Creep behavior of copper-chromium <Emphasis Type="Italic">in-situ</Emphasis> composite

Creep deformation and fracture behaviors were investigated on a deformation-processed Cu-Cr in-situ composite over a temperature range of 200 °C to 650 °C. It was found that the creep resistance increases significantly with the introduction of Cr fibers into Cu. The stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) were observed to be 5.5 and 180 to 216 kJ/mol, respectively. The observation that the stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) are close to those of pure Cu suggests that the creep deformation of the composite is dominated by the deformation of the Cu matrix. The high stress exponent at low temperatures (200 °C and 300 °C) is thought be associated with the as-swaged microstructure, which contains elongated dislocation cells and subgrains that are stable and act as strong athermal obstacles at low temperatures. The mechanism of damage was found to be similar for all the creep tests performed, but the distribution and extent of damage were found to be very sensitive to the test temperature.     

Microstructure and sliding-wear behavior of tungsten-reinforced W-Ni-Si metal-silicide <Emphasis Type="Italic">in-situ</Emphasis> composites

W-Ni-Si metal-silicide-matrix in-situ composites reinforced by tungsten primary grains were fabricated using a water-cooled copper-mold laser-melting furnace by the LASMELT process. Main constitutional phases of the W/W-Ni-Si in-situ composites are the tungsten primary phase, peritectic W₂Ni₃Si, and the remaining W₂Ni₃Si/Ni₃₁Si₁₂ eutectics, depending on the alloy compositions. The sliding-wear resistance of the W/W-Ni-Si intermetallic composites was evaluated at room temperature and 600 °C. Wear mechanisms of the W/W₂Ni₃Si in-situ composites were discussed based on morphology observations of the worn surface and wear debris. Results show that the W/W-Ni-Si composites have excellent wear resistance under both room- and high-temperature sliding-wear-test conditions, because of the high yield strength and toughness of the tungsten-reinforcing phase and the high hardness and the covalent-dominated intermetallic atomic bonds of the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides. Tungsten-reinforcing grains played the dominant role in resisting abrasive-wear attacks of microcutting, plowing, and brittle spalling during the sliding-wear process, while the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides are responsible for the excellent adhesive wear resistance.     

Phase evolution during laser <Emphasis Type="Italic">in-situ</Emphasis> carbide coating

With the help of laser surface engineering, in-situ carbide composite coating on the surface of plain carbon steel was achieved. Energy dispersive spectroscopy (EDS) in supplement with X-ray diffractometry indicated the evolution of TiC, Fe-Cr, and M₇C₃ as major phases in the coating. A variation in the evolution of M₇C₃ phase was observed with respect to the laser power over the range of 900 to 2100 W (3 mm ×600 μm rectangular beam spot) during processing. Computational techniques were employed with the aim of studying possible reasons for phase evolution, stability of phases, solidification path, and optimization of parameters to stabilize the M₇C₃ phase and hence tailor properties.     

Development of electroless composite coatings by using <Emphasis Type="Italic">in-situ</Emphasis> co-precipitation followed by co-deposition process

This article presents the studies carried out on the in-situ coprecipitation followed by codeposition of Al₂O₃ and ZrO₂ solids along with Al₃Zr within electroless Ni-P matrix. The present pioneered route for synthesis of composite electroless coating, Ni-P-X (X=ZrO₂-Al₂O₃-Al₃Zr), is developed and studied as a function of concentration of coprecipitation reactants and bath loading factors on coating rate. The coating has been found to grow laterally and vertically to form a continuous layer, and fine second-phase particles, from the coprecipitation reaction, are seen to get entrapped in the Ni-P matrix. The coating rate of the composite coating has been observed to be higher than that of Ni-P alloy coatings for the identical experimental conditions. The morphology, phase analysis, thermal response, and hardness studies of the composite coatings have been reported. This article also includes the characterization of coprecipitated powder obtained. Different analysis methods used in this study include scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDXA) and differential thermal analysis (DTA).     

<Emphasis Type="Italic">In-situ</Emphasis> formation of SiC-reinforced Al-Si alloy composites using methane gas mixtures

Theoretical and experimental studies on the in-situ formation of an Al-Si alloy composite using a methane gas mixture were carried out. An Al-Si alloy composite with in-situ formed SiC as a reinforced phase was produced by bubbling methane gas at temperatures from 1223 to 1423 K. An optical microscope, scanning electron microscope (SEM), and electron microprobe were used for the product characterization. Primary and eutectic silicon were observed in the samples taken from the top part of the crucible, and only eutectic silicon was observed in the samples taken from the bottom of crucible. The SiC formation rate increased with the decrease in the bubble size. A silicon concentration gradient existed at different vertical positions of the liquid alloy. The silicon concentration close to the top of the liquid alloy was higher than that at the bottom. The SiC concentration was very low in the bulk alloy. The bubbling of the gas mixture in the melt resulted in the formation of a layer of foam on top of the crucible. Formed SiC particles were enriched in the foam and carried out of the crucible by the overflow foam to a composite collector located under the crucible. The foam in the composite collector was broken, and composites in the foam contained up to 30 wt pct SiC. The particle size of the SiC is in the range of 1 to 10 μm. The bubbling process resulted in the unevenness of the silicon concentration and the different crystallizing processes. The SiC formation rate was found to be about 12.5 mg/(L·s). A kinetic model was developed. The model-predicted results are in very good agreement with the experimental results.     

Thermodynamic effect of alloying addition on <Emphasis Type="Italic">in-situ</Emphasis> reinforced TiB<Subscript>2</Subscript>/Al composites

From the viewpoint of thermodynamics, using the Wilson equation and an extended Miedema model, the effect of the alloying element on the stability of the precipitated phases during the fabrication of in-situ reinforced TiB₂/Al composites was evaluated. The result shows that additions of alloying elements, such as Mg, Cu, Zr, Ni, Fe, V, and La, can promote the formation of Al₃Ti and TiB₂ phases. Particularly, Zr has the most pronounced effect among these alloying elements. In addition, alloying elements can hinder the formation of AlB₂ to a small extent. The calculation results also show that it is easier for magnesium to react with the salts to form TiB₂ than aluminum during the fabrication of in-situ reinforced TiB₂/Al using the flux-assisted synthesis (FAS) technology.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

Application of electromagnetic separation of phases in alloy melt to produce <Emphasis Type="Italic">In-situ</Emphasis> surface and functionally gradient composites

The principle of electromagnetic separation of phases (primary phase) in alloy melt is that the electromagnetic force scarcely acts on the primary phases due to its low electric conductivity as compared to the melt. As a result, a repulsive force acts on the primary iron-rich phases to push them to move in the direction opposite to that of the electromagnetic force. The in-situ surface composite and the functionally gradient composite reinforced by primary Si are produced when the hypereutectic Al-Si alloy solidifies under electromagnetic force induced by static magnetic field and DC current. Similarly, the Al-Si-1.20 pct Fe-1.60 pct Mn alloy in-situ surface composite reinforced by primary iron-rich phase is produced. Based on this, a new method for production of in-situ multigradient composite with several layers, by electromagnetic separation of phases and directional solidification technique, is proposed.     

Quality assessment of artificially aged A357 aluminum alloy cast ingots by introducing approximate expressions of the quality index <Emphasis Type="Italic">Q</Emphasis><Subscript><Emphasis Type="Italic">D</Emphasis></Subscript>

The mechanical performance of A357 cast ingot aluminum alloy specimens subjected to 25 different artificial aging heat treatments has been experimentally investigated. For the quality evaluation of the artificially aged alloys, the quality index Q _D , proposed by the authors, has been involved. The index Q _D interprets quality as the potential of an alloy to offer combinations of tensile strength, ductility, and toughness at certain levels of values. The evaluation of the alloy quality using Q _D relies on the availability of the materials tensile flow curve. To facilitate the exploitation of Q _D for quality assessment, approximations of Q _D are developed. In the proposed approximations, Q _D is formulated as a function of ultimate tensile strength, yield strength, and elongation to fracture, or alternatively, as a function of Rockwell hardness E and Charpy impact energy values. Accurate estimations of the quality index Q _D are compared against Q _D values obtained by using the proposed approximations.     

Magnetic contribution to the interdiffusion coefficients in bcc (<Emphasis Type="Italic">α</Emphasis>) and fcc (<Emphasis Type="Italic">γ</Emphasis>) Fe-Ni alloys

The interdiffusion coefficients in bcc (α) and fcc (γ) Fe-Ni alloys below their Curie temperatures have been calculated based on the magnetic contribution to the free energy for interdiffusion. The free energy for interdiffusion due to magnetic ordering in bcc Fe-Ni alloys is positive. The calculated interdiffusion coefficients in bcc Fe-Ni alloys fit the experimental data quite well. In fcc Fe-Ni alloys, the magnetic contribution to interdiffusion depends on both temperature and composition and is abnormal for Ni compositions in the Invar region. The free energy of vacancy formation is positive and the free energy of vacancy migration is negative, due to the effect of magnetic ordering. The interdiffusion coefficient in the ferromagnetic phase is lower than that extrapolated from the paramagnetic phase for Ni compositions of 50 at. pct and greater and is higher than that extrapolated from the paramagnetic phase for Ni compositions of 40 at. pct and lower.     

Optimization of the johnson-mehl-avarami equation parameters for <Emphasis Type="Italic">α</Emphasis>-ferrite to <Emphasis Type="Italic">γ</Emphasis>-austenite transformation in steel welds using a genetic algorithm

A nonisothermal Johnson-Mehl-Avarami (JMA) equation with optimized JMA parameters is proposed to represent the kinetics of transformation of α-ferrite to γ-austenite during heating of 1005 steel. The procedure used to estimate the JMA parameters involved a combination of numerical heat-transfer and fluid-flow calculations, the JMA equation for nucleation and growth for nonisothermal systems, and a genetic algorithm (GA) based optimization tool that used a limited volume of experimental kinetic data. The experimental data used in the calculations consisted of phase fraction of γ-austenite measured at several different monitoring locations in the heat-affected zone (HAZ) of a gas tungsten arc (GTA) weld in 1005 steel. These data were obtained by an in-situ spatially resolved X-ray diffraction (SRXRD) technique using synchrotron radiation during welding. The thermal cycles necessary for the calculations were determined for each monitoring location from a well-tested three-dimensional heat-transfer and fluid-flow model. A parent centric recombination (PCX) based generalized generation gap (G3) GA was used to obtain the optimized values of the JMA parameters, i.e., the activation energy, pre-exponential factor, and exponent in the nonisothermal JMA equation. The GA based determination of all three JMA equation parameters resulted in better agreement between the calculated and the experimentally determined austenite phase fractions than was previously achieved.     

Comparison of microstructural evolution in laser-deposited and arc-melted <Emphasis Type="Italic">In-Situ</Emphasis> Ti-TiB composites

In-situ Ti-TiB composites have been processed via two different routes: arc by melting elemental Ti and B and by direct laser deposition of a blend of elemental Ti and B powders using the laser-engineered net-shaping (LENS^™) process. The conventionally cast composite exhibits a significantly coarser-scale microstructure as compared with the LENS^™-deposited composite and consists of primary proeutectic TiB precipitates dispersed in an eutectic matrix. The microstructure of the LENS^™-deposited Ti-TiB composite consists of a fine-scale homogeneous dispersion of primary TiB precipitates in an α-Ti matrix. In addition, a nanometer-scale dispersion of secondary TiB precipitates is formed in the α matrix. The hardness and modulus of these composites have been measured using nanoindentation techniques. The ability to produce such a fine dispersion of TiB precipitates in near-net-shape, near-full-density Ti-TiB composites processed using LENS^™ could potentially be highly beneficial from the viewpoint of applicability of these metal-matrix composites.     

Calciothermic reduction of titanium oxide and <Emphasis Type="Italic">in-situ</Emphasis> electrolysis in molten CaCl<Subscript>2</Subscript>

A concept for calciothermic direct reduction of titanium dioxide in molten CaCl₂ is proposed and experimentally tested. This production process consists of a single cell, where both the thermochemical reaction of the calciothermic reduction and the electrochemical reaction for recovery of the reducing agent, Ca, coexist in the same molten CaCl₂ bath. A few molar percentages of Ca dissolve in the melt, which gives the media a strong reducing power. Using a carbon anode and a Ti basket-type cathode in which anatase-type TiO₂ powder was filled, a metallic titanium sponge containing 2000 ppm oxygen was produced after 10.8 ks at 1173 K in the CaCl₂ bath. The optimum concentration of CaO in the molten CaCl₂ was 0.5 to 1 mol pct, to shorten the operating time and to achieve a lower oxygen content in Ti.     

Interdiffusion behavior of Pt-modified <Emphasis Type="Italic">γ</Emphasis>-Ni + <Emphasis Type="Italic">γ</Emphasis>′-Ni<Subscript>3</Subscript>Al alloys coupled to Ni-Al-based alloys

The effect of platinum addition on the interdiffusion behavior of γ-Ni + γ′-Ni₃Al alloys was studied by using diffusion couples comprised of a Ni-Al-Pt alloy mated to a Ni-Al, Ni-Al-Cr, or Ni-based commercial alloy. The commercial alloys studied were CMSX-4 and CMSX-10. Diffusion annealing was at 1150 °C for up to 100 hours. An Al-enriched γ′-layer often formed in the interdiffusion zone of a given couple during diffusion annealing due to the uphill diffusion of Al. This uphill diffusion was ascribed to Pt addition decreasing the chemical activity of aluminum in the γ + γ′ alloys. For a given diffusion couple end member, the thickening kinetics of the γ′ layer that formed increased with increasing Pt content in the Ni-Al-Pt γ + γ′ alloy. The γ′-layer thickening kinetics in diffusion couples with Cr showed less of a dependence on Pt concentration. Inference of a negative effect of Pt and positive effect of Cr on the Al diffusion in this system enabled explanation of the observed interdiffusion behaviors. There was no or minimal formation of detrimental topologically close-packed (TCP) phases in the interdiffusion zone of the couples with CMSX-4 or CMSX-10. An overlay Pt-modified γ + γ′ coating on CMSX-4 showed excellent oxidation resistance when exposed to air for 1000 hours at 1150 °C. Moreover, the Al content in the coating was maintained at a relatively high level due to Al replenishment from the CMSX-4 substrate.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Reactive synthesis of <Emphasis Type="Italic">in-situ</Emphasis> ZrAl<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al composites

In this work, a reactive synthesis process is proposed to obtain ZrAl₃-Al₂O₃ particulate-reinforced aluminum matrix composites. The process involves the in-situ formation of Al₂O₃ and ZrAl₃ from Al-ZrO₂ green compacts. Upon compact heating, it is found that reduction of ZrO₂ by molten aluminum occurs at temperatures above 750 °C, leading to the development of ZrAl₃ and Al₂O₃ phases. Thermodynamically, it is found that the reduction of zirconium oxide is driven mainly by the dissolution of Zr in molten aluminum. Because the solubility of Zr in liquid aluminum is extremely small, the formation of ZrAl₃ is favored after relatively small Zr dissolutions. The first Zr-Al intermetallics to form at the lowest temperatures seem to be metastable, as infered from the measured atom ratios for Al : Zr of 2.83 : 1. At increasing temperatures, the reaction comes into completion, resulting in the formation of equilibrium intermetallic ZrAl₃ phases. The results obtained from differential scanning calorimetry (DSC) indicate that by increasing the scanning rates, both the reaction temperature and the exothermic peak intensity also increase. Alternatively, it is found that by reducing the amount of ZrO₂ in the green compact, the in-situ reaction temperatures also shift toward higher values.     

Phase evolution during laser <Emphasis Type="Italic">In-Situ</Emphasis> carbide coating

With the help of laser surface engineering, in-situ carbide composite coating on the surface of plain carbon steel was achieved. Energy dispersive spectroscopy (EDS) in supplement with X-ray diffractometry indicated the evolution of TiC, Fe−Cr, and M₇C₃ as major phases in the coating. A variation in the evolution of M₇C₃ phase was observed with respect to the laser power over the range of 900 to 2100 W (3 mm×600 μm rectangular beam spot) during processing. Computational techniques were employed with the aim of studying possible reasons for phase evolution, stability of phases, solidification path, and optimization of parameters to stabilize the M₇C₃ phase and hence tailor properties.     

Dynamic phase transformation during superplastic deformation of Nb/Nb<Subscript>3</Subscript>Al <Emphasis Type="Italic">in-situ</Emphasis> composite

Nb_ss/Nb₃Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nb_ss) and A15 ordered Nb₃Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10⁻⁴ s⁻¹ for as-forged specimens. Phase transformation from Nb_ss to Nb₃Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.     

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of <Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript>+<Emphasis Type="Italic">γ</Emphasis> titanium aluminides

The stability of a lamellar structure consisting of α ₂ and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other α ₂+γ alloys.     

Some studies on the thermal-expansion behavior of C-fiber,SiC<Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>, and <Emphasis Type="Italic">In-situ</Emphasis> Mg<Subscript>2</Subscript>Si-reinforced AZ31 Mg alloy-based hybrid composites

Magnesium alloy-based hybrid composites with carbon-fiber, SiC _p , and in-situ Mg₂Si reinforcements have been prepared by the squeeze-infiltration technique. The results of the studies done on the measurement of the coefficient of thermal expansion after thermal cycling of these composites show that the thermal cycling initially leads to rapid linear expansion of the composite. However, the expansion becomes stabilized after a few cycles, pointing toward formation of the stable interfaces due to the formation of stable precipitates. The model for the growth kinetics of these precipitates at the interface shows a rapid initial growth of the precipitates with the number of thermal cycles, which becomes staturated after a few thermal cycles. The thermal treatment of the composite lowers the coefficient of linear thermal expansion, which can be explained on the basis of further stabilization of the interfaces after the thermal treatment.