Wolfgang A. Kaysser

Analysis of particle growth by coalescence during liquid phase sintering

Abstract A statistical approach has been applied to particle coarsening during liquid phase sintering assuming direct particle coalescence as basic growth mechanism instead of Ostwald ripening. The coalescence process controlled by diffusion through the melt results in an increase of the average particle size proportional to the cube root of sintering time. After a short initial sintering interval the particle size distribution approaches a unique normalized form which is considerably broader than forms predicted by Ostwald ripening theories. The effect of preferred coalescence possibilities for definite particle size ranges and the effect of concurrent coalescence and Ostwald ripening are treated and discussed.

Particle growth by coalescence during liquid phase sintering of Fe-Cu

Abstract Particle growth during liquid phase sintering of Fe-30 wt% Cu and Fe-60 wt% Cu was measured and compared with a recently developed coalescence theory. Measured normalized particle size distribution, time-dependence of average particle size and dependence of growth rate on solid phase volume fraction agree well with calculations based on the assumption of a diffusion-controlled coalescence process where all component particles are assumed to have equal coalescence frequency. Slight discrepancies can be explained as the results of nonspherical particle shape and of favored coalescence opportunity for larger particles.

Melting of CuIn solid solutions at small superheating by droplet formation and liquid film migration

Abstract The melting of homogenized, polycrystalline α-CuIn solid solutions was studied by Differential Thermal Analysis and metallographic methods. At small superheating melt forms by two mechanisms. The formation of homogeneously distributed droplets in the interior of the grains and the subsequent diffusion of the excess indium to these nuclei dominates, if the solid solution is heated up rapidly (≈ 20 K s−1) up to 50 K above its solidus temperature. At small heating rates of ≈ 8.3·10−1 K s−1, homogenization takes place by the formation of thin liquid films at the grain boundaries and their subsequent migration. The moving liquid films accumulate the excess indium from the dissolved supersaturated solid solution and leave a solid solution of equilibrium concentration in their wake. From the correspondence between the experimental migration velocities and calculated values it is concluded, that coherency strains in the solid adjacent to the liquid film provide the driving force for the second mechanism.

GROWTH OF MO GRAINS AROUND AL2O3 PARTICLES DURING LIQUID-PHASE SINTERING

Abstract The growth behavior of Mo grains in contact with Al 2 O 3 particles during liquid phase sintering was studied in 95.4 Mo-4 Ni-0.6 Al 2 O 3 (wt%). Sintering of the powder compacts at 1460°C was repeatedly interrupted by cooling to 1300°C and reheating to 1460°C. Layers grown on Mo grains during each sintering interval between the interruptions were revealed by etch boundaries. The Mo grains grew around Al 2 O 3 particles by solution-reprecipitation. In contact areas the Mo grains and the Al 2 O 3 -particles were separated by thin liquid films. Deposition or loss of material in contact areas resulting from material transport in the thin liquid film was found to be slow. During prolonged sintering some Al 2 O 3 particles were trapped within growing Mo grains.

Grain Boundary Migration in Iron During Zincification

Diffusion of a solute into a polycrystalline sample at temperatures where only grain boundary diffusion is active has been observed to induce otherwise stable grain boundaries to migrate. This phenomenon has been observed in many binary metal systems1–5 under a variety of experimental conditions. The region swept by the boundary has a much higher solute concentration than can be accounted for by volume diffusion. The solute is left behind as the boundary migrates, while the region ahead of the migrating boundary remains solute free. Since no large scale size change in the samples is observed (above that due to a change in the molar volume), solvent atoms must leave the interior of the sample and diffuse to the surface via grain boundaries, or cause dislocation motion or void formation. Besides this net flux of solvent atoms from the grain boundary area, a net flux of the solvent atoms across the boundary is necessary for boundary motion. While it is clear that the driving force for this boundary motion must be the reduction in the total energy of the system that accompanied mixing, how the driving force couples with the atomic transport necessary for boundary motion is not known4.

A Comparative Study on the Influence of Nb and Ti Additions to Different Processed Atomized Nial Powders.

Prealloyed powders of NiAl, NiAl-5Ti, NiAl-5Nb and NiAl-5Ti-5Nb (at.%) were gas atomized with an average particle size of 90 μm and cooling rates of > 10 4 K/s. The powders were attritor milled under Ar atmosphere reducing the average particle size to 1.5 μm. Sintering CIP compacts developed microstructure and density distributions, which allowed subsequent containerless HIPing to near full density (>98.5%) and a final grain size lc , values from 3.8 to 14.4 and 15.3 MPa√m, respectively. For comparison, HIPed materials from the as-atomized powders were also tested.

Hip and Sinter-Hip of Ternary NiAl-X Alloys

Prealloyed powders of NiAl, NiAl-5Ti and NiAl-5Nb (at.-%) were gas atomized with an average particle size of 90 μm and attritor milled under Ar atmosphere reducing the average particle size to 1.5 μm. Sintering CIP compacts developed microstructure and density disributions, which allowed subsequent containerless HIPing to near full density (>98.5%) and a final grain size < 8 μm. Alloying of NiAl with 5at% Nb or Ti increased its hardness, Young’s modulus, toughness and creep resistance. The room temperature fracture path changed from primarily intergranular (NiAl) to primarily transgranular (Nb, Ti alloyed), increasing the KIC values from 3.8 to 14.4 and 15.3 MPa\( \sqrt m \), respectively. For comparison, HIPed materials from the as-atomized powders were also tested.

The Prediction of HIP Parameters for Intermetallic Prealloyed Ni — Al Powder

Rapid solidification processes have received considerable attention as a suitable method to produce spherical powders with fine grain size, increased chemical homogeneity and extended solid solubility. Depending on the type of process the material’s response to rapid heat extraction differs widely. Microstructures of high-grade homogeneity can be obtained if process control during atomization allows a high undercooling of the particles prior to solidification. Undercooling controls the solidification rate. The solidification front velocity must exceed the diffusivity of atoms in the melt at solidification temperature if segregation is to be avoided. High pressure Ar gas atomization is one of the preferred processes for producing polycrystalline, narrow size distribution spherical powder particles with inner grain sizes of only a few micrometers. These attributes are beneficial for HIP consolidation. Transferred to the production of fine grained intermetallic compounds these attributes offer promising properties for technological applications. In a previous paper experiments were described where Ni50Al50, Ni75Al25 and nonstoichiometric prealloyed powders were produced by Ar-gas atomization (1). The consolidation of these fine grained intermetallics with ordered lattice structure by plastic deformation was limited to the very beginning of the densification. Grain boundary and volume diffusion were also limited by the ordered structure. Full density material was achieved only if HIP cycles reach approx. 0.8 Tm. Grain growth decreased the ductility (2) and, hence, cycle times should be optimized to a minimum length to obtain full density and fine grained HIPed material. The purpose of this paper is to demonstrate the collection of data from an actually used material tuning the sensitive parameters in HIP diagram calculations (3).