Discontinuous Precipitation Colonies Research Articles

Phase reversion kinetics of thermally decomposed (α + γ′) phases to γ-phase in U – 10 wt% Mo alloy

Thermally induced phase reversion kinetics of decomposed α + γ′ phases to γ phase was investigated for the U – 10 wt% Mo (U–10Mo) hypoeutectoid alloy using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The as-cast U–10Mo alloy was homogenized to the γ phase at 1073 K for 96 h, and decomposed at sub-eutectoid temperature of 773 K as a function of time from 240 to 1200 h. The decomposition of γ phase initiated via two mechanisms: (1) discontinuous precipitation (DP) or cellular precipitation along the grain boundaries and inclusions; and (2) continuous precipitation (CP) within the γ grains as γ′ needles. After 720 h at 773 K, the DP colonies completely consumed the continuous γ matrix, after which discontinuous coarsening (DC) was observed. The completely-decomposed U–10Mo alloy samples were then annealed at 843, 853, and 863 K as a function of time to document the kinetics of α+γ’ → γ phase reversion. The reference intensity ratio analysis was employed to XRD patterns to quantify the phase amount after each heat treatment. A combinatorial kinetic model, consisting of the classical JMAK nucleation and growth, and spherical particle dissolution, was employed to describe the α+γ’ → γ phase reversion as a function of time and temperature. This phenomenological model provided a good approximation for the phase reversion within the temperature and time ranges examined in this study. Furthermore, a diffusion enhancement approach was employed for a set of typical irradiation parameters to estimate the irradiation effect on the α+γ’ → γ phase reversion kinetics.

An electron backscatter diffraction analysis of grain boundary initiated discontinuous precipitation in U–10Mo

The effect of varied thermomechanical processing on discontinuous precipitation (DP) in U–10Mo was investigated, with specific emphasis on understanding the role of grain boundary misorientation in DP. Varied prior homogenization heat treatment and thermomechanical processing resulted in differences in both the fraction of DP and the colony width, and was attributed to variations in the grain boundary misorientation distribution. Regardless of the degree of DP-based transformation, extensive growth of DP colonies was dominant on 30°–45° misorientation boundaries. Interestingly, misorientation histograms of the deformed and annealed specimens suggest the processing steps involved may have inhibited DP colony growth along a small fraction of these high angle boundaries. Large-area electron backscatter diffraction montages coupled with high resolution mapping suggest symmetric {110}-type interfaces may be important considerations for mitigating extensive DP growth in this alloy system.

Grain boundary precipitation phenomena in an alloy 33 (Cr-Fe-Ni-N) subjected to direct-aging treatments (700 °C and 900 °C)

Precipitation behavior of alloy 33 upon isothermally aging at 700 °C and at 900 °C was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (XEDS). The name, alloy 33, reflects the quasi-equal atomic content of the base elements Fe, Ni and Cr close to 33%, with the addition of Mo and N. The most remarkable features of the microstructural evolution resulting from the aging heat treatments are described as follows: aging at 700 °C leads to homogeneous and heterogeneous grain boundary precipitation, where the discontinuous mode is dominant at this temperature. Conversely, the opposite behavior was observed when the alloy is aged at 900 °C: predominantly homogeneous precipitation together with grain boundary precipitation, however with rare evidence for discontinuous precipitation (DP), even after longer aging times. In the present study, the topology and content of the DP colonies generated by the combined effect of grain boundary migration diffusion and precipitation under isothermal aging at 700 °C were particularly investigated. A variety of morphological features were observed: mainly (1) classical-type microstructure with straight and parallel lamellar precipitates with regular interlamellar spacing and (2) another microstructure with irregular and complex DP colonies. According to our analytical electron microscopy observations, the latter shows that there are more than one precipitate phase within the same colony, which is new to the current theory and experimental evidence on this subject.

Effect of Co on Discontinuous Precipitation Transformation with TCP Phase in Ni-based Alloy Containing Re

The effect of Co on discontinuous precipitation (DP) transformation involving the formation of topologically close-packed (TCP) phase was investigated in three Ni-Cr-Re model alloys containing different levels of Co. One typical TCP phase, σ, was generated within DP cellular colonies along the migrating grain boundaries in experimental alloys during aging treatment. As a result of the increased solubility of Re in the γ matrix and enlarged interlamellar spacing of σ precipitates inside of growing DP colonies, Co addition suppressed the formation of σ phase and associated DP colonies. This study suggests that Co could potentially serve as a microstructural stabilizer in Re-containing Ni-base superalloys, which provides an alternative method for the composition optimization of superalloys.

A quantitative approach to investigate discontinuous precipitation on grain boundary of Ni-based single crystal superalloys

We designed a [001] tilt grain boundary by self-diffusion bonding technique in a single crystal superalloy. The effects of the grain boundary plane as well as its misorientation on discontinuous precipitation (DP) were investigated quantitatively. The anisotropic formation of DP colonies was associated with the anisotropic mobility of grain boundary plane. The critical misorientation for DP transformation is in between 20 and 25° for SXG3 alloy. The method proposed in this work can be used to investigate the misorientation tolerance of grain boundaries that occurs during casting of single crystal turbine blade.

Kinetics of discontinuous precipitation in Cu–20Ni–20Mn alloy

The morphology and growth kinetics of discontinuous precipitation (DP) in a Cu–20Ni–20Mn alloy were investigated in the temperature range of 523–673 K by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A lamellar mixed structure consisting of alternating lamellae of a matrix and NiMn phase was observed in DP colonies. The volume fraction of regions formed by a DP reaction was determined by quantitative metallographic measurements. The kinetics of DP was evaluated on the basis of the Johnson–Mehl–Avrami–Kolmogorov equation, which resulted in a time exponent of approximately 1.5. We confirmed that the nucleation of the discontinuous precipitate was confined to grain edges or boundaries at an early stage of the reaction. The activation energy of DP process was determined to be approximately (72.7 ± 7.2) kJ/mol based on the Arrhenius equation; this result suggests that DP is controlled by grain boundary diffusion. The hardness values exhibited good correlation with the volume fraction of DP; this correlation was attributed to the presence of the ordered NiMn phase.

Some observations on deformation-related discontinuous precipitation in an Al-14.6at.%Zn alloy

The discontinuous precipitation (DP) in a supersaturated Al-14.6at.%Zn alloy in relation to different forms of deformation structures has been investigated with optical and electron microscopy. It has been found that intense surface scribing, followed by short-term ageing at 65 °C, resulted in a recrystallized duplex structure with nanoscale equiaxed β-Zn particles and α-Al grains. In the absence of recrystallization and shear banding, moderate surface grinding increased the transformation kinetics of DP on the alloy surface by an order of magnitude compared with that of the undeformed counterpart. The enhanced transformation kinetics is attributed to intragranular nucleation and growth of DP colonies associated plausibly with dislocation cell wall structures induced by the surface strain. In contrast, bulk deformation by means of cold-rolling (13–66% reduction) and in situ stress-ageing (∼1% strain) both suppressed the development of DP in the alloy. The role of deformation bands as nucleation sites of DP and the driving force determining the development of DP colonies in deformed matrices are discussed.

A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy

The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution.

Kinetics of discontinuous precipitation and type I discontinuous coarsening in Zn–4 at % Ag alloy

A detailed study of the kinetics of discontinuous precipitation (DP) and type I discontinuous coarsening (DCI) in Zn-4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Both DP and DCI are characterized by a predominantly lamellar morphology of the precipitate phase, statistically constant interlamellar spacing and steady state reaction front (RF) velocity at a given temperature. The interlamellar spacing increases with temperature. The RF velocity shows a C-curve behavior for DP, but increases monotonically for DCI, as a function of temperature. DCI is distinguished from DP by a 3-5 times larger interlamellar spacing and 1-2 orders of magnitude lower RF velocity than those of DP under comparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kinetic analysis of DP using the models of Turnbull, Cahn, Hillert, and Petermann and Hornbogen, and of DCI using the modified Petermann and Hornbogen model (by Fournelle) have yielded grain boundary diffusivity data in the temperature range 353-573 K. Subsequent Arrhenius analysis shows that the activation energy of the DP and DCI processes lies between 50-66 kJ mol -1 . The latter is comparable with the activation energy of grain-boundary self-diffusion of Zn and is nearly half that of tracer impurity diffusion (volume/bulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy.

Interface Multiplication Through the Combination of Interface and Lattice Diffusion-Controlled Transformations in Alloys

ABSTRACTA procedure for obtaining interface and grain boundary multiplication, without externally applied deformation, in alloy systems is described. It is shown that an essential prerequisite is the occurrence of, and the possibility of controlling, discontinuous precipitation (DP) reactions in the alloy. Several alloy systems have been studied and experimentally generated, in a controlled volume fraction, DP lamellar products in the following model alloys: Al%Ag; Al-22at%Zn; Cu-7at%In and Ni-8at%Sn. The resulting microstructures have been observed in detail through conventional and analytical electron microscopy, confirming that DP is a transformation controlled by solute diffusion along moving grain boundaries. Although the precipitates correspond to the equilibrium phase, the transformation, as a whole, does not reach thermodynamic equilibrium since high-resolution microanalysis of aged and quenched microstructures reveal that a significant amount of supersaturation is retained in the depleted lamellar matrix. In addition, TEM observations have revealed that the DP product, besides generating a high density of interfaces, is able to incorporate a significant amount of strain energy. On the other hand, the dissolution of the lamellar microstructures has been observed to proceed in both continuous and discontinuous fashions. The former, dominating at high dissolution temperatures, is controlled by volume diffusion and gives rise to the development of new microstructures in the original DP colonies: the generation of dislocations and their dynamic re-arrangement into a cellular sub-structure in Al – base alloys; the formation of new grains, in Cu-In and in Ni-Sn alloys. Operating mechanism are proposed and the nature of the driving force for the observed phenomena is discussed in terms of its chemical, interfacial and strain energy components.

Discontinuous precipitation in a Cd-6 at.% Ag alloy

Discontinuous precipitation (DP) in a Cd-6 at.% Ag alloy has been investigated for the first time. The precipitate phase maintains a lamellar morphology and statistically constant interlamellar spacing under a given isothermal condition in the temperature range studied (333–523 K). The interlamellar spacing increases with an increase in isothermal temperature. The reaction front velocity registers a typical C-curve variation with the inverse of temperature. The reaction rate is maximum at 470 K. The predicted upper limit of DP occurrence in this alloy is 23 K lower than the concerned equilibrium solvus temperature. Continuous precipitation accompanies DP at all temperatures, especially beyond a certain time, and adversely affects the growth kinetics of DP colonies by reducing the local chemical driving force and/or posing physical hindrance to the reaction front migration. An extensive kinetic analysis of DP using the models by Turnbull, Aaronson and Liu, and Petermann and Hornbogen has yielded the grain boundary chemical diffusivity data in Cd-6 at.% Ag for the first time, the activation energy of which lies in the range 55–77 kJ/mol.

Mechanism and kinetics of type II discontinuous coarsening in a Zn-4 at% Ag alloy

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The present study concerns mechanism and kinetics of DCII in a Zn-4 at% Ag alloy in the range 353–513 K following DP at 393 K for 60 h. DCII colonies prefer to initiate either from one or both sides of the interfaces between the former DP colonies. A suitable comparison of the kinetic data reveals that interlamellar spacing (λ,) and steady-state growth velocity (v) values in DCII are significantly different than those in DP. On the other hand, the kinetics of DCIvis-a-vis DCII in terms of λ andv are comparable to each other, though the calculated values of the driving forces between them differ marginally. A detailed kinetic analysis of DCII through the Livingston-Cahn model leads to an underestimation of the activation energy (Qb) of grain boundary chemical diffusion of Ag in Zn-Ag (=30.7 kJ mol−1), whereas the same obtained from the modified Petermann-Hornbogen model (=61.0 kJ mol−1) compares well with that for DP/DCI (reported elsewhere by us), and grain boundary self diffusion of Zn. Considering thatQb in DCII is nearly 50% of the activation energy for volume/matrix diffusion of Ag in Zn, it appears that DCII in the present alloy is a boundary diffusion controlled process.