Ni-free Ti-based Bulk Metallic Glasses: Glass Forming Ability and Mechanical Behavior

Metallic glasses are amorphous alloys that do not possess long-range structural order in contrast to crystalline alloys. Ni-free Ti-based bulk metallic glasses (BMGs) have potential for biomedical applications due to their attractive properties such as high strength, good corrosion resistance and excellent micro-formability, which cannot be obtained for conventional crystalline alloys. In this PhD thesis, Ni-free Ti-based BMGs, i.e. Ti40Zr10Cu34Pd14Sn2 and Ti40Zr10Cu36-xPd14Inx (x = 0, 2, 4, 6, 8), were prepared in the shape of rods by suction casting. Both alloy classes were systematically characterized in terms of glass forming ability, thermal stability, phase formation and mechanical properties. The largest diameter obtained in the fully glassy state for Ti40Zr10Cu34Pd14Sn2 alloy is 3 mm and for Ti40Zr10Cu36-xPd14Inx (x = 2, 4, 6, 8) alloys is 2 mm. Base alloy (Ti40Zr10Cu36Pd14) contains some crystalline phase(s) in the glassy matrix for a 2 mm diameter rod. The structural transformations of Ti40Zr10Cu34Pd14Sn2 BMG upon heating were thoroughly analyzed by utilizing different combination of methods. Firstly, we used differential scanning calorimetry (DSC), X-Ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate the first crystallization event. The main products of the first crystallization are possibly -(Ti, Zr) and Cu3Ti (orthorhombic) phases. Secondly, we employed in situ x-ray diffraction in transmission mode using synchrotron beam to deeply study the thermally-induced structural changes like relaxation, glass transition and crystallization. Since the first peak in the diffraction patterns reflects the structure of the glassy phase on the medium-range scale, the position, width and intensity of this peak in diffraction patterns are fitted through Voigt function below 800 K. All the peak position, width and intensity values show a nearly linear increase with increasing temperature to the onset temperature of structural relaxation, Tr = 510 K. However, these values start to deviate from the linear behavior between Tr and glass transition temperature Tg. The changes in the free volume, which was arrested during rapid quenching of the BMG, and the coefficient of volumetric thermal expansion prove that the aforementioned phenomenon is closely related to the structural relaxation. Above 800 K, three crystallization events are detected and the first exothermic event is due to the formation of metastable nanocrystals. For the Ti40Zr10Cu34Pd14Sn2 alloy, 2 mm diameter rods exhibit the best combination of mechanical properties (e.g. large plastic strain and high yield strength) among all the diameters (ø2, ø3 and ø4 mm) under the room-temperature compression tests. With the aim to improve its room-temperature mechanical properties, the processes of pre-annealing and cold rolling have been applied for the 2 mm diameter rods. Annealed and quenched specimens below Tg and in the supercooled liquid region (between Tg and onset crystallization temperature Tx) do not lead to the enhancement of the plasticity compared to as-cast alloys due to annihilation of excess free volume and crystallization. Cold rolling can effectively improve the plasticity of this BMG by inducing structural heterogeneities. Rolled samples up to a thickness reduction of 15% result in the largest plasticity of 5.7%. Low yield strength and visible work hardening ability are observed in the both 10%-rolled and 15%-rolled samples. The deformation behavior of Ti40Zr10Cu34Pd14Sn2 BMG at the elevated temperatures slightly below Tg and in the supercooled liquid region has been investigated. The stress-strain relations for this BMG over a broad range of temperatures (298 ~716 K) and strain rates (10-5 to 10-3 s-1) were established in uniaxial compression. Under compression tests at the highest test temperature of 716 K, the Ti-based BMG partially crystallizes and low strain rates can lead to the formation of larger volume fractions of crystals. In order to further improve the plasticity of Ti-Zr-Cu-Pd BMGs and simultaneously reduce the content of Cu (considering harmful element for the human body), the Ti40Zr10Cu36-xPd14Inx (x = 2, 4, 6, 8) BMGs have been newly developed with different short- or medium-range order in the structure. The compressive global strain of Ti40Zr10Cu36-xPd14Inx (x = 0, 2, 4, 6, 8) can be significantly improved from 4.5% for the In-free alloy to 10.2% for x = 4. However, a further increase of the indium content to 8 at.% results in a decrease of the plasticity. Among all the monolithic Ni-free Ti-based BMGs reported so far, the novel Ti40Zr10Cu32Pd14In4 BMG shows the largest plasticity. Inspired by the dislocation concept in crystalline materials, we propose a strategy for the design of ductile BMGs through minor substitution using relatively large atoms, which make the bonding nature become more metallic and with it less shear resistant. Such a locally modified structure results in topological heterogeneity, which appears to be crucial for achieving enhanced plasticity. This strategy is verified for Ti-Zr-Cu-Pd glassy alloys, in which Cu was replaced by In, and seems to be extendable to other BMG systems. The atomic-scale heterogeneity in BMGs is somewhat analog to defects in crystalline alloys and helps to improve the overall plasticity of BMGs