Evolutions Of Structure And Mechanical Properties Zr-based Bulk Metallic Glasses During Plastic Deformation

Characterized by long-range disorder and short-range order, amorphous alloys possess many peculiar performances in mechanical, physical and chemical aspects. Therefore, amorphous alloys are regarded to be greatly potential in application. However, as being thermodynamically metastable, their structures may change when subjected to heating and/or loading. In order to properly use amorphous alloys and improve their performances, it is of crucial importance to understand the structural change.This dissertation investigated the evolutions of the structures and mechanical properties of a series of Zr-based amorphous alloys during cold rolling deformation using X-ray diffractometer (XRD), differential scanning calorimeter (DSC), high-resolution transmission electron microscopy (HRTEM), as well as digital microhardness tester. Besides, pre-annealing of the as-cast amorphous alloy prior to rolling was performed to further investigate the relationship between thermal and mechanical stabilities of the Zr-based alloys. The obtained main results are stated as follows:Thickness reduction up to 96% can be achieved during rolling the Zr₆₀Cu₁₀Al₁₀ sample at a strain rate of 5.0×10^-2 s^-1. There is neither phase separation nor nanocrystallization to occur in the deformed sample. But calculations of the coordination number (N), first coordination radius (r1) and correlation radius (rc) from the radial pair distribution function RDF(r) and pair correlation function g(r) indicate that the short- and medium-range orders change due to the deformation. As a result, the crystallization during the subsequent annealing is accelerated.The thermal stability of the ternary Zr₆₀Cu₁₀Al₁₀ alloy decreases as a fourth element Pd is added. Nanocrystallization occurs when this quaternary alloy is rolled to the maximum thickness reduction of 96% at the strain rate of 5.0×10-2 s-1. The deformation induced nanocrystals are identified to be the Zr2(Cu,Pd) phase that can be induced by annealing as well. The appearance of these nanocrystals leads to an increase of microhardness in Zr₆₀Cu₂₀Al₁₀Pd10.When the elements of Ni and Ti are added to Zr₆₀Cu₁₀Al₁₀, rolling of the formed Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti5 amorphous alloy at different strain rates at room temperature leads to quite different microstructures and microhardnesses. There is no phase transformation to occur during rolling at a lower strain rate of 5.0×10^-2 s^-1. The rolling-introduced shear bands makes the material softer. However, at a higher strain rate of 5.0 s-1, phase separation and nanocrystallization take place inside and near the shear bands, where the material has been subjected to a much heavier deformation compared to the regions far from shear bands. Additionally, although the phase separation and nanocrystallization, to some extent, might bring about a reinforcement of the material, the measured microhardness is found still to decrease as the deformation proceeds, indicating that the softening due to the introduction of shear bands dominates the microhardness evolution.Pre-annealing of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti5 amorphous alloy at 680 K for 0.5 h leads to phase separation. If the annealing duration is prolonged to 1.5 h, nanocrystallization accompanied by the formation of metastable f.c.c. Zr-Cu phase occurs. Rolling of both types of pre-annealed samples triggers nanocrystallization. For the sample pre-annealed for 1.5 h, the volume fraction of nanocrystals increases as the rolling proceeds, whereas the Zr-Cu phase vanishes. This phenomenon indicates that the rolling transforms the metastable Zr-Cu phase into a stable nanocrystalline one. The fact that the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti5 samples pre-annealed for 0.5 and 1.5 h do not break during rolling even if the thickness reduction reaches 96% demonstrates that the appearance of phase separation and nanocrystallization does not decrease the ductility of the material. However, the brittle fracture occurs when the 3 h annealed sample is rolled up to 10% thickness reduction. This can be explained by the remarkable increase in the volume fraction of nanocrystals after the prolonged pre-annealing. Pre-annealing for different time and then rolling of the amorphous alloy prove that a critical volume fraction of nanocrystals of 36% controls the ductility, above which it is likely for brittle fracture to occur.The fully crystallized products of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti5 amorphous alloy contain Zr₂Ni, Zr₂Cu, ZrCu and ZrAl phases. Using the Johnson-Mehl-Avrami (JMA) transformation kinetics equation, the Avrami exponents for two crystallization peaks has been obtained. It is revealed that the first crystallization peak corresponds to a nanocrystallization process with an increasing nucleation rate, while the second one reflects the growth of nanocrystals with a gradually decreasing nucleation rate. Both pre-annealing and rolling of the amorphous alloy decrease the activation energies of glass transition and crystallization.