Preparation, Microstructure And Mechanical Properties Of Zn-22wt.%Al Based Alloys And Tubes

Zn-22wt.%Al based alloys (abbreviated as Zn-22A1) are typical high temperature (100-250 ℃) superplastic alloys, which show high elongation, no neck and good formability. The alloys produce less calorific value during deformation at room temperature, and the mechanical behaviours at room temperature are somewhat similar to its high temperature state. These characteristics are comparable to those of toxic lead antimony alloys (Pb-Sb), which have been currently widely used as delay detonator bodies in commercial explosive industry. On the other hand, the Zn-22A1 alloys have also high damping vibration resistance. In order to develop new environmental friendly materials for the applications in the commercial explosive industry and the related fields, in this thesis, based on phase equilibrium principle, the first principles calculation, phonon spectrum calculation and computational thermodynamics methods were employed to study the phase structure, phase transformation and thermodynamics of Zn-Al-M alloys, part of the computational work is involved in this thesis, and then a series of alloys were investigated experimentally. Using different heat treatment and plastic processing technology, the alloy tubes and virtual delay detonator bodies (take Al2O3 as delay explosive) were prepared. Furthermore, the relationship between the alloy microstructure and mechanical properties were studied under different preparation and processing conditions. The mechanisms of high plasticity, low hardness and work-softening were investigated. The main results were obtained as follows.The phonon spectrum of Al-Sc binary system shown that these compounds were structural stable in the view of thermodynamics, and the calculated thermodynamic properties agreed well with the experimental data. The enthalpies of formation calculated by quasi-harmonic approximation combined with the thermal electronic vibration were temperature-dependent, and the slope was about -3.4J/mol/K,-2.3J/mol/K,-0.8J/mol/K and -2.7J/mol/K for B2 AlSc, C15 Al2Sc, B82 AlSc2and Ll2 Al3Sc, respectively. Comparing with the results calculated by harmonic approximation and quasi-harmonic approximation, the lattice and electronic thermal vibration should not be neglected. The enthalpies of formation of AB3-Ll2 structure in Zn-Al binary system were of also temperature-dependence. However, the variable was quite small, for example, the phase transition temperature of α+η to α2 was 558 K by calculating from the thermodynamic database of end-members at ground state (OK), and 560K from temperature involved thermodynamic database, respectively. So the ground state thermodynamic database of end-members was used to predict the site fractions and phase transition of Zn-22A1 alloys. The results shown that the structure of D019 disappeared at 558 K and the structure of Ll2 became a single phase above this temperature, which agreed well with the Zn-Al binary phase diagrams. The Cu and Ag microalloying could increase the phase transition temperature of α+η→α2, but have less effects on the ordering behavior of Zn and Al in the sub lattices.The microstructure and phase analysis by OM, SEM, TEM and XRD of alloy samples under different plastic processing states and heat treatment processes shown that the α+η was stable phase at room temperature. For the tubes obtained by slow speed extrusion (SE)、medium speed extrusion (ME) and fast speed extrusion (FE) and survived different annealing processes, there were two obvious different phenomenons:with the increase of annealing temperature, one is annealing softening and the other is annealing hardening. The annealing softening mainly was attributed to the low temperature annealing releasing the residual stress and dynamic recovery. The annealing hardening mainly was attributed to that the high temperature annealing promote the softer α2 phase decompose into the harder α+η phase and grain grow bigger. There is a critical annealing temperature or quenching in superplastic heat treatment temperature on the hardness. When the queching temperature is below the critical temperature, with the increase of quenching temperature the hardness decreased. Because the SE, ME and FE tubes produced more α2 phase at high temperature and more residual α2 phase at room temperature, with the increase of quenching temperature. While when the queching temperature is beyond he critical temperature, the α2 grain grown bigger with the increase of quenching temperature, and eutectoid structure was bigger at room temperature, so the hardness increased. When fixed the annealing and quenching temperature on the minimum hardness of the SE, ME and FE tubes, with the extension of time, the elongation increased and the tensile strength decreased gradually. After 6h and 9h the mechanical properties keep nearly stable which was attributed to the combined effects of grain size and the α2 phase volume fraction. Under optimized processes, the tubes with the lowest hardness and considerably high plasticity have been drawn to 05.8 mm. The work-softening of alloys were attributed to the dynamic recovery, dynamic recrystallization and grain refinement.