Dynamic Mechanical Properties And Adiabatic Shear Behavior Of Zr And Zr Alloys

Adiabatic shear failure was the common phenomenon when materials were loaded at high strain rate and it had important theoretical and practical significance to study of it. In the present study, dynamic loading experiments were applied to Zr and Zr alloy by split Hopkinson pressure bar(SHPB) and rolling mills. The effect of composition and microstructure on dynamic deformation behavior and adiabatic shear sensitivity of the alloys was studied. Failure process of the alloy cuased by shear band as well as the microstructure evolution in the shear band was observed and discussed.Annealed and as-cast pure zirconium was loaded by SHPB. The influence of microstructure and loading conditions on the evolution process of the shear band was analyzed. Johnson-cook constitutive model was selected to describe the dynamic behavior of the pure Zr. For the first time, we established a model for calculating the temperature within the shear band, and thermal diffusion effect was considered in this model. Based on TEM observation results and calculation results of the temperature rise within shear band, the microstructure evolution process within shear band was clearly described. First, elongated dislocation cells and subgrains were formed at the initial stage of shear deformation. Second, the fine recrystallized grains were formed by the the rotation of subgrains’ boundaries in the subsequent deformation process. At last, the grain size was not obvious increased at the following cooling stage. We proposed a phenomenological model for the nucleation and evolution of shear band based on the results obtained by molecular dynamics simulation and the correlation analysis of the dynamic constitutive equation. Firstly, localized shear deformation initiated at the regions of stress concentration or material defects during high strain rate deformation, then the shear band developed rapidly and penetrated the sample along the direction of maximum resolved shear stress. Finaly, the width of the shear band increased gradually during subsequent deformation process.In order to improve the impact resistance of the materials, Zr40Ti60 alloy was prepared. Dynamic mechanical properties of as-cast and hot-rolled Zr40Ti60 alloy were tested using SHPB. The alloy had the relatively higher strength due to a lot of acicular a phase in it, but the dynamic plasticity of the alloy was very low, leading to the adiabatic shearing sensitivity of this alloy relatively high. Based on experimental results, the failure process of the material caused by shear band could be described as follows. Firstly, microvoids nucleated at the center of the shear band, secondly, these microvoids coalesced and generated micro-cracks along the shear band path, and fnaly, the micro-cracks developed into full blown crack and caused fragmentation of the material.Zr34Ti51Al10V5 alloy was prepared and its dynamic mechanical properties were tested by SHPB. The adiabatic shear sensitivity of the alloy as forged and solution-aged was very high because these samples contain a lot of acicular a phase, which led to the dynamic plasticity lower than 3%. The samples after solution treatment were composed of pure β phase and they have the preferable match of dynamic strength and dynamic plasticity. The alloy after solution treatment at 750°C had the lowest adiabatic shear sensitivity. The dynamic strength and dynamic plasticity was 1700 MPa and 23%, respectively.The effect of rolling parameters on formation and evolution of the shear band were studied for as-cast pure Zr and as cast Zr40Ti60 alloy. The results showed that shear band was easily produced during single pass rolling process, and the width of the shear band increased with the increase of the thickness reduction ratio. The temperature within shear band was estimated using the model proposed in this paper. The results showed the temperature within the shear band was only slightly higher than the surrounding matrix because of the strong influence of thermal diffusion effect. TEM observation results showed that the shear band was composed of elongated dislocation cells and substructures, and there were no equiaxed grains formed in it. The reason for this was that the temperature within the shear band caused by deformation was lower than dynamic recrystallization temperature.