Melting and crystallization in Ni nanoclusters: The mesoscale regime

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms (5.7 nm) using molecular dynamics with the quantum-Sutten–Chen many-body force field. We find a transition from cluster or molecular behavior below ~500 atoms to a mesoscale nanocrystal regime (well-defined bulk and surface properties) above ~750 atoms (2.7 nm). We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N = Tm,bulk–alphaN^–1/3, dropping from Tm,bulk = 1760 K to Tm,336 = 980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N>400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character

Critical cooling rate and thermal stability of Zr–Ti–Cu–Ni–Be alloys

The critical cooling rate as well as the thermal stability are measured for a series of alloys in the Zr–Ti–Cu–Ni–Be system. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr70Ti30)55(Ni39Cu61)25Be20 to (Zr85Ti15)55(Ni57Cu43)22.5Be27.5 show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for some compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the composition-dependent glass forming ability and thermal stability is discussed with reference to a chemical decomposition process

Criteria for formation of metallic glasses: The role of atomic size ratio

We consider metallic alloys of Cu*, Cu, and Cu** in which the atoms differ only in their atomic radii and examine how the size ratio affects the local orders in the alloy systems. These studies use molecular dynamics simulations in which the atomic interactions are modeled with a Sutton–Chen many-body potential. Considering rapid cooling of these binary and ternary alloys from the melt, we find three regimes defined by the magnitude of atomic size ratio lambda (lambda<=1.0): with (i) large size ratios of 0.95<lambda<=1.0, crystallization occurs; (ii) with moderate size ratios of 0.60<=lambda<=0.95, a glass phase forms; and (iii) with small size ratios of lambda<0.60, the alloy phase separates into pure phases and crystallize. From analyzing the structures of these binary and ternary alloys, we find that the liquid phase is characterized by local structures in which bonded atoms have local fivefold symmetry, which becomes more prominent as the glass phase forms. For phases that crystallize this local fivefold symmetry disappears as the long-range order of the crystalline phase dominates. The fivefold symmetry in the glass phase is mainly due to the icosahedral cluster formation. Energetically, the formation of icosahedral cluster is favored at the atomic size ratio of lambda~0.85, which is close to the lambda at which our analyses shows the maximum in the fivefold symmetry and the number of icosahedral clusters. As lambda decreases further, the phase separation is observed. The fivefold symmetry character and the number of icosahedral cluster shows the local minimum at this onset of the phase separation

Formation of Two Glass Phases in Binary Cu-Ag Liquid

The glass transition is alternatively described as either a dynamic transition in which there is a dramatic slowing down of the kinetics, or as a thermodynamic phase transition. To examine the physical origin of the glass transition in fragile Cu-Ag liquids, we employed molecular dynamics (MD) simulations on systems in the range of 32,000 to 2,048,000 atoms. Surprisingly, we identified a 1st order freezing transition from liquid (L) to metastable heterogenous solid-like phase, denoted as the G-glass, when a supercooled liquid evolves isothermally below its melting temperature at deep undercooling. In contrast, a more homogenous liquid-like glass, denoted as the L-glass, is achieved when the liquid is quenched continuously to room temperature with a fast cooling rate of ∼10¹¹ K/sec. We report a thermodynamic description of the L-G transition and characterize the correlation length of the heterogenous structure in the G-glass. The shear modulus of the G-glass is significantly higher than the L-glass, suggesting that the first order L-G transition is linked fundamentally to long-range elasticity involving elementary configurational excitations in the G-glass

Alien Registration- Johnson, William A. (New Limerick, Aroostook County)

https://digitalmaine.com/alien_docs/34435/thumbnail.jp

Strain Rate Induced Amorphization in Metallic Nanowires

Using molecular dynamics simulations with a many-body force field, we studied the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K. For all strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, we find that for both systems the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order and a near vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction. This amorphization which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities exhibits a new mode of amorphization

Quality Uncertainty and Grain Merchandising Risk: Vomitoxin in Spring Wheat

Marketing, Risk and Uncertainty,