Coarsening Rate Constant Research Articles

Effects of Temperature and Time on Microstructural Evolution of Semisolid 5A06 Aluminum Alloy: Preparation of Semisolid Billets in Ellipsoid Solid Phase

To shorten the semisolid billet process and reduce its cost, the effects of isothermal temperature and soaking time on the microstructural evolution and coarsening rate during directly heating room temperature hot-rolled 5A06 aluminum alloy to semisolid temperature were investigated. Samples cut from the hot-rolled plates were heated to semisolid temperatures ranging from 580 to 620 °C, and soaking for 5-30 min. The microstructure of samples was characterized by optical microscopy, scanning electron microscopy with energy-dispersive x-ray spectroscopy, and Image-Pro Plus software. The results showed that the microstructure contained ellipsoidal solid grains with the quantitative parameters of the average grain size of 143 µm and averaged shape factor of 0.55 under an optimal condition of 615 °C for 20 min. Recrystallization occurred within 5 min of holding time, increasing temperature can promote recrystallization, which is beneficial to spheroidization of solid phase, and it will also accelerate melting and the growth of solid particles. Extending soaking time, the liquid phase increased gradually and the change of solid particles was complicated. Abnormal accumulation of liquid phase was found, which will lead to the formation of irregular structure. The reasonable coarsening rate constants ranged from 1319 to 3454 μm3 s−1 under the desired treatment parameters. The segregation of Mg occurred at the grain boundaries, as well as around the inclusions within the grains.

Coarsening kinetics of γ′ precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics of γ′ precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900, 950 and 1000 °C. After heat treatment, well-defined cuboidal γ′ precipitates with low misfit was obtained within the experimental alloy. Then coarsening rate constants and particle size distribution (PSD) of γ′ phases were calculated and specified based on the measured precipitate sizes for varying periods of aging times from 100 to 2000 h. After aging for 2000 h, γ′ precipitates maintained cubical shape at 900 °C, while exhibited sphere at 950 and 1000 °C. Coarsening models based on diffusion-controlled process with a functional relationship of r3 vs. t (classic Lifshitz-Slyozov-Wagner coarsening model) and interface-controlled model with a function of r2 vs. t (trans-interface diffusion-controlled coarsening model) were investigated to fit between the experimental results and theoretical analysis. It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model, while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through the γ/γ′ interface. The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models. Finally, coarsening mechanism could be divided into four regimes: (i) coarsening by diffusion-controlled; (ii) coarsening by diffusion and interface co-controlled; (iii) coarsening by interface-controlled; (iv) coarsening by interface-controlled accompanied with γ′ coalescence.

Modeling of dendrite arm fragmentation and dendrite arm coarsening⋆.

A two-dimensional quantitative cellular automaton (CA) model is employed to simulate dendrite arm fragmentation and dendrite arm coarsening in mushy zones. The phenomenon of dendrite arm fragmentation of an Al-Cu alloy during heating is well represented by the CA simulation, and it is analyzed in detail by comparing the local actual concentration and local equilibrium concentration. The CA simulations for the dendritic microstructures of SCN-ACE alloys during isothermal holding in a mushy zone reproduce the typical dendrite coarsening features as observed in experiments. The effects of holding temperature and alloy composition on the microstructures and dendrite coarsening kinetics are investigated. It is found that the melting of small dendrite arms and interdendritic groove advancement are the two main mechanisms in dendrite coarsening. The mechanism of coalescence by joining arm tips is more likely to take place at a lower temperature or for a lower alloy composition, while the dendrite arm fragmentation mechanism tends to occur at a higher temperature. The coarsening rate constant is found to decrease with increasing holding temperature and alloy composition.

Effects of Cr content on compositional evolution and precipitation kinetics of γ′ phase in Ni–Al–Cr alloy: 2D phase-field simulation

The two-dimensional phase-field simulation coupled with the sublattice model is utilized to simulate the γ′ precipitation in Ni-12 Al-x Cr at.% (x = 8, 9, 10) alloys aged at 1073 K for 831 h. The compositional evolution of Cr and Al, and the precipitation kinetics of γ′ phase are studied. At the nucleation and growth stage, the concentration distribution of Cr and Al is characterized by an Al enrichment and Cr depletion at the γ/γ′ interface. As Cr content increases from 8 to 10 at.%, the partition ratio of Al and Ni into γ′ phase is promoted, and the equilibrium volume fraction of γ′ phase is increased, while the Al and Ni atoms at the corner site of L12-Ni3(Al, Cr) sublattice are replaced by Cr atoms partly. Besides, the coarsening rate constants of steady-state coarsening (166.2 < t < 831 h) are less than that of growth and coarsening stage (4.2 < t < 50.0 h), the increased Cr content promotes the coarsening of γ′ phase, and the average radius of γ′ phase obeys the time exponent 1/3 well for a high Cr content alloy.

Coarsening behavior of γʹ precipitates and compression deformation mechanism of a novel Co–V–Ta–Ti superalloy

The coarsening behavior of γʹ precipitates at different temperatures and the compression deformation mechanism of a novel Co–V–Ta–Ti superalloy are investigated. It is found that the coarsening behavior of the γʹ precipitates follows the classical Lifshitz-Slyozov-Wagner (LSW) model based on measured results of the temporal evolution of the mean radius, volume fraction, and number density of the γʹ precipitates. The coarsening rate constant, K, of the γʹ precipitates is strongly dependent on the aging temperature, and it increases nearly ten times with the aging temperature rising from 800 to 900 °C. Within this temperature range, the activation energy for coarsening of the γʹ precipitates is determined to be 230 kJ/mol. This value is close to the activation energy for the diffusion of V in Co, suggesting that the coarsening rate of γʹ precipitates is limited by the diffusion of V element. A lot of slip bands are observed in the alloy after compression deformation, which shows the crystal slip is the primary deformation mechanism for the Co–V–Ta–Ti alloy. It is found that the dislocations bypass the γʹ precipitates below 700 °C in the alloy, and pairs of dislocations shear the γʹ precipitates with a high density of stacking faults (SFs) above the peak temperature of 700 °C.

Thermoelectric performance of thermally aged nanostructured bulk materials—a case study of lead chalcogenides

The performance of nanostructured bulk thermoelectric material is shown to be enhanced because of phonon scattering. However, it is well-known that microstructures evolve by thermal aging, and their stability at the operating temperature is of vital importance to the success of the device. Here, we investigate the effect of long-term annealing (up to 8 weeks) on thermoelectric performance and microstructure of a high-efficiency Na-doped multiphase quaternary Pb chalcogenides.A zT of ~1.7–1.9 in the 700–850 K temperature range is achieved at all stages of thermal aging. A redistribution of Na-dopant between phases above ~630 K is believed to explain an increasing electrical resistivity in aged samples. This is balanced by a reduced total thermal conductivity as thermal aging time increases, possibly because of an increased concentration of nanoscale precipitates in samples aged for 8 weeks. Intermediate annealing of nanostructured lead chalcogenides has been shown to be an effective means of preparing thermally stable, high-performance thermoelectric materials. We have also determined the precipitates coarsening rate constant, k, to be 0.0206 μm/h1/3.

Quantitative Phase-Field Simulation of Composition Partition and Separation Kinetics of Nanoscale Phase in Fe-Cr-Al Alloy

Phase separation of the Cr-enriched nanoscaleα′phase in the Fe-38 at.% Cr-10 at.% Al alloy is studied by utilizing phase-field simulation. The partition of elements in theαandα′phases is clarified with the composition evolution through theα/α′phase interface, and the separation kinetics is quantitatively investigated by the temporal evolution of the size and volume fraction of theα′phase. Aluminum partitions into the Fe-enrichedαphase and depletes in theα′phase, and the partition coefficient decreases as the temperature changes from 720 K to 760 K for the steady-state coarsening stage. As the temperature increases, the initial change rate of the volume fraction of theα′phase is faster, indicating an accelerated phase separation. At the coarsening stage, the average particle distance and coarsening rate constant of theα′phase increase with increased temperature, and the ratio of the Ostwald ripening is dominating compared with coalescence coarsening. The element partition and kinetics evolution of theα′phase with temperature are helpful for the morphology and property predication of nanoscale precipitates.

Research on the coarsening mechanism of precipitations and its effect on toughness for nickel-based weld metal during thermal aging

Abstract

Investigation of intrinsic correlation between microstructure evolution and mechanical properties for nickel-based weld metal

The microstructure evolution was characterized in detail for nickel-based weld metal with multi-layers manufactured by narrow gap arc welding technology after thermal exposure at 750 °C up to 12,000 h. The coarsening kinetics of precipitations and its intrinsic correlation with mechanical properties were investigated and revealed. As exposure time lasted, the hardness of weld metal increased. The toughness decreased dramatically and the ripening rate of γ´ phases dropped with a slow rate of 0.0047 nm/h, finally kept stable rate when exposure time exceeded 8000 h. The coarsening rate constant of 3.86 × 10−29 m3/s for γ′ phases indicated lower coarsening kinetics and higher ripening resistance, which contributed to the good thermal stability for the weld metal. A mass of needle-like M6C carbides was found among interdendritic regions and on the grain boundaries, which became coarser with increasing exposure time due to the segregation of element Mo. Another finding was that spherical γ′ phases grew at the expense of the smaller ones and achieved stabilization at long-term exposure time. As the main factor for reducing toughness, the growth of needle-like shape M6C could be inhibited by the γ′ phases around them via preventing Mo and Cr as M6C-forming elements diffusing, giving rise to the stable toughness.

Microstructural Evolution of ZL104 Aluminum Alloy Semi-Solid Billet Fabricated by RAP Process

In this study, ZL104 aluminum alloy supplied in cold rolled state was introduced in recrystallization and partial melting (RAP) process to fabricate semi-solid billets. During the RAP process, samples cut from cold rolled ZL104 aluminum plate were heated to different semi-solid temperatures, and the effects of isothermal treatment parameters on the microstructures of semi-solid billets were investigated. Results showed that, with the increase of isothermal holding temperature and time, both the average grain size and the shape factor were increased. Namely, the shape of solid grain was more and more spherical, but the size of solid grain was larger and larger, which may be not suitable for semi-solid forming. The size of liquid droplets was increased while the number of liquid droplets was decreased with increasing the isothermal holding temperature and time. Microstructural coarsening of solid grain were attributed to coalescence and Ostwald ripening mechanisms, however, the latter one played a more and more important role with the increase of isothermal holding time and temperature. Additionally, The optimal isothermal holding temperature and time are 570 °C and 5 min, respectively, and the coarsening rate constant is 1357.2 μm3/s at 570 °C.

Microstructural evolution and mechanical properties of thixoformed 7075 aluminum alloy prepared by conventional and new modified SIMA processes

Abstract The microstructural evolution during semi-solid processing and thixoformability of a 7075 alloy prepared by conventional and new modified strain induced melt activation (SIMA) processes were comparatively investigated in this paper. The semi-solid slurries were thixoformed at 600 °C, at which temperature the solid fraction was estimated to be 0.8. The coarsening process of the semi-solid samples was described using Lifshitz–Slyozov–Wagner theory and the effect of pre-deformation on the coarsening kinetics of the solid particles was discussed. The coarsening rate constant of the new modified SIMA sample showed a remarkable decrease compared to that of the conventional SIMA sample. Microstructural and mechanical investigations indicated that the sample with a near-equiaxed microstructure deforms through the plastic deformation of solid grains mechanism. However, the sliding of solid grains and flow of liquid incorporating solid grains mechanisms were dominant in the sample with a globular microstructure. Also, it was observed that the yield and ultimate strengths and hardness of the sample prepared by the new modified SIMA process after thixoforming and T6 heat treatment increased by about 15 %, 10 % and 25 % respectively, compared to those of the conventional SIMA sample.

Formation of spheroidal microstructure of semisolid Al–Zn–Mg–Cu alloy prepared by RAP and modified SIMA

The aim of this paper is to characterize the evolution of fine and spheroidal microstructure of Al–Zn–Mg–Cu alloy by strain-induced melting activation (SIMA) and recrystallization and partial remelting (RAP) routes. The phase transformation and microstructural evolution were investigated during the isothermal treatment in a wide semisolid temperature range from 550 to 610 °C. Effects of isothermal temperatures and processing routes on coarsening rate constant were also analyzed. The results show that during the semisolid isothermal treatment, recrystallization process occurred and intermetallic particles were gradually dissolved into the liquid matrix once the heating temperature was above the solidus. Further extension of soaking time led to the coarsening and spheroidization of the solid grains. The increase in the soaking temperatures raised the liquid fraction and accelerated the spheroidization process. Smaller spheroidal grains and better spheroidization effect were obtained in SIMA microstructure in comparison to RAP microstructure. The coarsening rate constants were, respectively, 319 and 454 μm3·s−1 in SIMA samples after soaked at 600 and 610 °C. The coarsening rate constants of 349 and 522 μm3·s−1 were achieved in RAP microstructure.

Coarsening behavior of γ′ precipitates in the γ'+γ area of a Ni3Al-based alloy

Abstract The morphological evolution and coarsening kinetics of γ′ precipitates in the γ'+γ area of a Ni3Al-based alloy during isothermal aging at temperature range of 800 °C to 1000 °C have been investigated. For the aging temperature above 900 °C, the average size of γ′ precipitates increases and the morphology of γ′ precipitates gradually changes from cuboidal to strip-like and L-shaped with increasing aging time up to 50 h. The temporal evolution of the average size of γ′ precipitates is evaluated by the experimentally measured data and it suggests that the cube rate law fits better for experimental data at the early stage coarsening, while the square rate law fits better for the later stage coarsening. Moreover, using these determined values of coarsening rate constants which determined by using a coarsening rate equation based on the LSEM model, the activation energy of the early stage and later stage coarsening have been calculated as 174.2 kJ/mol and 186.3 kJ/mol, respectively.

Effect of Coherent Lattice Mismatch on the Morphology and Kinetics of Ordered Precipitates

The morphological evolution and precipitation kinetics of ordered precipitates under the influence of lattice mismatch were studied by using the phase field model. The variation of the average aspect ratio of the precipitates and their distributions was investigated quantitatively, and the relationship of particles shape and particles fraction with the variation of the aspect ratio was determined. As the lattice mismatch increases, the coarsening rate constants and the particles number density decrease, the particles size distribution (PSD) becomes narrower, and the peak value of the PSD becomes lower than the value obtained by the Lifshitz–Slyozov–Wagner theory, while it is close to the value obtained by the Brailsford–Wynblatt theory. The change rate of the particles number density shows four-stage characteristics and decreases as the lattice mismatch and time increase, indicating that the precipitation and coarsening are retarded for a large lattice mismatch and that a large lattice mismatch results in fewer particles being dispersed in the matrix phase. The morphology and the particles number density show that a suitable lattice mismatch can enhance the mechanical properties of the alloys.

Temporal evolution of a model Co-Al-W superalloy aged at 650 °C and 750 °C

The temporal evolution of a γ(f.c.c.)/γ’ (L12) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min–4096 h) and 750 °C (10 min–256 h) is studied utilizing atom-probe tomography (APT), scanning electron microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L12) volume fraction, and mean precipitate radius, <R2-D(t)>, are determined. Coarsening rate constants and temporal exponents are calculated for <R-D(t)> of the γ’ (L12)-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of <R2D(t)> of the γ’ (L12)-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L12)-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L12) volume fractions for aging at 650 °C and 750 °C being ϕ = 53% and 54%, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L12)-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L12)-nanoprecipitates form in a non-uniform or multimodal distribution.

Morphology and kinetics evolution of γ′ phase with increased volume fraction in Ni–Al alloys

The morphological evolution and precipitation kinetics of γ′ phase in Ni–Al alloys with the variation of volume fraction are studied by using phase-field simulation. The variation of morphology, particles number density and coarsening rate constants are quantitatively clarified with increased volume fraction. The shape of γ′ phase changes from square to rectangular and then to connected strip structure, so the average aspect ratio of γ′ phase increases. The change rate of particles number density has an uniformly increase at the nucleation and growth stage, while it increases firstly and then decreases for volume fraction greater than 30% at the coarsening stage. The coarsening rate constants decrease for the volume fraction less than 20%, while increase when the volume fraction is greater than 30%. In addition, the width of the particles size distribution (PSD) broadens and the peak value decreases, the peak position shifts from 1.0 to 0.5, the PSDs deviate from the theoretical value by the Lifshitz-Slyozov-Wagner for the large volume fraction.

Influencing factors of the coarsening behaviors for 7075 aluminum alloy in the semi-solid state

The kinetics of microstructural coarsening for the semi-solid wrought 7075 aluminum alloy was determined. The variation of the coarsening rate constant K with the increasing liquid fractions and the corresponding coarsening mechanisms were determined for the recrystallization and partial remelting-processed sample. The effect of plastic pre-deformation on the value K was considered for equal channel angular pressing-based stain-induced melting activation-processed sample. A succinct review of the attempts to understand the various parameters involved in grain growth in this study and some similar literature was also provided. The results show that the rate of grain growth depends on the liquid content, temperatures, alloy composition and processing routes. The volume fraction of liquid influences both the liquid–solid interfacial area and the mean diffusion distance. The actual coarsening rate constant is the summation of independent solid and liquid contribution to the atoms diffusion. Three different coarsening mechanisms, viz. coalescence, inhibited Ostwald ripening and classical Ostwald ripening, are dominant for the elevated liquid fractions, respectively. A greater strain in the solid state or shearing rate in the liquid state usually leads to a lower coarsening rate for the alloys in the semi-solid state due to the facilitated nucleation–growth rate ratio. Further, the wrought aluminum alloys exhibit lower coarsening rate than the cast aluminum alloys due to the inhibited coarsening process by the intermetallic precipitates.

Nanoscale Phase Evolution during Continuum Decomposition of Fe-Cr Alloys.

The continuum decomposition of the Fe-Cr alloys from initial phase separation to steady-state coarsening with concentrations varying from 25 at % Cr and 30 at % Cr to 33 at % Cr aged at 750 K was studied by utilizing three-dimensional phase-field simulations. The dynamic stages of separation of nanoscale Cr-enriched α′ phase were distinguished by the evolution of the volume fraction, particle number density and the average particle radius of the α′ phase. The stage of steady-state coarsening was characterized with an equilibrium volume fraction and decreasing particle number density. The coarsening rate constant by linear fitting of the cube of average radius and aging time shows an increase with the increasing Cr concentration. The time exponents decrease from the growth and coarsening stage to the steady-state coarsening stage and show a dependence on the particles number density at different concentrations. The quantitative evolutions of α′ phase via nucleation growth and spinodal decomposition are theoretically helpful for understanding the microstructure evolution with aging time in Fe-Cr alloys.

Modeling of particle coarsening and precipitation free zones

Previous simulation studies of the effect of volume fraction on the precipitate coarsening process have treated precipitates as point sources or sinks of solute atoms. These studies have demonstrated that, although the average particle size varying with time as is still valid, the coarsening rate constant is an increasing function of volume fraction. In this study we extend these simulation methods to model particle coarsening near grain boundaries. An essential feature of the new model is the solute depletion that occurs at grain boundaries. The simulation results are shown to quantitatively reproduce the following aspects of particle coarsening near grain boundaries in certain alloys: (a) precipitate free zones (PFZs) form near grain boundaries, (b) the width of PFZs increases with time and is proportional to the square root of time, (c) particles at the edge of PFZs are larger than those inside the grain. This novel model is shown to be well suited to describe particle coarsening near grain boundaries. In addition, it reinforces the credibility of the theories built into our mathematical model, i.e., the formation of PFZs near grain boundaries is caused by diffusion of solute atoms.

Effect of Y Addition on the Semi-Solid Microstructure Evolution and the Coarsening Kinetics of SIMA AZ80 Magnesium Alloy

Semi-solid feedstock of AZ80 magnesium alloy modified by trace rare-earth Y element (0, 0.2, 0.4, 0.8 wt. %) was fabricated by strain-induced melting activation (SIMA) in the form of extrusion and partial remelting. The effect of Y addition on the microstructure evolution of the extruded and isothermally heat treated alloy was observed by using an optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and quantitative analysis. The results show that the Y addition can refine the microstructure and make the β-Mg17Al12 phases agglomerate. During the subsequent isothermal treatment at 570 °C, the average solid grain size, shape factor and liquid fractions increased with the prolonged soaking time. The smaller spheroidal solid grains and larger shape factor were obtained in the semi-solid microstructure due to Y addition. The coalescence and Ostwald ripening mechanism operated the coarsening process of solid grains simultaneously. The coarsening rate constants of AZ80M1 (0.2 wt. % Y addition) of 164.22 μm3 s−1 was approximately four times less than the un-modified AZ80 alloy of 689.44 μm3 s−1. In contrast, the desirable semi-solid structure featured, with fine and well globular solid grains, an appropriate liquid fraction, and shape factor was achieved in AZ80M1 alloy treated at 570 °C for 20–30 min.