Coherent equilibrium in alloys containing spherical precipitates

Transactions of the Indian Institute of Metals, 2018

The present study reports the microstructure evolution in multicomponent Al-Ni-X (X = Fe, Cr) alloys. The X-ray diffraction characterization technique and electron microscope attached with energy-dispersive spectroscopic analysis have been used to understand the structural and microstructural evolution in the present multicomponent Al-Ni-X (X = Fe, Cr) alloys. Al 98.6-Ni 0.5 Fe 0.9 alloy shows the presence of (Al) ss , Al 3 Fe and Al 9 FeNi phases. It is observed that the microstructure of Al-Ni-Fe alloy shows the existence of primary dendritic phase of (Al) ss and two eutectics (i.e. globular eutectic: L ? (Al) ss ? Al 3 Fe and lamellar eutectic: L ? (Al) ss-? Al 9 FeNi). Al-Ni-Fe alloy shows superb compressive strength (* 200 MPa) and plasticity (* 70%) at room temperature, while the microstructure of multicomponent Al 92 Ni 4 Cr 4 alloy exhibits the presence of dendritic phases of Al 7 Cr and Al 3 Ni plus ternary eutectic (i.e. L ? (Al) ss-? Al 3 Ni ? Al 7 Cr). Al-Ni-Cr alloy shows good room temperature ultimate compressive strength (* 300 MPa) and plasticity (* 50%). It is important to note that there is no fracture during mechanical testing of investigated Al-Ni-X (X = Fe, Cr) alloys. Keywords Al-based alloys Á SEM Á XRD Á Microstructural evolution Á Mechanical properties & Sandeep Jain

Acta Materialia, 2007

The coarsening behavior of Ni-Al solid-solution precipitates in an Ni 3 Al matrix was investigated in alloys containing 22.0-22.8 at.% Al aged at 650-800°C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol À1 , agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitatematrix interface, controls the kinetics.

Intermetallics, 2012

The effects of alloying elements (Ti, Nb, Ta, Cr, W, Re, Co and Si) on the phase equilibria among Ni (A1), Ni 3 Al (L1 2) and Ni 3 V (D0 22) phases were studied by means of microstructure observation and energy dispersive spectroscopy analysis on thin foils. It was found that the alloying elements can be classified into four categories: (a) Ti stabilizing the phases in the order: L1 2 > D0 22 > A1, (b) Nb and Ta stabilizing the phases: D0 22 > L1 2 > A1, (c) Cr, W, Re and Co stabilizing the phases: A1 > D0 22 > L1 2 , (d) Si stabilizing the phases: Al > L1 2 > D0 22. The elements in the categories (a) and (b) raise the temperature of a eutectoid-type reaction (A1 / L1 2 þ D0 22) while the elements in the category (c) reduce the temperature. The alloying effects were examined in terms of mixing enthalpy and electron concentration.

2006

ABSTRACT Purpose: This research was aimed to investigate the mechanism of Ni 3 Al phase precipitation during longterm process of ageing Cu-Ni-Al. type alloys with particular account of the precipitates morphology changes, including the changes in their size with varying temperature and ageing time, so as to determine an effect of the elastic strain energy on these changes. Design/methodology/approach: Samples of cold-rolled strips from the CuNi16Al5 alloy were solution -treated at 900°C for 1h in argon atmosphere, water quenched and next aged at the temperatures of 450 and 550°C for up to 380 and 760 hours, respectively. Their microstructure was investigated by transmission electron microscopy. Findings: It was found that decomposition of supersaturated solid solution proceeds by nucleation and growth of the coherent precipitates of the L1 2 -Ni 3 Al phase. Their morphology changes as a result of competitive influence of an elastic strain energy, surface energy on the matrix-precipitate inter-phase boundary, and the energy of elastic interaction between precipitates. The L1 2 -Ni 3 Al precipitates nucleate as the spherical ones and grow, forming intermediate sub-structures, until they reach a cubic form with the planes parallel to the{100} planes of a matrix and take privileged positions along the <110> directions. Clear deviations from the LSW coagulation theory and its modifications, demonstrated by the slow-down of the process, have been observed. In the extreme case, growth of the precipitates can be completely stopped in some time ranges of the ageing process. Research limitations/implications: Further research should be concentrated on the precipitation kinetics within a wider range of volumetric fraction of the Ni 3 Al phase in a copper matrix. Practical implications: This effect can be used in practice to stabilise mechanical properties at elevated temperature. Originality/value: The paper contributes to better understanding of the precipitation mechanism in the alloys examined.

Acta Materialia, 2008

The kinetic pathways resulting from the formation of coherent L1 2 -ordered γ'-precipitates in the γmatrix (f.c.c.) of Ni-7.5 Al-8.5 Cr at.% and Ni-5.2 Al-14.2 Cr at.% alloys, aged at 873 K, are investigated by atom-probe tomography (APT) over a range of aging times from 1/6 to 1024 hours; these alloys have approximately the same volume fraction of the γ'-precipitate phase. Quantification of the phase decomposition within the framework of classical nucleation theory reveals that the γmatrix solid-solution solute supersaturations of both alloys provide the chemical driving force, which acts as the primary determinant of the nucleation behavior. In the coarsening regime, the temporal evolution of the γ'-precipitate average radii and the γ-matrix supersaturations follow the predictions of classical coarsening models, while the temporal evolution of the γ'-precipitate number densities of both alloys do not. APT results are compared to equilibrium calculations of the pertinent solvus lines determined by employing both Thermo-Calc and Grand-Canonical Monte Carlo simulation. 6/26/2007

ABSTRACT Purpose: This research was aimed to investigate the mechanism of Ni 3 Al phase precipitation during longterm process of ageing Cu-Ni-Al. type alloys with particular account of the precipitates morphology changes, including the changes in their size with varying temperature and ageing time, so as to determine an effect of the elastic strain energy on these changes. Design/methodology/approach: Samples of cold-rolled strips from the CuNi16Al5 alloy were solution -treated at 900°C for 1h in argon atmosphere, water quenched and next aged at the temperatures of 450 and 550°C for up to 380 and 760 hours, respectively. Their microstructure was investigated by transmission electron microscopy. Findings: It was found that decomposition of supersaturated solid solution proceeds by nucleation and growth of the coherent precipitates of the L1 2 -Ni 3 Al phase. Their morphology changes as a result of competitive influence of an elastic strain energy, surface energy on the matrix-precipitate inter-phase boundary, and the energy of elastic interaction between precipitates. The L1 2 -Ni 3 Al precipitates nucleate as the spherical ones and grow, forming intermediate sub-structures, until they reach a cubic form with the planes parallel to the{100} planes of a matrix and take privileged positions along the <110> directions. Clear deviations from the LSW coagulation theory and its modifications, demonstrated by the slow-down of the process, have been observed. In the extreme case, growth of the precipitates can be completely stopped in some time ranges of the ageing process. Research limitations/implications: Further research should be concentrated on the precipitation kinetics within a wider range of volumetric fraction of the Ni 3 Al phase in a copper matrix. Practical implications: This effect can be used in practice to stabilise mechanical properties at elevated temperature. Originality/value: The paper contributes to better understanding of the precipitation mechanism in the alloys examined.

Acta Metallurgica et Materialia, 1992

The kinetics of coarsening of ?' precipitates in binary Ni AI alloys containing nominally 5.72, 5.74 and 5.78 wt% A1 and aged at 630 C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630uC. The volume fractions of 7' were low, between 0.02 and 0.03. The pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent 7' precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constants decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%, Additionally, the rate constants exceed the values expected from the literature by factors of 10~40. The distributions of particle sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the 7,' precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction.

Metallurgical and Materials Transactions A, 1999

The stiffness constants, c ij , of monocrystalline Ni 3 Al of three different compositions, 23.2, 24.0, and 25.0 at. pct Al, were measured over the temperature range from 300 to 1100 K using the rectangular parallelepiped resonance (RPR) method. The bulk modulus, as well as the shear modulus, Young's modulus, and Poisson's ratio for randomly oriented polycrystalline stoichiometric Ni 3 Al, were derived from the stiffness constants. The data indicate that c 44 is essentially independent of composition, decreasing slightly with increasing temperature for all three alloys. The values of c 11 and c 12 , however, decrease with increasing aluminum content, the difference being small at room temperature but becoming larger at higher temperatures. We find that c 11 and c 12 are not as sensitive to aluminum concentration as is implied by previous results. A comparison of different shear moduli of Ni 3 Al and the saturated Ni-Al solid solution in equilibrium with it indicates that the ordered phase is generally elastically stiffer than the solid solution over the range of temperatures at which coarsening of the Ni 3 Al precipitate has been heavily investigated.

Ni-Al alloy system exhibits particular solidification behavior due to chemical long-range order of the intermetallic compounds Ni 3 Al (L1 2) and NiAl (B2). Using rapid solidification technique their slow solidification kinetics leads to the entrapment of solute atoms and/or disorder, respectively. In the present work containerless processing by applying electromagnetic levitation technique is used to analyze the aspects of dendrite growth, as observed in solidification of undercooled Ni-Al melts. Sharp-interface modeling is used to illustrate the effect of chemical order on the solidification kinetics of intermetallic phases. Both experimental results and theoretical work denote the role of chemical order in solidification of Ni-Al intermetallics.