Metals2014, 4(4), 490-502; doi:10.3390/met4040490 - published 30 October 2014 Show/Hide Abstract
Abstract: High pressure shock compression induces a large temperature increase due to the dissipation within the shock front. Hence, a solid sample subjected to intense shock loading can melt, partially or fully, either on compression or upon release from the shocked state. In particular, such melting is expected to be associated with specific damage and fragmentation processes following shock propagation. In this paper, we show that laser driven shock experiments can provide a procedure to investigate high pressure melting of metals at high strain rates, which is an issue of key interest for various engineering applications as well as for geophysics. After a short description of experimental and analytical tools, we briefly review some former results reported for tin, then we present more recent observations for aluminum and iron.
Metals2014, 4(4), 477-489; doi:10.3390/met4040477 - published 29 October 2014 Show/Hide Abstract
Abstract: Interest in ultrasonic treatment of liquid metal has waxed and waned for nearly 80 years. A review of several experiments representative of ultrasonic cavitation treatment of Al and Mg alloys shows that the theoretical mechanisms thought to be responsible for grain refinement are (1) cavitation-induced increase in melting temperature predicted by the Clausius-Clapeyron equation and (2) cavitation-induced wetting of otherwise unwetted insoluble particles. Neither of these theoretical mechanisms can be directly confirmed by experiment, and though they remain speculative, the available literature generally assumes that one or the other or both mechanisms are active. However, grain size is known to depend on temperature of the liquid, temperature of the mold, and cooling rate of the entire system. From the reviewed experiments, it is difficult to isolate temperature and cooling rate effects on grain size from the theoretical effects. Ultrasonic treatments of Al-A356 were carried out to isolate such effects, and though it was found that ultrasound produced significant grain refinement, the treatments also significantly chilled the liquid and thereby reduced the pouring temperature. The grain sizes attained closely correlated with pouring temperature suggesting that ultrasonic grain refinement is predominantly a result of heat removal by the horn and ultrasonic stirring.
Metals2014, 4(4), 465-476; doi:10.3390/met4040465 - published 15 October 2014 Show/Hide Abstract
Abstract: This paper demonstrates a molding technique for producing spheres composed of eutectic gallium-indium (EGaIn) with diameters ranging from hundreds of microns to a couple millimeters. The technique starts by spreading EGaIn across an elastomeric sheet featuring cylindrical reservoirs defined by replica molding. The metal flows into these features during spreading. The spontaneous formation of a thin oxide layer on the liquid metal keeps the metal flush inside these reservoirs. Subsequent exposure to acid removes the oxide and causes the metal to bead up into a sphere with a size dictated by the volume of the reservoirs. This technique allows for the production and patterning of droplets with a wide range of volumes, from tens of nanoliters up to a few microliters. EGaIn spheres can be embedded or encased subsequently in polymer matrices using this technique. These spheres may be useful as solder bumps, electrodes, thermal contacts or components in microfluidic devices (valves, switches, pumps). The ease of parallel-processing and the ability to control the location of the droplets during their formation distinguishes this technique.
Metals2014, 4(3), 455-464; doi:10.3390/met4030455 - published 29 August 2014 Show/Hide Abstract
Abstract: We have performed first-principles calculations to obtain magnetic moment, magnetocrystalline anisotropy energy (MAE), i.e., the magnetic crystalline anisotropy constant (K), and the Curie temperature (Tc) of low temperature phase (LTP) MnBi and also estimated the maximum energy product (BH)max at elevated temperatures. The full-potential linearized augmented plane wave (FPLAPW) method, based on density functional theory (DFT) within the local spin density approximation (LSDA), was used to calculate the electronic structure of LPM MnBi. The Tc was calculated by the mean field theory. The calculated magnetic moment, MAE, and Tc are 3.63 μB/f.u. (formula unit) (79 emu/g or 714 emu/cm3), −0.163 meV/u.c. (or K = −0.275 × 106 J/m3) and 711 K, respectively. The (BH)max at the elevated temperatures was estimated by combining experimental coercivity (Hci) and the temperature dependence of magnetization (Ms(T)). The (BH)max is 17.7 MGOe at 300 K, which is in good agreement with the experimental result for directionally-solidified LTP MnBi (17 MGOe). In addition, a study of electron density maps and the lattice constant c/a ratio dependence of the magnetic moment suggested that doping of a third element into interstitial sites of LTP MnBi can increase the Ms.
Metals2014, 4(3), 445-454; doi:10.3390/met4030445 - published 28 August 2014 Show/Hide Abstract
Abstract: The paper presents a new method for the production of the closed-cell Al foams of improved sound absorbing ability. Final heat treatment procedure including heating below the solidus temperature followed by water quenching is proposed as an alternative method to machining, which is used commonly for improvement of the sound absorption coefficient. Several kinds of foams based on AlZnMg-alloys comprising brittle eutectic domains of interdendritic redundant phase have been produced by the Alporas-like melting process to realize the method above. Opening of the closed cell structure required for ensuring high sound absorption ability has been achieved by cracking the walls between neighboring cells, making them gas permeable. They ultimately looked like Helmholtz micro-perforated resonators. Processing parameters and other variables that are favorable both for foaming regime and for final heat treatment are discussed and specified.
Metals2014, 4(3), 428-444; doi:10.3390/met4030428 - published 27 August 2014 Show/Hide Abstract
Abstract: The use of a stress-strain constitutive relation for the undamaged material and a traction-separation cohesive crack model with softening for cracking has been demonstrated to be an effective strategy to predict and explain the size-scale effects on the mechanical response of quasi-brittle materials. In metals, where ductile fracture takes place, the situation is more complex due to the interplay between plasticity and fracture. In the present study, we propose a multi-scale numerical method where the shape of a global constitutive relation used at the macro-scale, the so-called hardening cohesive zone model, can be deduced from meso-scale numerical simulations of polycrystalline metals in tension. The shape of this constitutive relation, characterized by an almost linear initial branch followed by a plastic plateau with hardening and finally by softening, is in fact the result of the interplay between two basic forms of nonlinearities: elasto-plasticity inside the grains and classic cohesive cracking for the grain boundaries.