Precipitation and Growth Kinetics in Mechanically Alloyed Ni–Al

Abstract

The precipitation and growth kinetics ofγ′ precipitates, which are strengthening factors in Ni-base oxide dispersion strengthened (ODS) superalloys, were investigated. The cuboidal-typeγ′ precipitates are formed in conventional arc-melted Ni–Al alloys, whereas spherical-type precipitates are formed in the mechanically alloyed (MAed) specimens. The morphology is controlled by a lattice misfit between theγ′ precipitates and the matrix at the aging temperature of 800°C. The growth kinetics of theγ′ precipitates can be followed by Ostwald ripening. The Arrhenius plot yielded a lower activation energy for the solute atom diffusion in MAed specimens, which is attributed to their high dislocation density and nanosized grains.

Evaluation of Processing Conditions on the γ′ Morphology of an Oxide Dispersion Strengthened Ni‐Based Superalloy

“…To study the microstructural evolution during annealing, the MAed powders were encapsulated in a silica tube sealed in vacuum. Then the annealing was conducted in a temperature range of 400-1300 C for 1 h. To compare the effect of aging treatments on the microstructure, three different heat treatment regimes were used based on similar superalloys: [19,23,36] one-step aging (900 C, 24 h), two-step aging (1150 C, 4 h-870 C, 24 h), and three-step aging (1150 C, 4 h-870 C, 24 h-760 C, 30 h). After different ageing conditions, Vickers hardness of the powders was measured by a HMV-Micro Hardness Tester, SHIMADZU, under a load of 980 mN and dwell time of 10-20 s. Ten indentations were conducted for each condition and the average value was reported.…”

“…[18] This morphology was previously observed in MAed Ni-Al alloys. [19] During aging treatment, the partitioning of alloying elements between the γ-γ 0 phases occurs and it influences the lattice misfit (δ) between the phases defined as [22]…”

“…As the volume fraction of γ 0 increases, the elastic strain energy plays a more dominant role over interfacial energy, thus stabilizing the morphology of the precipitates. [18,19] Ricks et al have observed that the morphology of γ 0 often evolves from spherical to cuboidal shape to arrays of cubes during aging due to elastic strains associated with lattice misfit. [20] Recently, we characterized a new morphology of γ 0 phase as the nanoscale lamellar structure in a new developed mechanically alloyed (MAed) ODS superalloy.…”

Microstructural Development and Coarsening Behavior of γ′ Precipitates in Co–Ni–Al–W-Base ODS Alloys

“…This value of activation energy is much lower than that for the coarsening of £A precipitates in Ni-base alloy (210 303 kJ/mol). 13,14,18) The relatively low activation energy is closely related to the microstructural characteristics of Cobase ODS alloys. It is speculated that the atomic diffusion to produce £A phase is effectively enhanced in mechanically alloyed materials due to the high dislocation density and extremely fine grains, resulting in the reduction of activation energy.…”

Synthesis and Characterization of Nickel-Alumina Composites from Recycled Nickel Powder

“…Especially, powder metallurgical nickelalloy products show improved properties over conventional cast and wrought alloy products. Moreover, nickel-base alloys in the form of superalloys have already proven their usefulness in demanding environments [17][18][19][20]. Alumina has low density, electrical resistivity and coefficient of thermal expansion, high hardness, and specific stiffness, and is one of the most stable oxides, providing excellent oxidation and corrosion resistance and good high-temperature mechanical properties.…”