Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S—B) twinning mechanism is conservative. The possible wider significance of the S—B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Lousiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels

A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between nucleation site distribution and the nucleation rate is demonstrated by computer simulation.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Modeling of grain growth characteristics in three-dimensional domains and two-dimensional cross sections

In the modeling of grain growth of isotropic, single-phase materials using three-dimensional (3-D) Monte Carlo (MC) Potts algorithm, the theoretically expected grain growth exponent was obtained only in the late simulation stages. This article addresses the grain growth simulated by a modified MC Potts model using simple cubic lattices. The grain growth kinetics was analyzed both for the 3-D domain and for two-dimensional (2-D) cross sections. Regression analyses of the grain size data averaged over time, multiple simulations runs, and three cross sections showed that both the Louat function and the log-normal function can be fitted to the data. It was clearly observed that the lognormal function allows a better fit to the 3-D simulation data, while the Louat function is more suited to the cross-sectional data. Furthermore, parabolic grain growth kinetics was obtained both for the 3-D domain and for the cross sections, but the grain growth rates calculated for these cross sections were smaller than that obtained for the 3-D domain. This article is based on a presentation made in the symposium entitled “Three Dimensional Materials Science” during the 2003 MS&T ′03: Materials Science & Technology Conference 2003 in Chicago, Illinois, on November 11 & 12, 2003, under the auspices of the ASM/MSCTS: Materials Science Critical Technology Sector Committee and the TMS/SMD: Structural Materials Division Committee.     

AnIn Situ transmission electron microscope deformation study of the slip transfer mechanisms in metals

The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Density measurements as an assessment of creep damage and cavity growth

The change in density during creep of several polycrystalline metals may be correlated through the expression —Δρ/ρ =B(∈t/d)(σ/G)^q exp (—Q _gb /RT) where —Δρ is the density change, p is the original density, e is the strain,t is the time,d is the linear intercept grain size,σ is the applied stress,G is the shear modulus,Q _gb is the activation energy for grain boundary diffusion,R is the gas constant,T is the absolute temperature, andB andq are constants withq ≃2 to 3. This expression is consistent with the theory of unconstrained grain boundary diffusion growth of cavities provided there is also concomitant strain-dependent nucleation. The expression does not support the power-law growth of cavities, growth by surface diffusion, or constrained grain boundary diffusion growth. Formerly Research Associate, University of Southern California.     

Neutron and Synchrotron X-ray Studies of Recrystallization Kinetics

An overview is given on recrystallization kinetics investigated by neutron and synchrotron X-ray methods. It is shown that during recrystallization, grains belonging to different texture components may have very different growth kinetics. Also, every single grain has its own kinetics different from the other grains. Effects hereof on recrystallization modeling are discussed and an outlook for future experiments and modeling is finally given. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S-B) twinning mechanism is conservative. The possible wider significance of the S-B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Al-TiC composites In Situ-processed by ingot metallurgy and rapid solidification technology: Part I. Microstructural evolution

The present work was undertaken to highlight a novel in situ process in which traditional ingot metallurgy plus rapid solidification techniques were used to produce Al-TiC composites with refined microstructures and enhanced dispersion hardening of the reinforcing phases. Microstructures of the experimental materials were comprehensively characterized by optical microscopy, electron microscopy, and X-ray diffraction. The results show that the in situ-synthesized TiC particles possess a face-centered cubic crystal structure with an atomic composition of TiC_0.8 and a lattice parameter of 0.431 nm. The typical ingot metallurgy microstructures exhibit aggregates of TiC particles segregated generally at the α-Al subgrain or grain boundaries and consisting of fine particles of 0.2 to 1.0 μm in size. The rapidly solidified microstructures formed under certain thermal history conditions contained a uniform, fine-scale dispersion of TiC phase particles with a size range of 40 to 80 nm in an α-Al supersaturated matrix of 0.30 to 0.85 μm in grain size. These dispersed TiC particles generally have a semicoherent relationship with the α-Al matrix. Based on the experimental results, a comprehensive kinetic mechanism of in situ TiC synthesis, which includes a solid-liquid interface reaction between the carbon particles and the Al melt and multiple nucleation and growth of TiC from the Al melt, was proposed. Then, the evolution of the aggregate TiC particles in a superheated melt before rapid solidification, i.e., dissolution, nucleation, and growth of the regenerated TiC dispersed particles, was analyzed. Furthermore, the behavior of rapid solidification kinetics, the nucleation of α-Al on TiC-dispersed particles, and the interaction between TiC particles and the solidification front were documented experimentally and theoretically. These studies provided the theoretical criteria and an experimental basis for the optimum design of this kind of composite.     

Reactive atomization of silicon to formin situ oxide sintering aids

The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder within situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N₂-5 pct O₂ mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y₂O₃ and Al₂O₃, which act asin situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 μm. The powder fines (<38 μm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that β-Si₃N₄ formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of thein situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

Recrystallization and grain growth in metastable beta III titanium alloy

The progress of recrystallization and subsequent grain growth has been systematically investigated in a metastable beta titanium alloy (Ti-11.5 Mo-6 Zr-4.5 Sn). Quantitative evaluation of the kinetics of these processes over a wide range of temperature, deformation, and initial grain sizes has been performed. For a given deformation, the average grain boundary velocity, decreasing with the reciprocal of annealing time, suggests the occurrences of recovery with second order kinetics concurrent with the recrystallization. The amount of deformation, varying from 20 to 80 pct cold reduction and proportional to the stored energy of deformation in the alloy, increases the average grain boundary migration rate during recrystallization by three orders of magnitude. The temperature dependence of the recrystallization rate, however, remains unaffected by the amount of deformation at 83 kcal/mole (347 kJ/mole). The isothermal grain growth kinetics follow the power law such that the time exponent of the process remains at a value of 0.35 at most annealing temperatures. The excellent agreement between the driving force exponent of recrystallization and the time exponent of grain growth based on a model which relates the driving force dependence of the rates of both processes, clearly suggests that the kinetics of these processes are controlled by a single mechanism,i.e. impurity dependent boundary migration. This paper is based on a presentation made at a symposium on “Recovery, Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Crack paths and hydrogen-Afinssisted crack growth response in AlSi 4340 steel

A study of the correlation between crack paths and crack growth response was undertaken to define better the elemental processes involved in gaseous hydrogen embrittlement. AISI 4340 steel fractured under sustained load in hydrogen and in hydrogen sulfide over a range of temperatures and pressures, whose crack growth kinetics have been well characterized previously, was chosen for study. Fractographic results showed that crack growth followed predominantly along prior-austenite grain boundaries, with a small amount of quasi-cleavage, at low temperatures. At high temperatures, crack growth occurred primarily by microvoid coalescence. The fracture surface morphology, which is indicative of the micromechanisms for crack growth, was essentially the same for hydrogen and hydrogen sulfide. Changes in fracture morphology,i.e., crack paths, corresponded to changes in crack growth kinetics, both of which depended on pressure and temperature. There was no evidence for crack nucleation in advance of the main crack, and this suggests that the fracture process zone is located within one prior-austenite grain diameter from the crack tip. The experimental results indicate that microstructure plays an important role in determining crack growth response. The prior-austenite grain boundaries are seen to be most susceptible to hydrogen embrittlement, followed by the (110)_α’ and (112)_α’ cleavage planes. The martensite matrix, on the other hand, is relatively immune. The observed changes in crack growth rate with temperature and pressure in the higher temperature region are explained in terms of the partitioning of hydrogen into the different microstructural elements and the consequent changes in the micromechanisms for fracture. Leave from the Department of Materials Science, Shanghai Jaio Tong University, Shanghai, People’s Republic of China. Formerly Research Associate, Department of Mechanical Engineering and Mechanics.     

Growth kinetics of solid-liquid Ga interfaces: Part I. Experimental

A technique based on the Seebeck effect was used to determine directly the solid-liquid (S/L) interface supercooling and toin situ monitor the interfacial conditions during growth of high-purity Ga single crystals from a supercooled melt. Using this nonintrusive technique, the growth kinetics of faceted (111) and (001) interfaces were studied as a function of the interface supercooling in the range of 0.2 to 4.6 K, corresponding to bulk supercoolings of about 0.2 to 53 K. In addition, the growth kinetics have been determined as a function of crystal perfection related to the emergence of dislocations at the S/L interface. The results show that at low super-coolings, the faceted interfaces grow with either of the lateral growth mechanisms: two-dimensional nucleation-assisted (2DNG) or screw dislocation-assisted (SDG), depending on the perfection of the interface. At increased interfacial supercoolings, however, both growth rates (2DNG and SDG) become a linear function of the supercooling. Application of the existing growth theories to the experimental results gives only qualitative agreement and fails to predict the observed deviation in the kinetics at high supercoolings. A theoretical treatment of the growth of faceted interfaces will be given in Part II of this series.¹ Formerly Research Assistant with the Department of Materials Science and Engineering, University of Florida. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

The formation mechanism(s), morphology,and crystallography of ferrite sideplates

The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy in a high-purity Fe—0.12 wt pct C—3.3 wt pct Ni alloy isothermally transformed at temperatures of 550 °C, 600 °C, 650 °C, and 675 °C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and the allotriomorphs with which they are associated. These results indicate that “secondary sideplates” and the allotriomorphs from which they evolve arenot composed of monolithic single crystals formed by a morphological instability mechanism but are instead composed of multiple crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel suggest that the findings presented here may be applicable to a number of other alloy systems as well. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Effects of electron irradiation on precipitation in Ni-Al alloys

Binary Ni-Al alloys have been irradiatedin situ by high voltage electron microscopy (650 kV) in order to investigate the effects on precipitation. Nucleation and growth of dis-location loops are observed during irradiation in the temperature range 20 to 650°C. In solution-treated specimens, large faulted loops form during irradiation at 500°C. Con-trast analysis shows that loops are Frank-type with l/3 〈HI〉 Burgers vectors and in-terstitial in character. At 650°C and above, loop formation and growth are not observed once γ′ precipitates have formed. In preaged specimens, much smaller loops form dur-ing irradiations at below 500°C. At 500°C and above, no loops are observed in specimens containing large volume fractions of precipitates. The spatial distribution of loops ap-pears rather uniform regardless of the presence of γ′ precipitates except when large in-coherent precipitates exist. The growth of γ′ precipitates is enhanced by irradiation in the temperature range 300 to 750°C. Precipitate coarsening under irradiation follows radiation-enhanced diffusion kinetics. Quantitative analysis demonstrates that the radia-tion-enhanced diffusion coefficient responsible for coarsening of γ′ is independent of the temperature between 300 and 650°C. This result is in agreement with theoretical pre-dictions for the case in which irradiation-produced point defects are lost by their anneal-ing at fixed sinks. Formerly with Department of Metallurgy and Materials Science, Case Western Reserve University, is now with Glidden Metals Group, SCM Corpora-tion, Cleveland, OH 44106 This paper is based on a presentation made at a symposium on “Radiation Induced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Mechanism of steady-state grain growth in aluminum

Grain growth, defined as the increase in volume of the average grain, is found, in its steadystate, to be directly proportional to the time of isothermal annealing. During steady-state grain growth the grain corners are found all to be quadruple points, the grain edges all triple lines and the ratio of corners to faces to edges to be 6:7:12. The rate constant for steady-state grain growth is shown to be calculable from first principles and from properties that can be measured independently of the growth observation. It is the product of four individual constants, namely: 1) a dimensionless topological constant ⊝ that is characteristic of steady-state grain growth in any material, 2) the mobility ώ of the average grain boundary in the specific material, 3) the surface tension y of the average grain boundary in the specific material and 4) a dimensionless structural constant σ which expresses the curvature of surface of the grain boundary in the array of grain forms obtaining in the specific specimen of the material and which can be determined metallographically. The topological changes that constitute steady-state growth are shown to exist as a logical sequence of simple events. Rewritten for publication from theAndrew Carnegie Lecture of the Pittsburgh Section of the American Society for Metals; presented by Frederick N. Rhines on November 9, 1972.     

The evolution of microstructure in Al-2 Pct Cu thin films: Precipitation,dissolution, and reprecipitation

The precipitation, dissolution, and reprecipitation processes of Al₂Cu (θ phase) in Al-2 wt pct Cu thin films were studied. The films were characterized in the as-deposited condition, after annealing at 425 °C for 35 minutes, and after rapid thermal annealing (RTA) at 345 °C, 405 °C, and 472 °C. In the as-deposited samples, the precipitates had a fine even distribution throughout the thin film both at aluminum grain boundaries and within the aluminum grains. Annealing below the solvus temperature caused the grain boundary precipitates to grow and precipitates within the center of aluminum grains to diminish. Annealing above 425 °C caused the θ-phase precipitates to dissolve. Upon cooldown, the θ phase nucleated at aluminum grain boundaries and triple points in the form of plates.In situ heating and cooling experiments documented this process in real time. Analytical microscopy revealed that there is a depletion of copper at the aluminum grain boundaries in regions free of precipitates. The θ-phase precipitates nucleated and grew at the grain boundariesvia a collector plate mechanism and drew copper from the areas adjacent to the aluminum grain boundaries. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.