From powders to bulk metallic glass composites
Résumé
One way to adjust the properties of materials is by changing its microstructure. This concept is not easily applicable on bulk metallic glasses (BMGs), because they do not consist of grains or different phases and so their microstructure is very homogeneous. One obvious way to integrate inhomogeneities is to produce bulk metallic glass composites (BMGCs). Here we show how to generate BMGCs via high-pressure torsion (HPT) starting from powders (amorphous Zr-MG and crystalline Cu). Using this approach, the composition can be varied and by changing the applied shear strains, the refinement of the microstructure is adjustable. This process permits to produce amorphous/crystalline composites where the scale of the phases can be varied from the micro-to the nanometer regime. Even mixing of the two phases and the generation of new metallic glasses can be achieved. The refinement of microstructure increases the hardness and a hardness higher than the initial BMG can be obtained. Bulk metallic glasses (BMGs) have advantages over crystalline metals including high hardness, high elastic energy storage and high corrosion resistivity, but have also some major drawbacks. BMGs are relatively brittle and especially show poor ductility in tensile testing 1. Changing the microstructure is a common way to tune properties of materials. It was shown for BMGs that changing the local short range order through rejuvenation by thermal or mechanical cycling will influence mechanical properties 2–4. Another way is to produce composites containing an additional amorphous or crystalline phase 5. One prominent route is to partly crystallize the amorphous sample by either choosing a slower cooling rate, another composition or reheating the BMG up to the crystallization temperature 6–11. Other routes are casting the BMG with crystalline parts as springs, tubes, particles or wires 12–17 or using warm extrusion of a mixture of powders 18. Drawbacks of these strategies are the inhomogeneous micro-structure and sometimes the brittle crystalline phase 19, 20. Another idea is to use powder metallurgy and sintering to fabricate bulk metallic glass composites (BMGCs) and despite problems with porosity, promising results have been published 21–23. For producing crystalline composites, also severe plastic deformation (SPD) has been used in the past 24, 25. Even though many different SPD techniques (equal channel angular extrusion, accumulative roll bonding and many others) have been developed, high pressure torsion (HPT) is used for this work because it has some major advantages especially for research. The applied strain can be easily varied by changing the number of rotations; many commonly brittle materials can be deformed due to its high nearly hydrostatic pressure and even powders are possible as initial material. By using powders, a wider range of compositions become feasible compared to conventional casting. Super saturated solid solutions are producible and unfavorable phases can be avoided 26–30. HPT was already used on MGs, on one hand to change the properties of BMGs by structural reju-venation 3, 4, 31–34 and on the other hand to fabricate BMGs and BMGCs 35–44. Zr-MG powder was compacted and deformed via HPT beforehand. It could be shown that fully dense and amorphous specimens without cracks and pores can be fabricated with sufficient applied strains and no crystallization occurred during the HPT process 45. The aim of this study is to show that new types of BMGCs can be produced via SPD. Questions addressed are: what are the obtainable limits of the metal-metallic glass composites in scale and content, and how far can we extend the field of bulk metallic glasses. The initial materials are powders (Zr-MG and crystalline Cu) that were mixed and then consolidated, welded together and refined by HPT. Four different compositions (Zr-MG Xwt% Cu, X = 20, 40, 60, 80) were produced as well as single phase Zr-MG samples as reference. To investigate the influence of the degree of deformation and the ratio of the two phases on the evolution of the microstructure and mechanical properties, scanning electron microscopy (SEM), X-ray diffraction (XRD) and hardness measurements were used.