Microstructural evolution during laser resolidification of Fe-25 atom percent Ge alloy

The microstructural evolution of concentrated alloys is relatively less understood both in terms of experiments as well as theory. Laser resolidification represents a powerful technique to study the solidification behavior under controlled growth conditions. This technique has been utilized in the current study to probe experimentally microstructural selection during rapid solidification of concentrated Fe-25 atom pct Ge alloy. Under the equilibrium solidification condition, the alloy undergoes a peritectic reaction between ordered α2 (B2) and its liquid, leading to the formation of ordered hexagonal intermetallic phase ε (DO19). In general, the as-cast microstructure consists of ε phase and ε-β eutectic and α2 that forms as a result of an incomplete peritectic reaction. With increasing laser scanning velocity, the solidification front undergoes a number of morphological transitions leading to the selection of the microstructure corresponding to metastable α 2/β eutectic to α2 dendrite + α2/β eutectic to α 2dendrite. The transition velocities as obtained from the experiments are well characterized. The microstructural selection is discussed using competitive growth kinetics

Microstructural evolution during laser resolidification of Fe-18 At. Pct Ge alloy

The technique of laser resolidification has been used to study the rapid solidification behavior of concentrated Fe-18 at. pct Ge alloy. The microstructural evolution has been studied as a function of scanning rate of laser beam. Scanning electron microscopy (SEM) reveals the formation of a two-layer (designated as "A" and "B") microstructure in the remelted pool. The A layer shows a band consisting of a network of interconnected channels and walls, quite similar to cell walls. The B layer shows dendritic growth. Transmission electron microscopic observations reveal the formation of bcc a-FeGe in the B layer. Laser melting has been found to play an important role in formation of the A layer. Microstructural evolution in B has been analyzed using the competitive growth criterion, and formation of bcc a-FeGe has been rationalized in the remelted layers

Microstructural evolution in laser-ablation-deposited Fe-25 at.% Ge thin film

Films with Fe-25 at.% Ge composition are deposited by the process of laser ablation on single crystal NaCl and Cu substrates at room temperature. Both the vapor and liquid droplets generated in this process are quenched on the substrate. The microstructures of the embedded droplets show size as well as composition dependence. The hierarchy of phase evolution from amorphous to body-centered cubic (bcc) DO3 to has been observed as a function of size. Some of the medium-sized droplets also show direct formation of ordered DO19 phase from the starting liquid. The evolution of disordered bcc structure in some of the droplets indicates disorder trapping during liquid to solid transformation. The microstructural evolution is analyzed on the basis of heat transfer mechanisms and continuous growth model in the solidifying droplets

Phase-field simulation of fusion interface events during solidification of dissimilar welds: effect of composition inhomogeneity

We investigate the events near the fusion interfaces of dissimilar welds using a phase-field model developed for single-phase solidification of binary alloys. The parameters used here correspond to the dissimilar welding of a Ni/Cu couple. The events at the Ni and the Cu interface are very different, which illustrate the importance of the phase diagram through the slope of the liquidus curves. In the Ni side, where the liquidus temperature decreases with increasing alloying, solutal melting of the base metal takes place; the resolidification, with continuously increasing solid composition, is very sluggish until the interface encounters a homogeneous melt composition. The growth difficulty of the base metal increases with increasing initial melt composition, which is equivalent to a steeper slope of the liquidus curve. In the Cu side, the initial conditions result in a deeply undercooled melt and contributions from both constrained and unconstrained modes of growth are observed. The simulations bring out the possibility of nucleation of a concentrated solid phase from the melt, and a secondary melting of the substrate due to the associated recalescence event. The results for the Ni and Cu interfaces can be used to understand more complex dissimilar weld interfaces involving multiphase solidification

Densification and microstructure development in spark plasma sintered WC-6 wt% ZrO<SUB>2</SUB> nanocomposites

In this paper, we report the results of a transmission electron microscopy investigation on WC-6 wt% Zr02 nanocomposite, spark plasma sintered at 1300 &#176; C, for varying times of up to 20 min. The primary aim of this work was to understand the evolution of microstructure during such a sintering process. The investigation revealed the presence of nanocrystalline Zr02 particles (30-50 nm) entrapped within submicron WC grains. In addition, relatively coarser Zr02 (60-100 nm) particles were observed to be either attached to WC grain boundaries or located at WC triple grain junctions. The evidence of the presence of a small amount of W2C, supposed to have been formed due to sintering reaction between WC and Zr02, is presented here. Detailed structural investigation indicated that Zr02 in the spark plasma sintered nanocomposite adopted an orthorhombic crystal structure, and the possible reasons for o- Zr02 formation are explained. The increase in kinetics of densification due to the addition of is believed to be caused by the enhanced diffusion kinetics in the presence of nonstoichiometric nanocrystalline Zr02

Microstructural evolution during remelting of laser surface alloyed hyper-monotectic Al-Bi alloy

The present investigation explores the possibility of synthesizing a two-phase microstructure consisting of a fine dispersion of bismuth particles in an aluminium matrix using the laser surface alloying technique. The possibility of controlling the size distribution of bismuth particles by subsequent remelting is also investigated. The microstructural analysis of the surface alloyed samples shows that the average size of the bismuth particles reduces with increase in laser scan speed. In order to understand the factors that determine the nature of the size distribution of the particles, a detailed model is developed. The model incorporates heat and fluid flow induced by the laser to arrive at the evolution of the temperature and velocity of the melt in three dimensions. Using these as inputs, a kinetic analysis of the nucleation, growth and coarsening induced by collision-controlled coalescence of the bismuth particles from the melt is carried out. Comparison with the experiments indicates that coalescence due to convection plays an important role in the evolution of the size distribution of bismuth particles

Laser cladding of quasi-crystal-forming Al-Cu-Fe-Bi on an Al-Si alloy substrate

We report here the results of an investigation aimed at producing coatings containing phases closely related to the quasi-crystalline phase with dispersions of soft Bi particles using an Al-Cu-Fe-Bi elemental powder mixture on Al-10.5 at. pct Si substrates. A two-step process of cladding followed by remelting is used to fine-tune the alloying, phase distribution, and microstructure. A powder mix Al64CU22.3Fe11.7Bi2 of has been used to form the clads. The basic reason for choosing Bi lies in the fact that it is immiscible with each of the constituent elements. Therefore, it is expected that Bi will solidify in the form of dispersoids during the rapid solidification. A detailed microstructural analysis has been carried out by using the backscattered imaging mode in a scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructural features are described in terms of layers of different phases. Contrary to our expectation, the quasi-crystalline phase could not form on the Al-Sisubstrate. The bottom of the clad and remelted layers shows there growth of aluminum. The formation of phases such as blocky hexagonal Al-Fe-Si and a ternary eutectic (Al + CuAl2 + Si) have been found in this layer. The middle layer shows the formation of long plate-shaped Al13Fe4 along with hexagonal Al-Fe-Si phase growing at the periphery of the former. The formation of metastable Al-Al6Fe eutectic has also been found in this layer. The top layer, in the case of the as-clad track, shows the presence of plate-shaped Al13Fe4along with a 1/1 cubicrational approximant of a quasi-crystal. The top layer of the remelted track shows the presence of a significant amount of a 1/1 cubicrational approximant. In addition, the as-clad and remelted microstructures show a fine-scale dispersion of Bi particles of different sizes formed during monotectic solidification. The remelting is found to have a strong effect on the size and distribution of Bi particles. The dry-sliding wear properties of the samples show the improvement of wear properties for Bi-containing clads. The best tribological properties are observed in the as-clad state, and remelting deteriorates the wear properties. The low coefficient offriction of the as-clad and remelted track is due to the presence of approximant phases. There is evidence of severe subsurface deformation during the wear process leading to cracking of hard phases and a change in the size and shape of soft Bi particles. Using these observations,we have rationalized possible wear mechanisms in the Bi-containing surface-alloyed layers

Non-equilibrium solidification of concentrated Fe-Ge alloys

The solidification of concentrated alloys containing ordered compounds is less well understood. These alloys often exhibit complex phase change like peritectic reaction during liquid to solid transformation. The Fe-rich part of Fe-Ge binary alloy system consists of several critical points and ordered-disorder transitions and can be used as a model system to study the effect of departure from equilibrium on the solidification microstructure. In order to understand the phase selection and morphological transitions, undercooling and recalescence behaviour; growth rate of the solidifying phases and microstructure need to be explored. In the present paper, we summarise the results obtained in several iron rich alloy compositions (Fe-(14-25) at.% Ge) using techniques of melt quenching, levitation and laser resolidification. These results provide insight to the current theories of dendritic growth and reveals possibility of a new pathway for phase evolution in peritectic alloys at high undercooling involving a massive transformation

Size effect on the lattice parameter of KCl during mechanical milling

The size effect on the lattice parameter of ionic KCl nanocrystals was studied systematically during mechanical milling of pure KCl powder under vacuum. The results suggest anomalous lattice expansion, with the lattice parameter increasing from 6.278&#197; at d=6&#956; m to 6.30307&#197; at d=85nm. The defects generated during ball milling of KCl and surface stress are deemed to be responsible for this lattice parameter expansion