R.T. Ott

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu―Zr alloys

We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35.5 liquids and glasses.

Computer simulation and experimental study of elastic properties of amorphous Cu-Zr alloys

Molecular-dynamics simulations were performed to determine the elastic constants of CuxZr100−x (33.3≤x≤64.5) metallic glasses at room temperature. The accuracy of the interatomic potentials used to obtain the model glass structures was tested by comparing to the total structure factors obtained from high-energy synchrotron x-ray diffraction and, more importantly, to acoustic velocities measured from melt spun ribbons. Both the simulated and measured acoustic velocities increased at comparable rates with increasing Cu concentration, but the former underestimated the latter by about 10%. Young’s moduli of the simulated models were determined by combining the ultrasonic data with densities that were obtained from simulations. In addition, the compositional dependence of Poisson’s ratio, shear modulus, and bulk modulus for this series of simulated metallic glasses was determined. Examination of partial-pair correlations deduced from simulated glass structures shows a correlation between higher bulk moduli in ...

Molecular dynamics simulation of diffusion in supercooled Cu–Zr alloys

Molecular dynamics (MD) simulations of diffusion in Cu–Zr alloys in their liquid and supercooled liquid states were performed using a recently developed Finnis–Sinclair many-body interatomic potential. To help assess how well the interatomic potential describes the energetics of the Cu–Zr system, the liquid structure determined by MD simulations was compared with wide-angle X-ray scattering measurements of the liquid structure for a Cu64.5Zr35.5 alloy. Diffusion was examined as a function of composition, pressure and temperature. The simulations reveal that the diffusion exhibits strong compositional dependence, with both species exhibiting minimum diffusivities at ∼70% Cu. Moreover, the MD simulations show that the activation volumes for Zr and Cu atoms exhibit a maximum near 70% Cu. Evidence is obtained that the glass transition temperature also changes strongly with composition, thereby contributing to the diffusion behaviour. The relationship between this minimum in diffusion and the apparent best glass-forming composition in the Cu–Zr system is discussed.

Deformation behavior of an amorphous Cu64.5Zr35.5 alloy: A combined computer simulation and experimental study

Molecular dynamics (MD) simulations were performed to examine the temperature-dependent elastic properties and high-temperature deformation behavior of a Cu64.5Zr35.5 amorphous alloy. From the simulations we find that the elastic constants of the amorphous solid and supercooled liquid exhibit an approximately linear temperature dependence. The predicted temperature dependence of the Young’s modulus for the amorphous solid obtained from the MD simulations is in good agreement with experimental measurements using dynamic mechanical analysis. Furthermore, the high-temperature plastic deformation behavior determined by MD simulations is qualitatively in good agreement with results from plastic deformation experiments performed on 1 mm diameter Cu64.5Zr35.5 metallic glass rods at 698 K. Notably, the MD simulations reveal that the flow softening regime of the stress-strain curve corresponds to an increase in the free volume in the atomic structure. Moreover, the simulations indicate that the atomic mobility sig...

Imprinting bulk amorphous alloy at room temperature

We present investigations on the plastic deformation behavior of a brittle bulk amorphous alloy by simple uniaxial compressive loading at room temperature. A patterning is possible by cold-plastic forming of the typically brittle Hf-based bulk amorphous alloy through controlling homogenous flow without the need for thermal energy or shaping at elevated temperatures. The experimental evidence suggests that there is an inconsistency between macroscopic plasticity and deformability of an amorphous alloy. Moreover, imprinting of specific geometrical features on Cu foil and Zr-based metallic glass is represented by using the patterned bulk amorphous alloy as a die. These results demonstrate the ability of amorphous alloys or metallic glasses to precisely replicate patterning features onto both conventional metals and the other amorphous alloys. Our work presents an avenue for avoiding the embrittlement of amorphous alloys associated with thermoplastic forming and yields new insight the forming application of bulk amorphous alloys at room temperature without using heat treatment.

Effect of geometrical constraint condition on the formation of nanoscale twins in the Ni-based metallic glass composite

We investigated the effect of geometrically constrained stress–strain conditions on the formation of nanotwins in α-brass phase reinforced Ni59Zr20Ti16Si2Sn3 metallic glass (MG) matrix deformed under macroscopic uniaxial compression. The specific geometrically constrained conditions in the samples lead to a deviation from a simple uniaxial state to a multi-axial stress state, for which nanocrystallization in the MG matrix together with nanoscale twinning of the brass reinforcement is observed in localized regions during plastic flow. The nanocrystals in the MG matrix and the appearance of the twinned structure in the reinforcements indicate that the strain energy is highly confined and the local stress reaches a very high level upon yielding. Both the effective distribution of reinforcements on the strain enhancement of composite and the effects of the complicated stress states on the development of nanotwins in the second-phase brass particles are discussed.

Casting Characteristics of High Cerium Content Aluminum Alloys

This paper compares the castability of the near eutectic aluminum-cerium alloy system to the aluminum-silicon and aluminum-copper systems. The alloys are compared based on die filling capability, feeding characteristics and tendency to hot tear in both sand cast and permanent mold applications. The castability ranking of the binary Al–Ce systems is as good as the aluminum-silicon system with some deterioration as additional alloying elements are added. In alloy systems that use cerium in combination with common aluminum alloying elements such as silicon, magnesium and/or copper, the casting characteristics are generally better than the aluminum-copper system. In general, production systems for melting, de-gassing and other processing of aluminum-silicon or aluminum-copper alloys can be used without modification for conventional casting of aluminum-cerium alloys.

Characterization of Near Net-Shape Castable Rare Earth Modified Aluminum Alloys for High Temperature Application

The modern family of aluminum alloys has a broad range of applications. However, there are currently few aluminum alloys capable of operating at elevated temperatures while maintaining mechanical properties necessary for high performance automotive and aerospace applications. The high temperature phase equilibrium of Al-REE-X (REE-Rare earth elements and X-traditional alloying elements) is not well understood yet this class of materials shows promise as a lightweight material for stiffness-driven high temperature applications. To date, we have determined RE alloys of Al to be castable across a broad range of temperatures and compositions. In addition, room temperature physical properties of Al-RE alloys appear to be in line with existing aluminum alloys with superior microstructural stability and associated properties at temperatures exceeding 150 °C. The CALPHAD work, microstructure, and compositional data are presented for representative example of Al-RE alloys at room and elevated temperatures.

Determining strain in amorphous alloys: Uncertainties with analyzing structural changes during deformation

Molecular dynamics simulations were utilized to test the reliability of strain values obtained from diffraction data for noncrystalline alloys. We found that in the case of a one-component system, the strain value obtained from the pair correlation functions underestimates the actual value because of a small degree of atomic relaxations, which minimize the effects of the applied deformation. In the case of multicomponent systems, the different pairs are affected by applied deformation to different extents; moreover, this implies that the strain value determined from diffraction data should depend on the type of scattering.

Magnetostrictive performance of additively manufactured CoFe rods using the LENS (TM) system

Magnetostrictive materials exhibit a strain in the presence of a variable magnetic field. While they normally require large, highly oriented crystallographic grains for high strain values, metal additive manufacturing (3D printing) may be able to produce highly textured polycrystalline rods, with properties comparable to those manufactured using the more demanding free standing zone melting (FSZM) technique. Rods of Co75.8Fe24.2 and Co63.7Fe36.3 have been fabricated using the Laser engineered net shaping (LENSTM) system to evaluate the performance of additively manufactured magnetic and magnetostrictive materials. The 76% Co sample showed an average magnetostriction (λ) of 86 ppm at a stress of 124 MPa; in contrast, the 64% Co sample showed only 27 ppm at the same stress. For direct comparison, a Co67Fe33 single crystal disk, also measured as part of this study, exhibited a magnetostriction value of 131 and 91 microstrain in the [100] and [111] directions, respectively, with a calculated polycrystalline v...