Solid Phase Transformation And Microstructure In Cw-Cr-Zr-Mg Alloy

The precipitation sequence, precipitate phase structrures and orientation relationships and solute atom clusters and their chemical composition in Cu-Cr-Zr-Mg alloy were studied by high resolution transmission electron microscopy and atom probe tomography(APT). The effects of different aging conditions on the microstructure and properties of the alloy were investigated. The following conclusions can be drawn:(1) The hardness of Cu-Cr-Zr-Mg alloy at 450 ℃ grew rapidly in early aging stage, but the rate of growth slowed down after 4 h. The hardness increasing rate slowed down with the increasing aging time and reached equilibrium after 2h. The conductivity and hardness followed the similar growth rule. The conductivity increased from 51.3% IACS to 83.2 % IACS after 2 h and then slowly increase after4 h.(2)The sequence of decomposition in the Cu-Cr-Zr-Mg alloy during the early stage of aging at 450 ℃ can be summarized as: supersaturated solid solution �' GP zone(fcc Cr-rich phase) �' ordered fcc Cr-rich phase �' ordered bcc Cr-rich phase(B2, or Heusler structure). The sequence of decomposition during aging at 550 ℃ is:supersaturated solid solution �' ordered fcc Cr-rich phase �' ordered bcc Cr-rich phase.(3)APT analysis reveals that significant dispersions of loose solute clusters formed during aging for 5 min at 450 ℃. After the aging for 4 h at 450 ℃, the mean measured composition of solute clusters with a high number density is: 73.57 % Cu,11.48 % Cr, 1.56 % Zr, 7.09 % Fe, 2.16 % Mg, and 3.69 % Si. The partitioning ratio of Cr increases with the aging time, while the ones of Zr, Fe, Mg and Si decreases.(4)“S” curves of the dynamics for phase transformation were well reflected the aging change rate in different between 450 ~ 550 ℃. “S” curves were drawn to describe the aging precipitation. And the Avrami-equation of phase transformationkinetics verified by the integral functions of the reactive mechanism of thermal decomposition of solid solution. It revealed that the transformation during aging at450 ~ 550 ℃ in this alloy was controlled by the three-dimensional diffusion mechanism.