Sintering kinetics of YAG ceramics

Solid state reactive （SSR） sintering kinetics was observed for YAG ceramics. There were two densification stages in sin- tering process due to its reaction. After the first stage, samples began to expand, then, the second densification stage began. At a heat- ing rate of 10 ℃/min, the sample warped down and warped back to straight. The apparent activation energy of the first densification process was about 522 kJ/mol for the initial shrinkage of A1203 and Y203 mixed powder green-body, which increased in the follow- ing process due to the solid state reaction. In the second densification stage, synthesis reaction of YAG still worked. Green-bodies processed with higher heating rate got more shrinkage at the same temperature than lower heating rate green bodies. And its kinetic field diagram was abnormal, compared with that of other reported ceramics, such as Al203. It was found that the reaction of YAG provided positive effect to the sintering driving force. The apparent activation energy for densification of SSR YAG sintered in ArH5 atmosphere was 855 kJ/mol at temperature holding sintering. And the apparent activation energy for grain growth was 1053 kJ/mol.     

Finite-Element Modeling of Titanium Powder Densification

A powder-level, finite-element model is created to describe densification, as a function of applied stress during uniaxial hot pressing, of CP-Ti and Ti-6Al-4V powders with spherical or spheroidal shapes for various packing geometries. Two cases are considered: (1) isothermal densification (in the α- or β-fields of CP-Ti and in the β-field of Ti-6Al-4V) where power-law creep dominates and (2) thermal cycling densification (across the α/β-phase transformation of Ti-6Al-4V) where transformation mismatch plasticity controls deformation at low stresses. Reasonable agreement is achieved between numerical results and previously published experimental measurements and continuum modeling predictions.     

Cavity sintering in pure nickel

Experimental measurements of the kinetics of cavity elimination by annealing are reported for pure nickel. A sample of nickel was crept to implant creep cavities in the grain boundaries. The cavity radius distribution in the sample was obtained after measuring approximately 3000 cavity diameters over an area of 4.7 × 10⁻⁵m². The sample was then vacuum annealed for 7 days at 800°C to cause the cavities to shrink, and the radii distribution of the remaining cavities was determined. From the cavity radii distributions, converted from area to volume, it was possible to determine the rate of cavity shrinkage as a function of the cavity radius. It is shown that the rate of cavity shrinkage is driven by the surface free energy, possibly affected by the internal gas pressure, and controlled by grain boundary self diffusion.     

Sintering densification and microstructural evolution of injection molding grade 17-4 PH stainless steel powder

Densification behavior is investigated by means of dilatometry for powder-injection-molded (PIM) and die-compacted 17-4 PH stainless steel during sintering in pure H₂ and H₂ + N₂. The corresponding microstructural evolution is examined by quenching in a vertical furnace at various stages of sintering. The results show that in comparison with pure H₂, the H₂ + N₂ atmosphere retards densification and produces a lower shrinkage rate above 1220 °C. The X-ray diffraction (XRD) analysis reveals the presence of a stabilized austenitic microstructure during H₂ + N₂ sintering. In pure H₂, however, δ ferrite forms and grows above 1220 °C, producing a higher shrinkage rate. In terms of the atomic diffusivity, δ-ferrite formation is beneficial to pore shrinkage and densification.     

Creep of laminated aluminum composites

The creep behavior of a laminate system consisting of alternate layers of pure aluminum and SAP (sintered aluminum powder) sheet has been examined in the temperature range 323 to 473 K and in the stress range 35 to 68 MN m⁻². It was observed that secondary creep strain in the laminates was greater than in elemental SAP; the secondary creep strain rate in laminates was lower than that in pure aluminum and the creep rate decreased with increasing fracture of SAP. A stress exponent (n) value of ∼20 was observed for most of the laminates and was reasonably constant for 3, 5, 7, and 9 ply laminates and volume fractionsV _f ) in the range 0.3 <V _f < 0.65. For higher volume fractions of SAP the mechanical behavior of the laminates was similar to that of SAP. The experimental activation energy for creep of 30.5 ± 5 Kcal mol⁻¹ correlates well with that for self-diffusion in aluminum. Laminating induced appreciable ductility to the SAP. Formerly Postgraduate Research Student, Department of Metallurgy, University of Manchester/UMIST     

Sintering behavior of bi-modal powder compacts

Time dependent sintering of a bi-modal powder compact, consisting of two regions which sinter at different rates, is analyzed in detail. Complete solutions are obtained for the internal stress and for the densification rate. The important feature of the analysis is that it combines densification with deviatoric creep, since creep serves to relax the stress concentration produced by incompatible densification. The structure of the bi-modal compact is assumed to consist of a sphere of one type of material embedded in a matrix of the other material. Two cases are considered. In one case the matrix, and in the other the sphere, is assumed to sinter faster. The maximum tensile stress generated by incompatible sintering is found to be sensitive to a parameter, β, which is the ratio of the rate constant for creep, and the rate constant for densification. A large value of β reduces the magnitude of the stress. The generation of flaws as a result of this stress is considered. The influence of inhomogeneous sintering on the overall rate of shrinkage is calculated; we find that unless β is large the densification rate of the composite will deviate significantly from the simple rule-of-mixtures.     

Plastic creep flow effects in the diffusive cavitation of grain boundaries

We analyze the growth of cavities along grain interfaces by the combined processes of grain boundary diffusion and plastic dislocation creep in the adjoining grains. It is shown that the coupling between the processes can be expressed in terms of a parameter L, which has the dimensions of length and which is a function of material properties, temperature and applied stress; L decreases with increasing temperature and stress and has, e.g., values in the range of 0.25 to 25 μm for various pure metals when stressed to 10⁻³ × shear modulus at 0.5 T_m. The contribution of dislocation creep to the cavity growth rate is shown to be negligible when L is comparable to or larger than the cavity spacing, but significant interactions occur, leading to growth rates very much in excess of those predicted on the basis of boundary diffusion alone, when L is comparable to or smaller than the cavity size. The coupling occurs because extensive dislocation creep allows local accommodation of matter diffused into the grain boundary from the cavity walls.The cavity growth rate is analyzed by formulating a new variational principle that governs combined processes of grain boundary diffusion and non-linear viscous creep, and by implementing this principle through the finite-element method to obtain numerical solutions. Results for the cavity growth rate are presented for a wide range of ratios of L to cavity spacing, and of cavity radius to spacing. Also, results are presented for the total growth time of cavities from an initial size to final coalescence.     

The effect of particle reinforcement on the creep behavior of single-phase aluminum

The effect of TiC particle reinforcement on the creep behavior of Al (99.8) and Al-1.5Mg is investigated in the temperature range of 150 °C to 250 °C. The dislocation structure developed during creep is characterized in these materials. The addition of TiC increases creep resistance in both alloys. In pure aluminum, the presence of 15 vol pct TiC leads to a factor of 400 to 40,000 increase in creep resistance. The creep strengthening observed in Al/TiC/15_p is substantially greater than the direct strengthening predicted by continuum models. Traditional methods for explaining creep strengthening in particle-reinforced materials(e.g., threshold stress, constant structure, and dislocation density) are unable to account for the increase in creep resistance. The creep hardening rate(h) is found to be 100 times higher in Al/TiC/15_p, than in unreinforced Al. When incorporated into a recovery creep model, this increase inh can explain the reduction in creep rate in Al/TiC/15_p. Particle reinforcement affects creep hardening, and thus creep rate, by altering the equilibrium dislocation substructure that forms during steady-state creep. The nonequilibrium structure generates internal stresses which lower the rate of dislocation glide. The strengthening observed by adding TiC to Al-1.5Mg is much smaller than that found in the pure aluminum materials and is consistent with the amount of strengthening predicted by continuum models. These results show that while both direct (continuum) and indirect strengthening occur in particle-reinforced aluminum alloys, the ratio of indirect to direct strengthening is strongly influenced by the operative matrix strengthening mechanisms. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Reactive hot pressing of nonstoichiometric uranium dioxide

High density UO_2+x pellets have been produced by reactive hot pressing uranyl oxalate at temperatures up to 700°C. Rapid densification occurred during the decomposition reactions resulting in densities in the range 90 to 92 pct of the theoretical. A density of 98 pct of the theoretical value was achieved by further hot-pressing at 650° to 700°C for 30 min. This densification behavior can be related to the nonstoichiometry and submicron sized particles of UO_2+x produced in the decomposition reactions. The kinetics of hot-pressing of powder compacts of this UO_2+x were studied in the temperature range 500° to 700°C. The results were analyzed utilizing models proposed by Fryer. The activation energy of 53 kcal per mole, obtained from this analysis is the same as that for creep of nonstoichiometric urania in the temperature range 975° to 1400°C, suggesting that the mechanism controlling the rate of the final stage of densification may be a creep process.     

Sintering Densification and Microstructure Formation Kinetics in Silicon Nitride Ceramics

Research has been done on how the shrinkage rate and microstructure in the Si₃N₄ Al₂O₃ Y₂O₃ are dependent on temperature and nitrogen pressure during sintering. The shrinkage rate curves alter as the activator content varies. Additional densification may occur above 1800°C because of diffusion through the increased amount of liquid.     

The effects of segregation on the kinetics of irrtergranular cavity growth under creep conditions

Intergranular cavity growth under creep conditions is examined with particular reference to the potential effects of segregation on cavity growth. When cavities are present on all of the boundaries in a polycrystalline solid, they are able to grow in an unconstrained manner. Under these conditions the rate of cavity growth may be controlled by grain boundary diffusion (D _GB), surface diffusion (D _S), or power law creep, which in turn is controlled by lattice diffusion (D _L). When only isolated boundaries are cavitated, cavity growth is constrained and may be completely limited by creep flow of the surrounding grains. The segregation of solute to the grain boundary and the cavity surface can influence the kinetics of cavity growth in several different ways. The reduction in surface energy associated with segregation can increase the rate of cavity growth when the cavities are crack-like. The effects of segregation on grain boundary and surface diffusion can also influence the rate of cavity growth. A phenomenological relation proposed by Borisovet al indicates thatD _GB /D_L decreases with segregation, thus causing grain boundary diffusion controlled cavity growth to be slowed by segregation. In some cases solute additions increaseD _L, thus increasing the rate of creep controlled cavity growth. When these effects are sufficiently large,D _GB can increase with solute additions, in spite of the effect of segregation. In some alloysD _S increases with segregation by several orders of magnitude. Similar effects on cavity growth are not expected even for surface diffusion controlled cavity growth because the rate of cavity growth is limited by other factors whenD _S is very large. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Investigating the Dislocation-Driven Micro-mechanical Response Under Non-isothermal Creep Conditions in Single-Crystal Superalloys

The creep responses of the superalloy CMSX-4 under thermal cycling conditions (900 °C to 1050 °C) and constant load (\( \sigma_{0} = 200 MPa \)) were analyzed using TEM dislocation analysis and compared to the modeled evolution of key creep parameters. By studying tests interrupted at different stages of creep, it is argued that the thermal cycling creep rate under these conditions depends on the creation of interfacial dislocation networks and their disintegration by the γ′-shear of dissimilar Burgers vector pairs.     

High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

The Effect of Aluminum Additions on the Microstructure and Thermomechanical Behavior of NiTiZr Shape-Memory Alloys

The microstructure, thermal cycling, and mechanical behavior of Ni_48.5Ti_31.5?x Zr₂₀Al _x (x?=?0, 1, 2, 3) alloys were studied in the solution-treated and aged condition using microscopy techniques, differential scanning calorimetry, and compression tests. The microscopy techniques used include optical, scanning, and transmission electron microscopy, and three-dimensional, atom?Cprobe microscopy. The results of this study indicated a strong dependence of the transformation behavior on alloy chemistry and thermal cycling. The aluminum additions served to decrease transformation behaviors from 351?K to 596?K (78?°C to 323?°C) and reduce thermal stability. Additionally, aluminum was shown to increase the plateau stress in the aged condition, whereas the formation of coarse-grained intermetallic phases caused the embrittlement of the microstructure, reducing its ductility. The addition of Al resulted in the refinement of the coarse, lenticular precipitates identified as Ni₄(Ti,Zr)₃.     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

Modeling of Bolt Joint Behavior of Cast Aluminum Alloy (A380-T5) by Coupling Creep and Plasticity in Finite Element Analysis

Aluminum casting alloys exhibit creep behavior when the materials are exposed to high temperature and load. In this article, the stress- and temperature-dependent creep behavior of a die casting A380-T5 aluminum alloy was simulated using a classical constitutive model. The bolt-load retention behavior of the material was analyzed in a head bolt joint in an aluminum engine under thermal cycle condition using the finite element method. In this simulation, transient thermal analysis was performed first to calculate the metal temperature at the head bolt joint as a function of time during engine thermal cycling. This temperature was then input as the thermal loading in the subsequent structural analysis to calculate its effect on the bolt-load retention. The finite element analysis (FEA) model for the bolt-load retention simulation includes not only the plasticity in all metal components but also the creep properties of head bolt threads in the cast aluminum engine block. The FEA model was validated by good correlation between the predicted head bolt-load loss and the experimental measurement during engine thermal cycling. The simulation results also indicated that creep in the head bolt threads of cast aluminum engine block was mainly responsible for the load loss in the head bolt joint. This article is based on a presentation given in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties” which occurred March 12–16, 2006 during the TMS Annual Meeting in San Antonio, Texas under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

The effect of rapid thermal cycling on the creep properties of alpha iron

The effect of rapid, low amplitude thermal cycling on the creep properties of alpha iron was investigated. It was found that creep rates under thermal cycling conditions are lower than the creep rates calculated from steady-state isothermal creep experiments. This relative decrease of the creep rate was dependent on the dwell time and the applied stress. The maximum decrease in creep rate occurred when the time at the lower temperature was very short. In single cycle creep experiments it was found that an abrupt temperature decrease was followed by a delay period of zero creep rate. When the temperature was again increased, a period of inverse transient creep was observed. It is concluded that the inverse transient creep is responsible for the relatively lower creep rates observed under thermal cycling conditions. Investigations of the dislocation substructure with the electron microscope did not show any significant changes attributable to thermal cycling. The lowering of the creep rate is tentatively explained on the basis of dislocation pinning by point defects during the cooling part of the cycle which inhibits subsequent dislocation motion during the rest of the cycle and hence decreases the overall creep rate. D. EYLON, formerly with the Department of Materials Engineering, Technion     

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe₃Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH _f ^°(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe₃Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe₃Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe₃Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.     

High temperature creep of silicon carbide particulate reinforced aluminum

The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC_p-6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n, is high (n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q_a, is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC_p-6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC_p-6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC_p-6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.