T. C. Hufnagel

Metallic glass matrix composite with precipitated ductile reinforcement

We report a composite material consisting of precipitated micron-scale Ta-rich solid solution particles distributed in a bulk metallic glass matrix. The reinforcing ductile particles are precipitated during melting of the master alloy of glass-forming (Zr70Ni10Cu20)82Ta8Al10, by using previously prepared metastable Zr–Ta solid solution binary ingots. Upon cooling from the melt, the matrix undergoes a glass transition to produce an amorphous phase while the particles of precipitated Ta solid solution are distributed in the glass matrix. The resulting material not only shows high strength (∼2.1 GPa), but also has dramatically enhanced plastic strain to failure in uniaxial compression relative to single-phase bulk metallic glasses. The composite also displays limited tensile ductility.

Free volume coalescence and void formation in shear bands in metallic glass

We have investigated the possibility of void nucleation from the coalescence of excess free volume generated in shear bands during deformation of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass. Excess free volume in a shear band results in excess free energy relative to a relaxed glass with less free volume. To calculate the free energy of the material in a shear band with excess free volume, we model it as having the same structure as a glass solidified at an elevated glass transition temperature, which we call the fictive temperature. This excess free energy can be correlated with a free volume chemical potential that provides a driving force for void nucleation. The results of this modeling indicate that any free volume generated in the shear band during deformation is unstable, with the consequence that voids are predicted to form spontaneously from the coalescence of free volume. These voids are then expected to coarsen. Under tension, void growth and linkage would be facilitated by a tensile...

Nanometre-scale defects in shear bands in a metallic glass

Abstract We have compared the structure of shear bands with that of undeformed regions of a metallic glass. Using quantitative high-resolution electron microscopy, we observed void-like defects, approximately 1 nm in diameter and at a concentration of one in 100nm3, which are generated as a result of plastic deformation in the shear bands. These defects may result from the coalescence of excess free volume upon cessation of the flow. By comparing the free energy of the shear band containing uniformly distributed free volume with that of the relaxed shear band with voids present, we show that the coalescence is thermo-dynamically possible.

Deformation and Failure of Zr 57 Ti 5 Cu 20 Ni 8 Al 10 Bulk Metallic Glass Under Quasi-static and Dynamic Compression

We have examined the mechanical behavior of Zr 5 7 Ti 5 Cu 2 0 Ni 8 Al 1 0 bulk metallic glass under uniaxial compression at strain rates from 10 - 4 to 3 x 10 3 s - 1 . The failure stress decreases with increasing strain rate, and shear-band formation remains the dominant deformation mechanism. A consideration of basic properties of adiabatic shear bands makes it appear unlikely that shear bands formed under quasi-static loading are adiabatic; in the dynamic case, the time scales of deformation and thermal conduction are similar, indicating that a more sophisticated calculation is required. In the dynamic tests, however, high-speed cinematography reveals evidence that the mechanism of failure involves an adiabatic component.

Relation between short-range order and crystallization behavior in Zr-based amorphous alloys

We have examined the effect of Ti and cooling rate on the crystallization of Zr62−xTixCu20Ni8Al10(0⩽x⩽10) amorphous alloys. Ti stabilizes an icosahedral phase in Zr62−xTixCu20Ni8Al10(0⩽x⩽10) alloys. Without Ti (x=0), crystallization produces cubic and tetragonal intermetallic phases, and the crystallization temperature shows no dependence on the cooling rate at which the amorphous alloy was produced. The alloys containing Ti (3⩽x⩽10) precipitate an icosahedral quasicrystalline phase upon annealing, and show a significant reduction of crystallization temperatures with decreasing cooling rates of casting. We propose that the undercooled melts and amorphous alloys have icosahedral short-range order. The degree of short-range order or medium-range order in the amorphous alloys increases with decreasing cooling rate. Crystallization is easier when the precipitating phase resembles the short-range order of the amorphous solid. Therefore, the crystallization temperature is reduced when the precipitates are icosa...

Controlling shear band behavior in metallic glasses through microstructural design

Plastic deformation in metallic glasses is governed by the initiation and propagation of shear bands. The successful use of bulk metallic glasses in structural applications will depend on controlling these processes to improve ductility and toughness. In Zr–Cu–Ni–Al metallic glasses, the addition of Ta can influence the structure of the material and hence the shear band behavior in two ways. At low Ta contents (<4 at.%), the material is amorphous but has enhanced order over length scales of 5–15 A Higher levels of Ta result in the precipitation of bcc Ta-rich solid solution particles in a metallic glass matrix. Under uniaxial compression, both of these materials show greater apparent plastic strain to failure than the glass without Ta. This appears to be the result of the influence of the structure on the initiation and propagation of shear bands in the amorphous matrix.

Mechanical properties of single electrospun drug-encapsulated nanofibres

The mechanical and structural properties of a surface play an important role in determining the morphology of attached cells, and ultimately their cellular functions. As such, mechanical and structural integrity are important design parameters for a tissue scaffold. Electrospun fibrous meshes are widely used in tissue engineering. When in contact with electrospun scaffolds, cells see the individual micro- or nanofibres as their immediate microenvironment. In this study, tensile testing of single electrospun nanofibres composed of poly(ε-caprolactone) (PCL), and its copolymer, poly(caprolactone-co-ethyl ethylene phosphate) (PCLEEP), revealed a size effect in the Youngs modulus, E, and tensile strength, σ(T). Both strength and stiffness increase as the fibre diameter decreases from bulk (∼5 μm) into the nanometre region (200-300 nm). In particular, E and σ(T) of individual PCL nanofibres were at least two-fold and an order of magnitude higher than that of PCL film, respectively. PCL films were observed to have more pronounced crystallographic texture than the nanofibres; however no difference in crystalline fraction, perfection, or texture was detected among the various fibres. When drugs were encapsulated into single PCLEEP fibres, mechanical properties were enhanced with 1-20 wt% of loaded retinoic acid, but weakened by 10-20 wt% of encapsulated bovine serum albumin. This understanding of the effect of size and drug and protein encapsulation on the mechanical properties of electrospun fibres may help in the optimization of tissue scaffold design that combines biochemical and biomechanical cues for tissue regeneration.

Joining bulk metallic glass using reactive multilayer foils

Abstract We have welded a zirconium-based bulk amorphous alloy using Ni/Al multilayer foils that are capable of producing a self-propagating exothermic reaction. The shear strength of the welded joints increases with both the foil thickness and the pressure that is applied during joining. We measured shear strengths as high as 480 MPa in a compressive single-lap specimen geometry.

Characterization and modeling of a martensitic transformation in a platinum modified diffusion aluminide bond coat for thermal barrier coatings

Abstract Phase transformations in a platinum modified nickel aluminide bond coat were investigated by in situ high temperature X-ray diffraction analysis. Three phases, L1 0 martensite, B2 (β-(Ni,Pt)Al) and L1 2 (γ′-Ni 3 Al), were identified at different temperature ranges. The martensite is stable at temperatures below 620 °C, and the β-phase is stable at elevated temperatures. The reversible transformation, M↔β, is the principal reaction occurring throughout the bond coat layer during thermal cycling. Quantitative measurements indicate that the molar volume of the β-phase is approximately 2% larger than that of the martensite. Finite element simulations incorporating the volume change associated with this transformation indicate that the transformation significantly influences the distribution of stresses and strains in TBC systems. The effect of the martensite on TBC life is sensitive to the transformation temperatures relative to the creep strength of the bond coat.

Microstructural evolution of platinum modified nickel aluminide bond coat during thermal cycling

Abstract Microstructural evolution induced by thermal cycling in a platinum modified diffusion aluminide bond coat was investigated with transmission electron microscopy and elevated temperature X-ray diffraction (XRD). Before thermal cycling, the structure of the as-received bond coat was confirmed to be an ordered B2 phase, but significant lattice strains were found which were associated with the formation of a modulated structure. Thermal cycling resulted in significant changes in the microstructure of the bond coat. The compositional development assisted by chemical diffusion during thermal cycling has been related to the transformation of the bond coat from its original B2 structure to a Ni-rich L1 0 martensite. The L1 0 martensite was found to be stable at temperatures below approximately 600 °C and the B2 parent phase stable at elevated temperatures. Quantitative XRD measurements indicated that the volume of the B2 phase is approximately 2% larger than that of the martensite, which produces a ∼0.7% linear transformation strain.