On the physical aspects that control mechanical deformation in bulk metallic glasses

Authors: John D Plummer (University of Sheffield)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: March 2012
Diamond Proposal Number(s): 876

Open Access Open Access

Abstract: In this thesis two analyses are performed to understand the intrinsic ability of bulk metallic glasses ( BMGs ) to deform plastically, based on their elastic properties. In the first of these, a Blackman diagram is utilised, which plots the ratios of second order elastic constants C 12 / C 11 and C 44 / C 11 . Two physically meaningful conditions are represented on such a p lot : 1) proximity to the Born mechanical instability criterion , and 2) deviation from the zero Cauchy relationship ( C 12 = C 44 ), which indicates whether central or non - central forces govern material behaviour. Those alloys with greatest tendency for plastic s train are found closest to the Born condition, which is also found to correspond to BMGs with a large kinetic glass fragility index, m . Additionally, that plastic alloys exhibit the largest positive deviation from the Cauchy condition suggests that non - dir ectional bonding is a feature of deformable BMGs. In the second anaylsis, three isomechanical groups are found to exist when a representative group of 33 BMGs was studied, by plotting the Young’s ( E ) and shear ( G ) moduli versus k b T g / Ω (Botlzmann constant, glass transition temperature and atomic volume respectively). An analysis of covariance reveals that the variation in the gradient between each group on such plot s are statistically meaningful, indicating fundamental differences in both bonding and structure . These are rationalis ed as resulting from the extent of non - directional bonding and the ability for enhanced structure diversity in the potential energy landscapes (PELs) of fragile glass formers, in com parison to strong glass formers. This abili ty for structure variation in the liquid state can be translated to the solid state via the non - ergodicity parameter, α . This ability for an enhancement in the variety of local minima in the PEL of fragile glass formers is further studied by comparing the ability of high, low and intermediate m BMGs to retain a low modulus and low surface hardness during suction casting in copper dies, observed by performing nanoindentation traverses a cross the cross - section of as - cast rods. The most fragile alloy ( Pd 77.5 S i 16.5 Cu 6 ) is found to show the greatest propensity to retain a soft surface, while the effec t is most limited in the strong composition ( Ce 70 Al 10 Cu 10 Ni 10 ) . Finally, the extended x - ray absorption fine structure (EXAFS) method is used to understand the sourc e of the mech a nical homogeneity observed across the as - cast rods, which is found to potentially not be controlled by nearest neighbour s. Instead, cluster rotation may control the variable mechanical properties (including plastic yield), and so length scale s beyond the first atomic shell appear to be critical.

Journal Keywords: metallic glass; deformation

Subject Areas: Materials, Technique Development

Instruments: I18-Microfocus Spectroscopy