Effects Of Room Temperature Rolling On Microstructure And Properties Of Metallic Glasses

Metallic glass has become one of the most attractive fields in the research of advanced materials. The main reason is that it has a wide array of unique properties including physical, chemical and mechanical, etc. The plastic deformation of metallic glasses at temperatures below the glass transition temperature and high strain rates is known to be inhomegeneous and highly localized to shear bands. A shear band, as a highly deformed region, has more free volume, i.e. a lower atomic density compared with the undeformed regions. Hence inhomogeneously distributed more free volume is introduced into metallic glasses endured inhomogeneous plastic deformation. The introduction of shear bands with more free volume inside will undoubtedly affect the microstructure and related properties of metallic glasses endured inhomogeneous deformation. On one hand, accompanied by the formation of shear bands, the free volume content is enhanced for metallic glasses endured inhomogeneous deformation. As the free volume plays an important role in characterizing the microstructure and in understanding the relationship between the structures and properties of metallic glasses, numerous attempts have been made to use differential scanning calorimetry (DSC) to qualitatively characterize the average free volume changes for metallic glasses endured inhomogeneous deformation. The quantitative determination of the absolute free volume value based on DSC experiments for a metallic glasses undergone inhomogeneous deformation has been seldom developed so far. The introduction of shear bands also affects the properties of metallic glasses endured inhomogeneous. For example, it is found that the room temperature plasticity can be improved due to the introduction of multiple shear bands. In addition, the strength and hardness are also found to change for metallic glasses after inhomogeneous deformation and the reason is also related to the formation and intersection of multiple shear bands. Although the possible mechanism for shear bands to affect the correlated mechanical properties of metallic glasses is still unclear and needs further investigation, the shear band evolution including the shear band density as well as their distribution etc has attracted much scientific interest. Furthermore, the minimum shear band spacing observed so far is usually in the range of around a few hundred nanometers to a few microns and this is usually identified by scanning electron microscopy (SEM). Higher density of shear bands with spacing of tens of nanometers has seldom been observed, especially by conventional transmission electron microscopy (TEM). In addition, the formation of shear bands with enhanced free volume enhances the disorder and the atomic diffusion ability of the materials. The re-ordering within shear bands caused by subsequent annealing may change the local atomic arrangement and thus probably affect the thermal stability as well as the crystallization behavior for metallic glasses endured inhomogeneous deformation. However, the effects of inhomogeneous deformation on the thermal stability and the crystallization behavior for metallic glasses have not been systematically studied so far. Systemic experimental investigation of the thermal stability evolution as well as the crystallization behavior of a metallic glass after inhomogeneous deformation is still necessary. Moreover, it is found that minor alloying addition may affect the microstructure and thus improve the glass forming ability, the thermal stability or the mechanical property of metallic glasses. Hence the effect of minor alloying addition on the microstructure and the related properties has received much attention. Some empirical explanations have been pointed out to investigate the change of the microstructure and the related properties. However, these empirical explanations cannot predict the change of the related properties of metallic glasses with the amount of element additions. Therefore, the experimental investigations on the effects of minor alloying additions on the microstructure and related properties are necessary.The main contents of this dissertation involve the following aspects:(1) Based on the free volume model and the relationship between the free volume change and the enthalpy variation during heating at a constant heating rate, an approach for the quantitative determination of the average free volume in metallic glasses is developed based on the measured enthalpy change during constant heating rate DSC measurements and the equilibrium free volume at the onset temperature of glass transition. The free volume of the as-cast and annealed Pd4oNi4oP2o BMG is quantitatively determined with this method.(2) By room temperature rolling deformation at strain rates of～4.0×10-1 and 4.0×10-2s-1, the strain as high as 99% has been achieved for the Pd40Ni40P20 BMG. Neither phase separation nor crystallization occurs in the RT-rolled Pd4oNi4oP2o BMG The degree of the short range order of atomic arrangements decreases with increasing strain. The shear band density increases with increasing strain. For the sample endured 99% deformation at 4.0×10-1 s-1, the average shear band spacing is about 31 nm; compared with the sample deformed at 4.0×10-2 s-1 and endured the same deformation degree, a higher density of shear bands is introduced into the sample deformed at 4.0×10-1 s-1. Associated with the increase of the shear band density, the quantified average free volume content increases monotonously as the strain increases during the room temperature rolling deformation. Compared with the free volume value of about 0.0559 for the as-cast sample, the value increases to 0.0750 for the sample endured 99% rolling deformation, i.e. as high as about 34% more free volume was created during the inhomogeneous plastic deformation;compared with the sample deformed at 4.0×10-2 s-1 and endured the same deformation degree, more amount of free volume was created into the sample deformed at a higher strain rate. The crystallization peak temperature and crystallization activation energy decreases during the deformation, i.e. the thermal stability deteriorates during the deformation. The deterioration of the thermal stability may be caused by the reduction of the viscosity and the increase of the average Gibbs free energy influenced by the increase of the free volume. The microhardness increases with increasing strain and the sample deformed at a higher strain rate has a higher microhardness value, this may be caused by the intersection of the shear bands.(3) High density of shear bands with spacings less than 100 nm are introduced in the Zr55Cu4oAl5 metallic glass ribbons endured room temperature rolling deformation. Ordered clusters of 3-5 nm in size form in the shear band regions for the 60% room-temperature rolled sample. The inhomogeneous deformation and subsequent annealing treatment at a temperature near the glass transition change the crystallization behavior from one single step which corresponds to the formation of Cu8Zr3 and Al3Zr eutectic phases, to a double step process which corresponds to the formation of the Cu8Zr3 and Al3Zr eutectic phases and the formation of the Cu10Zr7 phase, respectively.(4) We studied the effects of minor Fe addition on the thermal stability and hardness of (Zr46Cu39.2Ag7.8Al7)100-yFey(y=0-7) BMGs. Both the supercooled liquid region width (ΔTx) and crystallization temperature of the (Zr46Cu39.2Ag78Al7)100-yFey(y=0-7) BMGs increase with Fe addition; and the largestΔTx of 114 K is achieved at y=4. The average free volume decreases with increasing Fe content. The hardness increases with increasing Fe content, which may be resulted from the decrease in free volume and the precipitation of nanocrystalline phases from glassy matrix due to Fe addition.