T. Z. Kattamis

Solidification processing and tribological behavior of particulate TiC-Ferrous Matrix composites

Abstract Particulate TiC-reinforced ferrous matrix composites were processed by solidification of (1) an FeTiC melt of appropriate composition from which particulate titanium carbide precipitates and (2) an FeC melt in which particulate titanium carbide is dispersed through electromagnetic stirring. The microstructure of composites processed by precipitating titanium carbide was controlled by controlling melt composition and homogeneity, as well as cooling rate. That of composites processed by dispersing titanium carbide powder in a steel or cast iron melt was controlled by controlling the original melt composition, volume fraction and particle size of titanium carbide added, mixing temperature and time, and cooling rate. The specific wear rate and the friction coefficient between a diamond stylus and the polished surface of composite specimens decreased with increasing volume fraction of titanium carbide and decreasing carbide particle size and spacing. They also decreased with increasing steel matrix microhardness, achieved through heat treatment and martensitic transformation.

Effect of solidification parameters on the growth geometry of MC carbide in IN- 100 dendritic monocrystals

Several dendritic monocrystals of IN-100 were prepared under six growth conditions with thermal gradient,G, varying between 4.5 × l03 and 16 × 103 K/m and growth rate,R, be-tween 8.3 × 10-6 and 67 × 10-6 m/s. Three types of carbide were identified: the unmelted, the faceted and the Chinese script-type. The volume percent of faceted carbide increased with increasingG/R ratio whereas that of the Chinese script-type exhibited the inverse variation. The average faceted carbide size and the average spacing of the Chinese script-type carbide decreased with increasing cooling rate, (G ×R), The specific perimeter of the faceted carbide increased with increasing cooling rate, indicating that coarsening is responsible for final geometry of this carbide. Detailed composition measurements showed that the unmelted carbide is rich in titanium, whereas the faceted is rich in mo-lybdenum and vanadium.

Tribological properties of SiCp-reinforced Al-4.5% Cu-1.5% Mg alloy composites

Abstract SiC p -reinforced Al-4.5% Cu-1.5% Mg composite specimens were processed through vigorous stirring of the carbide in a semi-solid alloy slurry, remelting and casting. The specimens were examined microstructurally and their tribological properties were evaluated both in the as-cast and heat-treated conditions by pin-on-disk tribometry and automatic scratch testing. The specific wear rate and the friction coefficient between a diamond stylus and the polished specimen surface decreased, and the composite microhardness increased with increasing volume fraction carbide, and decreasing carbide particle size and spacing. A solution and ageing treatment led to an increase in wear resistance and composite microhardness, and a reduction in friction coefficient.

Rheology of semisolid Al-4.5%Cu-1.5%Mg alloy

Abstract The rheological behavior of semisolid Al-4.5%Cu-1.5%Mg alloy slurry was investigated by using a slightly modified Couette viscometer to conduct three different types of experiments. In continuous cooling experiments the apparent viscosity of the sheared slurry increased with increasing volume fraction solid and increasing cooling rate, and decreased with increasing shear rate. The average particle size of the α phase decreased with increasing shear rate, increasing cooling rate, and increasing copper content in the alloy. The rate of decrease in particle size with increasing shear rate decreased with increasing cooling rate. In isothermal experiments at constant volume fraction solid and shear rate, the apparent viscosity decreased with increasing isothermal holding time, reaching a “steady state” value after about 1000s. The normalized specific particle surface area also decreased with isothermal holding time, reaching “quasisteady state” values. In isothermal experiments at constant volume fraction solid and variable shear rate, the steady state apparent viscosity of a semisolid alloy slurry decreased with increasing shear rate and decreasing volume fraction solid, while the normalized steady state specific particle surface area decreased and the normalized steady state average particle size increased. A state equation is proposed which fits reasonably well the experimental points obtained in this third group of experiments.

Cast microstructure of Inconel 713C and its dependence on solidification variables

The dependence of cast microstructure of Inconel 713C on solidification variables was investigated over a wide range of local cooling rates, ∈, and thermal gradients in the liquid at the solid-liquid interface,G. The shape of MC carbide particles was found to depend greatly on: 1) theG/R ratio at the solid-liquid interface, whereR is growth rate, through the effect of this ratio on the solid phase,γγ, growth morphology. Under planar front growth conditions the carbide particles were octahedral, under cellular growth conditions they were plate-like, elongated along the cellular growth direction, and under dendritic growth conditions they were irregularly shaped; 2) the local cooling rate, ∈, when γ was dendritic, with a transition from octahedral to dendritic with increasing ∈. The size of MC carbide particles was found to be controlled by coarsening and to become finer with increasing ∈. In this alloy the composition of the MC carbide was established as (Nb0.63Ti0.31Mo0.06) C and was practically independent of local cooling rate. Other observations were that the precipitation of γ′ and the formation of nonequilibrium eutectics, such as MC-γ, γ-γ′ or MC-γ-γ′ were suppressed at splat-cooling rates. Also, microsegregation of all alloying elements with the exception of aluminum was normal, with concentration increasing from the dendrite center-line to the dendrite arm boundary. Aluminum behaved in the opposite manner. Within the cooling rate range used herein, this variable had only a slight effect on microsegregation.

Electrical resistivity of metal matrix composites

Theoretical models for predicting the electrical resistivity of metal matrix composites reinforced with continuous fibers, short fibers, and particulates were developed by integrating thin slices of composite cells. The experimental electrical resistivity of aluminum, copper, and silver matrix composites was measured and compared with theoretical values derived from the models. Experimental resistivity of composites followed the trend of theoretical prediction, increasing with increasing volume fraction and decreasing size of reinforcement. Deformation regions containing residual stresses and dislocations formed around the reinforcement and raised the resistivity of composites. The magnitude of residual stresses and the dislocation density were found to depend on the type, size and shape of reinforcement, as well as the matrix type. The effective size of the deformation regions varied due to their overlapping and better fitted the calculated curves through empirical modification. Theoretical prediction of resistivity that takes into account the effect of residual stresses and dislocations, and the overlapping of deformation regions agreed reasonably well with experimental results.

Dendrite coarsening during directional solidification of Al-Cu-Mn alloys

Abstract Steady state directional solidification of Al–3.75%Cu–1.5%Mn and Al–1%Cu–1.5%Mn dendritic monocrystals was interrupted for different lengths of time prior to quenching and dendrite coarsening kinetics were evaluated versus temperature. For a given alloy composition, the isothermal coarsening rate, expressed as the time variation of the normalized specific dendrite surface area, increased with temperature and decreased with time. It also decreased with increasing copper concentration. The secondary dendrite arm spacing increased with time and for shorter times, with decreasing temperature. For Al–3.75%Cu–1.5%Mn alloy this trend was reversed for times exceeding about 1.5 min. For all times the rate of increase of the spacing increased with temperature. Secondary dendrite arm spacing increased with time during solidification and with decreasing growth rate. The dependence of specific surface area on time was predicted reasonably well by combining a coarsening model which is valid for the earlier stages of coarsening with a model which is valid for later stages. Copper diffusion through the liquid was found to be the rate controlling step in dendrite coarsening. Experimental measurements of secondary arm spacing during isothermal coarsening, as well as during solidification agreed reasonably well with model predictions.

Mechanism of establishment of cast microstructure during solidification of highly undercooled melts

Abstract A microstructural similarity between highly undercooled 440C alloy steel and the same alloy solidified from a nonundercooled but vigorously stirred melt indicated the role which remelting during recalescence may play in the formation of final cast microstructure in highly undercooled alloys. An experiment was conducted, which firmly established this role for undercooled iron-25% nickel alloy. A remelting model was proposed, predicting that the average length of dendritic segments formed by remelting during recalescence is inversely proportional to the square root of the recalescence rate. This relationship was experimentally verified for iron-25% nickel specimens which were undercooled 150° C and allowed to recalesce at rates ranging from 10° C/sec to 75° C/sec.

Dendritic coarsening during solidification

Abstract A model for dendritic coarsening during solidification was developed, assuming: dissolution and shrinkage of small dendrite arms; lateral growth of large dendrite arms; applicability of the Scheil equation to solidification; deposition of solidified material on large arms. A reasonable agreement was found between calculated and measured values of surface-to-volume ratio, S v , versus time. It was concluded that: solidification growth contributes more to the decrease of S v than does coarsening; coarsening contributes to the disappearance of dendrite arms and, hence, enhances the effect of solidification growth on S v .

Coarsening kinetics during solidification of Ni-Al-Ta dendritic monocrystals

Several dendritic monocrystals of nickel-rich Ni-Al-Ta alloys were directionally solidified at about 0.25 m/h−1 under a gradient of 8 × 10−3 K/m−1. The solid-liquid interface was fossilized at a given moment by rapidly quenching the remaining liquid. In some specimens crystal pulling was interrupted for various lengths of time prior to quenching. The quenched solid-liquid interfaces were used for a convenient and rapid evaluation of: 1) isothermal coarsening kinetics of the dendritic solid at a temperature between the liquidus and the eutectic temperatures and; 2) dendrite coarsening kinetics during solidification. It was found that extension to the ternary Ni-Al-Ta system of a model previously developed for binary systems predicted isothermal dendrite coarsening kinetics in close agreement with experimental results. Agreement for coarsening kinetics during solidification was less good. An increase in tantalum or aluminum contents slowed down coarsening, yielding finer microstructures. At equal atomic percental increase in concentration, the effect of tantalum was more significant than that of aluminum.