Oleg N. Senkov

Structural Evolution and Kinetics in Cu-Zr Metallic Liquids from Molecular Dynamics Simulations (Postprint)

Abstract : The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass-forming ability in the Cu-Zr binary system. In this paper, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.

A Coupled Thermal/Material Flow Model of Friction Stir Welding Applied to Sc‐Modified Aluminum Alloys

A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanism for the formation of onion rings within the weld zone is presented.

Localized Einstein modes in Ca-based bulk metallic glasses

Low-temperature specific heat and elastic moduli measurements are reported for Ca-based bulk metallic glasses. The deviation from the Debye behaviour observed in the specific heat is modelled with a simple Einstein oscillator with characteristic temperature θ E = 80 K. The presence of this local mode can also account for the deviation from normal ‘Varshni behaviour’ observed in the temperature dependence of the elastic moduli.

Atomic structure of Ca40+XMg25Cu35−X metallic glasses

The atomic structures of four Ca40+XMg25Cu35−X (X = 0, 5, 10, and 20 at. %) ternary metallic glasses have been determined using a synergistic combination of neutron diffraction, ab initio molecular dynamics (MD) simulation, and constrained reverse Monte Carlo modeling. It is described as close-packing of efficiently packed Cu-centered clusters that have Ca, Mg, and Cu atoms in the first coordination shell. The close-packed arrangement of the clusters provides a characteristic medium range order in these alloys. An average coordination number (CN) of 10 (with about 5–7 Ca, 2–3 Mg, and 1–2 Cu atoms) is most common for the Cu-centered clusters. The average coordination numbers around Mg and Ca are 12–13 (∼6–8 Ca, 3 Mg, and 1–4 Cu) and 13–15 (7–9 Ca, 3–4 Mg, and 2–5 Cu), respectively, and they are composition dependent. Strong interaction of Cu with Mg and Ca results in pair bond shortening. Icosahedral short range order does not dominate in these amorphous alloys, although polytetrahedral packing and five-fo...

Crystallization kinetics of an amorphous Al85Ni10Y2.5La2.5 alloy

Abstract Amorphous Al 85 Ni 10 Y 2.5 La 2.5 alloy powder with the particle size of −500 mesh was produced by gas atomization. Crystallization kinetics of the amorphous phase were studied using differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy. Crystallization occurred in the temperature range 230–450°C in three exothermic reaction steps. The onset and peak temperatures for these reactions shifted toward higher temperatures when the heating rate was increased; this allowed the activation energies of the mechanisms controlling each step of the crystallization to be determined as 148, 336, and 274 kJ/mol, respectively. XRD and SEM analysis of the powder heated to different temperatures showed that precipitation of Al-based particles occurred during the first exothermic reaction, crystallization of the Al 4 La phase and an increase in the volume fraction of the Al-based phase took place during the second exothermic reaction, and development of the Al 3 Ni phase occurred during the third exothermic reaction. After annealing at 550°C, the powder consisted of Al-based matrix reinforced with very fine intermetallic particles of different morphologies. The mechanism of crystallization is discussed.

Effect of Process Variables on the Inertia Friction Welding of Superalloys LSHR and Mar-M247

The effect of inertia friction welding process parameters on microstructure evolution, weld plane quality, and the tensile behavior of welds between dissimilar nickel-base superalloys was established. For this purpose, the fine-grain, powder metallurgy alloy LSHR was joined to coarse-grain cast Mar-M247 using a fixed level of initial kinetic energy, but different combinations of the flywheel moment of inertia and initial rotation speed. It was found that welds made with the largest moment of inertia resulted in a sound bond with the best microstructure and room-temperature tensile strength equal to or greater than that of the parent materials. A relationship between the moment of inertia and weld process efficiency was established. The post-weld tensile behavior was interpreted in the context of observed microstructure gradients and weld-line defects.

Characterisation of friction stir welded 7042- T6 extrusions through differential scanning calorimetry

Abstract Extruded and T6 tempered plates of 7042, an Sc modified Al–Zn–Mg–Cu alloy, were joined by friction stir welding (FSW) at a constant weld velocity and various pin rotation speeds (PRSs). At a low PRS, hardness decreased from each edge of the weld to a local minimum at the weld centre, but at a high PRS, the hardness initially decreased from each edge of the weld, but then rose towards the weld centre. The transition in the hardness profile was related to different heating and cooling conditions during FSW. Differential scanning calorimetry of baseline and welded samples revealed that the volume fraction of strengthening particles within the weld regions strongly depended on the weld conditions. For FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. The maximum welding temperature during FSW increased from ∼250 to 450°C as the PRS increased from 175 to 400 rev min−1.

FRICTION STIR WELDING OF SC-MODIFIED AL-ZN-MG-CU ALLOY EXTRUSIONS

Small additions of scandium to Al-Zn-Mg-Cu 7000 series alloys can significantly improve mechanical properties and augment the strength retention at low and elevated temperatures. This research program evaluates the residual properties of Sc-modified Al-Zn-Mg-Cu alloy extrusions joined through friction stir welding (FSW). Mechanical and corrosion testing were performed on the baseline material and on panels friction stir welded at 175, 225, 250, 300, 350 and 400 RPM (all other weld parameters held constant). A thermal model of friction stir welding is developed that utilizes an energy-based scaling factor to account for tool slip. The proposed slip factor is derived from an observed, empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures over a range of energy levels, and the mechanical and corrosion behavior is correlated to the temperature distribution predicted by the model.Copyright

Mechanical Behavior and Microstructure Evolution during Superplastic Deformation of the Fine-Grained AlCoCrCuFeNi High Entropy Alloy

Characteristics of mechanical behavior during superplastic flow and associated microstructural evolution in the wrought AlCoCrCuFeNi high-entropy alloy were studied. The alloy had complex microstructure with fine grain/particle size of ≈2.1 μm. 4 different phases with volume fractions from 7% to 46% and different deformation characteristics were found in the alloy. Very high tensile elongations of up to 1240% were observed during deformation at temperatures of 800°C–1000°C and at strain rates of 10-4 s-1–10-1 s-1 despite presence of pronounced softening stage followed by steady state flow stage. Microstructure of the alloy after tensile testing was studied in detail. Phase transformations were analyzed employing thermodynamic modeling and their role in strain accommodation is discussed.

Correlation Between Thermodynamic and Kinetic Properties of Glass-Forming Liquids

Correlations between three characteristic temperatures: glass transition, T g , Kauzmann, T k , and Vogel-Fulcher-Tammann, T o , were identified from the analysis of more than 60 metallic and non-metallic glass-forming materials. It was found that T g ≥ T k ≥ T o and T k is the geometric mean of T g and T o . The relation T k ≥ T o indicates that the excess total entropy of a super-cooled liquid Δ S approaches zero at a higher temperature than the configurational entropy Δ S conf , and such behavior was explained by the stronger temperature dependence of the excess vibrational entropy of the liquid, Δ S vib , than that of the corresponding glass, . A relationship between the fragility index m , reduced excess heat capacity Δ C p ( T g )/ S m , and reduced glass transition temperature, T rg , was identified using the found correlation between the characteristic temperatures.