Effects of annealing on the microstructure and giant magnetoresistance of Co-Cu-based spin valves

The effect of annealing on the microstructure and giant magnetoresistive (GMR) properties of NiO/Co/Cu/Co bottom spin valves is investigated using conventional and high resolution transmission electron microscopy. The value of the GMR of these spin valves is observed to decrease from 12.2 to 2.7 pct after annealing in a vacuum for 30 minutes at 335 °C. This decrease is attributed to an increase in the roughness of the Co and Cu layers. In annealed specimens, grain boundary grooving is also observed in the antiferromagnetic NiO pinning layer at the NiO/Co interface, and the location of these grooves correlates with waviness in the Co/Cu interfaces. An increase in the Néel “orange-peel” coupling between the ferromagnetic Co layers, resulting from the increased roughness of the Co/Cu interfaces, accompanies the degradation of the GMR.     

Instability of grain boundary grooves due to equilibrium grain boundary diffusion

All previous theoretical investigations on grain boundary grooving are based on the assumption, that the grain boundary does not participate in the material transport. In the present paper it is shown theoretically and experimentally for the first time, that by removing this restriction one obtains completely different groove profiles in comparison to those predicted by the classical grooving theory. In couples of an Al-bicrystal contacting an In-A-melt and Cu-bicrystals in contact with a BiCu-melt instabilities of the grain boundary groove are observed. Deep, channel-like grooves at the grain boundary intersection with the solid-liquid interface appear after isothermal annealing. Diffusion of In into Al and Bi into Cu grain boundaries was detected. With a mathematical model it is shown, that the observed instabilities are result of the superposition of two processes: classical grooving due to curvature dependent morphological rearrangement of the grain boundary intersecting the solid-liquid interface and grain boundary diffusion of solute (In or Bi) occurring simultaneously. Profiles of the instabilities and the kinetics of the process are calculated.     

Thermally stable nanomultilayer films of Cu/Mo

Thermal decomposition of nanoscaled (5, 50, and 100 nm) multilayer films has been studied in an immiscible Cu and Mo system. While the onset of nanolayer instability is by thermal grooving at elevated temperatures, the entire nanomultilayer film decomposition can be differentiated into three distinct stages, over a range of temperatures (848 to 1073 K). Stage I is characterized by the onset of grooves, which appear as minor perturbations in otherwise flat interfaces. This is followed by the occurrence of prominent grooves in stage II. Stage III consists of a complete breakdown of the layered structure, with the microstructure composed of grains of Cu and Mo. However, a good layer stability was observed in some of these nanomultilayers (50 Mo:5 Cu and 5 Mo:5 Cu), and stage II is retained up to long times at elevated temperatures. This is attributed to the large difference in the individual layer melting temperatures, combined with unequal film thickness (and hence volume fractions), which inhibits the attainment of an equilibrium groove configuration, up to extended periods of time. Analytical models for thermal grooving in bulk polycrystalline materials were applied to the case of thin film nanomultilayers. The predicted instability kinetics were found to corroborate with the experimentally observed stability of the nanomultilayers, except at very small size ranges (5 nm). A methodology for generating stable nanomultilayer films is suggested as an outcome of this study.     

Thermally stable nanomultilayer films of Cu/Mo

Thermal decomposition of nanoscaled (5, 50, and 100 nm) multilayer films has been studied in an immiscible Cu and Mo system. While the onset of nanolayer instability is by thermal grooving at elevated temperatures, the entire nanomultilayer film decomposition can be differentiated into three distinct stages, over a range of temperatures (848 to 1073 K). Stage I is characterized by the onset of grooves, which appear as minor perturbations in otherwise flat interfaces. This is followed by the occurrence of prominent grooves in stage II. Stage III consists of a complete breakdown of the layered structure, with the microstructure composed of grains of Cu and Mo. However, a good layer stability was observed in some of these nanomultilayers (50 Mo∶5 Cu and 5 Mo∶5 Cu), and stage II is retained up to long times at elevated temperatures. This is attributed to the large difference, in the individual layer melting temperatures, combined with unequal film thickness (and hence volume fractions), which inhibits the attainment of an equilibrium groove configuration, up to extended periods of time. Analytical models for thermal grooving in bulk polycrystalline materials were applied to the case of thin film nanomultilayers. The predicted instability kinetics were found to corroborate with the experimentally observed stability of the nanomultilayers, except at very small size ranges (5 nm). A methodology for generating stable nanomultilayer films is suggested as an outcome of this study.     

Thermally stable nanomultilayer films of Cu/Mo

Thermal decomposition of nanoscaled (5, 50, and 100 nm) multilayer films has been studied in an immiscible Cu and Mo system. While the onset of nanolayer instability is by thermal grooving at elevated temperatures, the entire nanomultilayer film decomposition can be differentiated into three distinct stages, over a range of temperatures (848 to 1073 K). Stage I is characterized by the onset of grooves, which appear as minor perturbations in otherwise flat interfaces. This is followed by the occurrence of prominent grooves in stage II. Stage III consists of a complete breakdown of the layered structure, with the microstructure composed of grains of Cu and Mo. However, a good layer stability was observed in some of these nanomultilayers (50 Mo∶5 Cu and 5 Mo∶5 Cu), and stage II is retained up to long times at elevated temperatures. This is attributed to the large difference, in the individual layer melting temperatures, combined with unequal film thickness (and hence volume fractions), which inhibits the attainment of an equilibrium groove configuration, up to extended periods of time. Analytical models for thermal grooving in bulk polycrystalline materials were applied to the case of thin film nanomultilayers. The predicted instability kinetics were found to corroborate with the experimentally observed stability of the nanomultilayers, except at very small size ranges (5 nm). A methodology for generating stable nanomultilayer films is suggested as an outcome of this study.     

The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb₅Si₃ microlaminates was investigated at temperatures ranging from 1200 °C to 1600 °C. Freestanding Nb/Nb₅Si₃ microlaminates were prepared by sputter deposition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable, with no evidence of layer pinchoff, even after annealing at 1600 °C. However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 °C and 1600 °C, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In terms of phase stability, there was no discernible dissolution of the Nb₅Si₃ layers and no silicide precipitates in the Nb layers following anneals at 1400 °C. Annealing at higher temperatures, though, resulted in the formation of non-equilibrium Nb₃Si on the Nb/Nb₅Si₃ interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nb layers. The most pervasive observed high-temperature breakdown mechanism was chemical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmosphere or by annealing in Ar.     

Faceting transformation and energy of a Σ3 grain boundary in silver

Grain boundary facets forming at the intersection between a grain boundary and the free surface in diffusion bonded Σ3〈011〉Ag bicrystals during prologed annealing have been characterized crystallographically by metallographic methods. It is shown that the observed faceting has qualitatively the same character as that in Σ3〈011〉 grain boundaries in Cu. The energy of an incoherent Σ3 grain boundary in Ag (210 mJ/m²) is determined from the dihedral angle of the thermal groove and the extrapolated literature data on the surface tension of Ag. The facet geometry is discussed with respect to computer simulation data on the inclination dependence of the energy of Σ3 grain boundaries in Cu. The geometrical stability of a grain boundary near the free surface is considered.     

Coarsening of cobalt grains dispersed in liquid copper matrix

Coarsening of spherical Co-rich grains dispersed in Cu-rich liquid matrix is investigated. The specimen compositions are 50 pct Co-50 pct Cu, 40 pct Co-60 pct Cu, and 30 pct Co-70 pct Cu by weight. The annealing temperatures have been varied between 1150 and 1300 °C. The specimens have been prepared by usual powder metallurgy technique from fine Co and Cu powders. Due to the equal density of Co and Cu, the Co-rich grains remain uniformly dispersed in the liquid matrix. The increase of average grain size with annealing time,t, follows closely thet ^1/3 law predicted for diffusion controlled mechanism by Lifshitz, Slyozov, Wagner (LSW) and Ardell. The observed increase of the growth rate with increasing solid grain fraction is a clear evidence for diffusion controlled mechanism. The linear intercept distribution of the grains agrees closely with the predictions for reaction controlled growth in LSW theory, but the result is also consistent with Ardell’s prediction that when the grains grow with small inter-grain distance under diffusion control, the grain size distribution is almost identical to that of the reaction controlled growth in LSW theory. The estimated values of the diffusion constant and its activation energy agree in order of magnitude with the typical values for diffusion in liquid metal. Formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science     

Effect of thermomechanical treatment on microstructure and mechanical properties of AISI 301LN stainless steel

Abstract

A wide range of cold thickness reduction (10–80%) and subsequent annealing were carried out on AISI 301LN stainless steel. X-rays and Feritscope MP30 were used to identify the strain induced α′-martensite phase and its volume fraction respectively. The microstructure was observed by optical micrograph and scanning electron microscope. The results show that shear bands were present and strain induced α′-martensite nucleated at their intersections. The volume fraction of α′-martensite increased with the increased cold reduction by the continuous growth of embryos, which resulted in the increasing yield and tensile strength. The reversion of α′-martensite to austenite occurred after subsequent annealing. The grain size variation of austenite was related to the annealing regime. A good combination of strength and ductility can be obtained after annealing at 650°C for 30 min. The effect of grain size on yield strength conformed with the Hall–Petch relationship in the entire range of our analysis.     

The constitution and phase stability of overlapping melt trails in Ag- Cu alloys produced by continuous laser melt quenching

Coatings consisting of overlapping trails melted with a scanning CW CO₂ laser have been produced on Ag-Cu alloys with the following compositions: Cu 17 at. pct Ag; Cu 37 at. pct Ag; Cu 61.7 at. pct Ag; Cu 71.8 at. pct Ag; and Cu 82 at. pct Ag. The laser beam was scanned at a velocity of 34 cm s¹ and with an intensity of 3.6 MW cm^-2. Selected trails were examined by X-ray diffractometry, optical microscopy, and scanning electron microscopy in the as-irradiated condition and after annealing for various periods of time in the temperature range 100 to 450 ° C. Time-temperature-transformation diagrams based on the annealing studies are presented. Significant amounts of the metastable extended solid solution (γ) were observed in the Ag-rich alloy trails. The silver rich terminal solid solution (α) was also detected, formed probably by solid state precipitation. An α’ phase with lattice parameter lying between that ofy and α was also observed in the Cu 61.7 at. pct Ag alloy. A metastable equilibrium diagram has been constructed and is employed to interpret these observations. The most striking microstructural feature of the trails are bands marking sequential positions of the melt-solid interface. We propose that these bands are evidence for planar, oscillating - steady-state, interface motion. The observation of a periodic cellular breakdown of the planar interface in the Cu 61.7 at. pct Ag alloy is attributed to a diffusional instability previously predicted by Baker and Cahn.     

Dissolution and interface reactions between the 95.5Sn-3.9Ag-0.6Cu, 99.3Sn-0.7Cu, and 63Sn-37Pb solders on silver base metal

Dissolution and intermetallic compound (IMC) layer development were examined for couples formed between 99.9 silver (Ag) and molten 95.5Sn-3.9Ag-0.6Cu (wt pct), 99.3Sn-0.7Cu, and 63Sn-37Pb solders, using a range of solder temperatures and exposure times. The interface reactions that controlled Ag dissolution were sensitive to the solder composition. The Ag₃Sn IMC layer thickness and interface microstructure as a whole exhibited nonmonotonic trends and were controlled primarily by the near-interface solder composition. The kinetics of IMC layer growth were weakly dependent upon the solder composition. The processes of Ag dissolution and IMC layer growth were independent of one another.     

Influence of Nitrogen Content on Thermal Stability and Grain Growth Kinetics of Cryomilled Al Nanocomposites

Nanocomposite powders of Al 5083/B₄C were produced via cryogenic milling (cryomilling) of boron carbide (B₄C) particles in Al 5083 matrix. The effect of milling time (up to 24 hours), and consequential nitrogen content, on grain growth in the nanocrystalline Al 5083 matrix was investigated. Thermal stability was studied at temperatures as high as ~0.96 T _m and annealing times of up to 24 hours. Average grain sizes increased with time and temperature and tended to stabilize after longer annealing times, regardless of nitrogen content. Higher thermal stability was observed in samples with higher nitrogen content, with the average grain size remaining in the range of 30 nm, even after exposure to the most extreme annealing conditions. This behavior was attributed to the retarding effect that nitrides have on grain growth, as a result of pinning grain boundaries. Kinetic studies based on the Burke equation showed two thermally activated grain growth regimes—a low-temperature regime with an activation energy of 15 kJ/mol and a high-temperature regime with an activation energy of 58 kJ/mol.     

Interfacial segregation of Ti in the brazing of diamond grits onto a steel substrate using a Cu-Sn-Ti brazing alloy

Diamond grits were brazed onto a steel substrate using a prealloyed Cu-10Sn-15Ti (wt pct) brazing alloy at 925 °C and 1050 °C. Due to the relatively high concentration of Ti in the brazing alloy, the braze matrix exhibited a composite structure, composed of β-(Cu,Sn), a Cu-based solid solution, and various intermetallic compounds with different morphologies. The reaction of Ti with diamond yielded a continuous TiC layer on the surfaces of the diamond grits. On top of the TiC growth front, an intermetallic compound, composed of Sn and Ti, nucleated and grew into a randomly interwoven fine lacey structure. An interfacial structure developed as the interwoven fine lacey phase was semicoherently bonded to the TiC layer, with the Cu-based braze matrix filling its interstices. The thickness of such a composite layer was increased linearly with the square root of isothermal holding time at 925 °C, complying with the law of a diffusion-controlled process. However, at 1050 °C, the segregation behavior of Ti and Sn to the interfaces between the TiC layer and the braze matrix diminished, due to the increased solubility of Ti in the Cu-based liquid phase. The enhanced dissolution of Ti in the Cu-based liquid phase at 1050 °C also caused the precipitation of rod-like CuTi with an average diameter of about 0.2 μm during cooling. SnTi₃ was the predominant intermetallic compound and existed in three different forms in the braze matrix. It existed as interconnected grains of large size which either floated to the surface of the braze matrix or grew into faceted grains. It also exhibited a nail-like structure with a mean diameter of about 1 μm for the rod section and a lamellar structure arising from a eutectic reaction during cooling.     

Effect of specimen thickness on mechanical properties of polycrystalline aggregates with various grain sizes

The flow stress of polycrystalline Al, Cu, Cu-13 at% Al and Fe have been investigated as a function of the grain size and the specimen thickness. The flow stress decreases with decreasing specimen thickness when the ratio of specimen thickness (t) to grain size (d) is smaller than a critical value. The critical value of t/d increases with decreasing both grain size and stacking fault energy.Based on the experimental results the radius of affected zone, in which individual grains strongly interact with each other caused by a deformed grain, is estimated using a simple model. The result shows that the long-range interaction among individual grains expands into a wide region across the first nearest-neighbor grains.     

Theory of grain boundary grooving in the convective-diffusive regime

A theory of grain boundary gooving is presented in which the material transport occurs by the combined action of convective and diffusive mechanisms. Groove profiles and the kinetics of the process are predicted as a function of time and fluid velocity. It is found the convection changes both the kinetics of the grooving process and the shape of groove profiles in comparison with the case of pure diffusion. The theoretical results are compared with experimental work carried out on Cu bicrystals in a lead-copper melt. It is shown that the effect of macro- and microconvection on grain boundary grooving can be described with the new theory. The agreement between theory and experimental results is satisfactory.     

The Influence of Grain Structure on Intermetallic Compound Layer Growth Rates in Fe-Al Dissimilar Welds

The thickness of the intermetallic compound (IMC) layer that forms when aluminum is welded to steel is critical in determining the properties of the dissimilar joints. The IMC reaction layer typically consists of two phases (η and θ) and many attempts have been made to determine the apparent activation energy for its growth, an essential parameter in developing any predictive model for layer thickness. However, even with alloys of similar composition, there is no agreement of the correct value of this activation energy. In the present work, the IMC layer growth has been characterized in detail for AA6111 aluminum to DC04 steel couples under isothermal annealing conditions. The samples were initially lightly ultrasonically welded to produce a metallic bond, and the structure and thickness of the layer were then characterized in detail, including tracking the evolution of composition and grain size in the IMC phases. A model developed previously for Al-Mg dissimilar welds was adapted to predict the coupled growth of the two phases in the layer, whilst accounting explicitly for grain boundary and lattice diffusion, and considering the influence of grain growth. It has been shown that the intermetallic layer has a submicron grain size, and grain boundary diffusion as well as grain growth plays a critical role in determining the thickening rate for both phases. The model was used to demonstrate how this explains the wide scatter in the apparent activation energies previously reported. From this, process maps were developed that show the relative importance of each diffusion path to layer growth as a function of temperature and time.     

Recrystallization and Grain Growth in NiAl

Experiments have established that upon annealing, warm-worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys. The recrystallization and grain growth kinetics, respectively, can be described by the expressionsX _v = 1 - exp(-Bt ^k ) and −d =Ct ⁿ where X_v is the fraction recrystallized,t is time, −d is the average grain size, andB, k, C, andn are parameters whose magnitudes depend upon temperature. G. R. HAFF, formerly Masters Student, Thayer School of Engineering     

Fabrication and evaluation of Nb/Nb5Si3 microlaminate foils

Alloys of Nb and Nb₅Si₃, and in particular Nb/Nb₅Si₃ microlaminates, have potential as high-temperature materials. In this study, microlaminates of Nb and amorphous Nb-37.5 at. pct Si are magnetron sputter deposited from elemental Nb and polycrystalline Nb₅Si₃ targets. The microlaminates are heat treated at high temperatures to produce crystalline layers of Nb and Nb₅Si₃ that are flat, distinct, and stable for at least 3 hours at 1200 °C. The layers consist of textured Nb grains and equiaxed submicron Nb₅Si₃ grains. Initial room-temperature tensile tests indicate that the microlaminates have strengths similar to cast and extruded alloys of Nb and Nb₅Si₃. The fracture mode of the Nb layers is dependent on the Nb layer thickness, with thin layers failing in a ductile manner and thick layers failing by cleavage. The Nb layers bridge periodic cracks in the Nb₅Si₃ layers, and using a shear lag analysis, the tensile strength of Nb₅Si₃ is estimated. The results indicate that microstructurally stable and mechanically robust microlaminates of Nb and Nb₅Si₃ can be fabricated by sputter deposition with a high-temperature heat treatment. The processing, microstructure, and mechanical properties of these microlaminates are discussed.     

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.