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BULK METALLIC GLASSES: FABRICATION
AND CHARACTERIZATION
At the end of this lab, you will be able to
know:
- (^) Introduction to BMG
- (^) Mechanism of BMG Formation
- (^) Deformation Mechanism/Micro-
mechanisms of Mechanical properties
- (^) Ways of Fabrication
- (^) Applications in various fields
BULK METALLIC GLASSES: FABRICATION
AND CHARACTERIZATION
Muhammad Mudasser
Khan
**1. Deep eutectic points in binary and ternary alloy systems
- Multi-component systems (at least >3 elements)** •. (^) Why multi-component systems have excellent glass **_forming abilities?
- Increased atomic packing density, as there are more atoms of the ‘right’_** size to fill free space in the randomly packed glass structure. (Denser **_packing leads to lower energy and thereby higher stability.)
- The ‘confusion principle’, i.e., the more elements involved, the lower the_** chance that the alloy can select viable crystal structures, and hence the greater the chance of formation of amorphous structure
Mechanisms of BMG Formation
What would you suggest for a system so that it could transform to amorphous state upon cooling?
- (^) Inoue suggested a comprehensive set of empirical rules for the
formation of BMGs
1. Alloys should be multicomponent systems consisting of more
than three elements
2. Significant atomic size mismatch
3. Main constituent elements should have negative heats of mixing
EMPIRICAL RULE
Inoue’s Rules for the formation of BMG
Illustrations of portions of a single cluster unit cell in the dense cluster packing model. (a) A two-dimensional representation of a dense cluster packing structure in the (100) plane of clusters illustrating the features of interpenetrating clusters and efficient atomic packing around each solute atom. (b) A portion of a cluster unit cell of a <12-10-9> model system representing a Zr-(Al,Ti)-(Cu,Ni)-Be alloy. Zirconium solvent spheres (pink) form relaxed icosahedra around each solute atom. There is no orientational order among the icosahedral clusters.
- (^) Efficiently packed solute-centered atomic clusters are retained as local structural units. An extended structure is produced by idealizing these clusters as spheres and efficiently packing these sphere-like clusters in fcc and hcp configurations to fill three-dimensional space.
- (^) Because of internal strains and topological frustration, the order of the cluster-forming solutes cannot extend beyond a few cluster diameters, and thus the disordered nature of metallic glasses can be retained beyond the nanoscale. STRUCTURAL ORIGINS OF METALLIC GLASS FORMATION
The plastic deformation of metallic glasses at room temperature is carried out by highly localized shear bands. Historically, several theories have been developed to describe the heterogeneous plasticity of metallic glasses.
These models are mainly based on two micromechanisms:
1. Deformation-induced dilatation or free volume mechanism , and
2. Cooperative shearing of atomic clusters termed shear
transformation zones (STZs)
Shear transformation theory and shear
transformation zones
BMGs have a wide supercooled liquid range when heated from room temperature. Within this temperature window, BMGs transform into a viscous supercooled liquid with significant softening and, thus, can be shaped under very small applied forces. This unique property of glassy materials endows BMGs with extraordinary formability and superplasticity Bulk metallic glasses are quite stable in the supercooled state and, after thermal processing, can be slowly cooled back to the strong glassy state. This outstanding thermal formability of BMGs has recently been exploited for applications in nanotechnology and MEMS Nanotechnology and MEMS
Formability and Superplasticity of BMGs
- (^) Utilizing the excellent thermal formability of BMGs, supercooled liquid fabrication provides an alternative and economic approach for the fabrication of micro- and nano-sized metallic parts and surface patterns for MEMS and micro- and nano-machines.
- (^) By applying plastic processing techniques, micro- or nano-patterns can be easily transferred to BMG surfaces from stiff dies and molds by thermal imprinting
- (^) The development of such nano-sized BMG structures may lead to a host of applications of BMGs in nanotechnology and bionanotechnology.
NANO-PATTERNS FABRICATION
- (^) The advantages of BMGs compared with conventional crystalline metals and alloys include **_1. Superior strength
- High elasticity
- Excellent wear and corrosion resistance_** •. (^) These are attributable to the lack of grain boundaries and crystal defects that usually lead to weakening of material strength, intergranular corrosion and stress- corrosion cracking in biological environments. •. (^) In vitro and in vivo experiments have demonstrated that BMGs are, in general, nontoxic to cells and compatible with cell growth and tissue function •. (^) BMGs possess attractive properties for load-bearing biomedical-implant applications •. (^) More recently, magnesium-based BMGs have been developed for application as biodegradable implants
Biomedical Applications
Implants, Stents etc.
- (^) BMGs have been used as one of the best soft magnetic materials for a wide range of applications
- (^) Combining high strength and low density, BMGs have also been exploited as light-weight structural materials for applications in which weight and strength are critical.
- (^) Dies
- (^) Golf sticks
Other Applications
