MPa in distilled water and in 5 g/L NaCl. The TGSCC velocity was 7×10-10-5×10-9 m/s. A delayed hydride-cracking(DHC) model for TGSCC was implemented using a finite element script in MATLAB and the model predictions were compared with experiment. A key outcome is that,during steady state TGSCC propagation,a high dynamic hydrogen concentration is expected to build up behind the crack tip. A number of recommendations are given for preventing SCC of Mg alloys in service. One of the most important recommendations might be that the total stress in service(i.e. the stress from the service loading + the fabrication stress + the residual stress) should be below a threshold level,which,in the absence of other data,could be(conservatively) estimated to be about 50% of the tensile yield strength.
