Evidence Of Recrystallization Research Articles

Heat treatment of twin-belt cast EN AW 7075 alloy

Abstract The potential of twin-belt cast EN AW 7075 sheet for structural sheet applications was investigated with a particular emphasis on its response to the standard industrial T6 heat treatment. The twin belt cast EN AW 7075 strip is dendritic with an α(Al)–Mg(Zn,Cu,Al) 2 eutectic and predominantly Mg 2 Si and Al 12 (Fe,Cr,Mn) 3 Si particles at interdendritic sites. The α(Al)–Mg(Zn,Cu,Al) 2 eutectic has largely transformed into a Mg(Zn,Cu,Al) 2 precipitate-rich network during the break down pass. The coarse intermetallic particles have fragmented while the fine intragranular precipitates have coarsened upon further hot rolling to a gauge of 2 mm. The predominant restoration process during rolling at 350 °C was dynamic recovery with no evidence of recrystallization. A fraction of Mg(Zn,Cu,Al) 2 was replaced with Al 2 CuMg. Both phases dissolved during the solution heat treatment at 470 °C while the hot band has entirely recrystallized. Subsequent ageing at 121 °C for 16 h has produced a T6 hardness of 170 HB, considerably higher than that reported for the conventionally processed counterpart (150 HB). In addition to a remarkable age hardening capacity, twin belt cast EN AW 7075 alloy has enjoyed a very high resistance to age softening as inferred from a nearly T6 hardness after the industrial T73 heat treatment with an additional ageing cycle at 160 °C.

Localized Recrystallization in Cast Al-Si-Mg Alloy during Solution Heat Treatment: Dilatometric and Calorimetric Studies

During heat treatment, the work piece experiences a range of heating rates depending upon the sizes and types of furnace. When the Al-Si-Mg cast alloy is heated to the solutionizing temperature, recrystallization takes place during the ramp-up stage. The effect of heating rate on recrystallization in the A356 (Al-Si-Mg) alloy was studied using dilatometric and calorimetric methods. Recrystallization in as-cast Al-Si alloys is a localized event and is confined to the elasto-plastic zone surrounding the eutectic Si phase; there is no evidence of recrystallization in the center of the primary Al dendritic region. The size of the elasto-plastic zone is of the same order of magnitude as the Si particles, and recrystallized grains are observed in the elasto-plastic region near the Si particles. The coefficient of thermal expansion of Al is an order of magnitude greater than Si, and thermal stresses are generated due to the thermal mismatch between the Al phase and Si particles providing the driving force for recrystallization. In contrast, recrystallization in Al wrought alloy (7075) occurs uniformly throughout the matrix, stored energy due to cold work being the driving force for recrystallization in wrought alloys. The activation energy for recrystallization in as-cast A356 alloy is 127 KJ/mole. At a slow heating rate of 4.3 K/min, creep occurs during the heating stage of solution heat treatment. However, creep does not occur in samples heated at higher heating rates, namely, 520, 130, and 17.3 K/min.

Preparation of spherulites from amylose–palmitic acid complexes

We conducted a systematic study of the formation of spherulites from amylose–palmitic acid complexes. The independent variables included final heating temperature, intermediate quench temperature, final quench temperature and cooling rates. Spherulites were characterized by different techniques: optical microscopy, scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and resistant starch content. Three distinct types of spherulite morphology were observed: spherical, sintered (“snowball”) and torus/disc shaped. Spherical-shaped particles were formed at higher endpoint heating temperatures and exhibited mixed B+V-type XRD patterns. Torus-shaped spherulites formed from samples heated to 140 °C followed by slow cooling, and exhibited V-6 I type XRD patterns. Resistant starch estimated for spherulites formed by heating to 180 °C followed by cooling to 40 °C at 1 °C/min vs. those formed by heating to 140 °C, cooled to 90 °C at 1 °C/min then to 40 °C at 0.04 °C/min was 28% and 39%, respectively. Morphology of the amylose–PA spherulites after resistant starch determination for spherical spherulite shows distinct pores and uneven surface texture. In contrast, the torus-shaped spherulites were degraded with evidence of recrystallization.

Capturing the texture changes in a zirconium alloy during the allotropic phase transformation

High-energy synchrotron X-ray diffraction was used to measure in situ the texture of a cold-rolled zirconium alloy during α to β phase transformation. Miniature samples were heat treated by resistive heating, during which diffraction patterns were captured continuously using a two-dimensional X-ray detector. High-quality quantitative texture information was obtained from single diffraction images. Experimental data of the high-temperature β texture is presented, as well as evidence of recrystallization and β grain growth.

Amorphous germanium under high-pressure conditions

In this work, we address the question of pressure-induced phase transitions in amorphous germanium. We discuss both the pressurization and depressurization cycles on an amorphous film, showing the appearance of various stable and metastable (LDA–HDA) phases by Raman scattering and X-ray absorption spectroscopy. In particular, we report evidence of recrystallization in the metastable ST12 phase driven by the density of voids of the film.

Strain-induced microstructural evolution and work softening behavior of Zn–15% Al alloy

Strain-induced microstructural evolution of a Zn–15% Al alloy was studied in order to elucidate the reason for its work softening behavior. Fully annealed microstructure of the Zn–15% Al alloy is distinctively characterized by primary η grains and (η + α) lamellar colonies, where η and α are Zn-rich HCP and Al-rich FCC phases, respectively. Hardness and yield strength decreased continuously with an increase in cold rolling up to 80%, exhibiting work softening behavior. During cold rolling, the (η + α) colonies with interlamellar spacing of ∼100 nm change into equiaxed η and α grains with sizes of ∼500 to ∼800 nm by dynamic recrystallization, whereas primary η grains are only elongated along the rolling direction without any evidence of recrystallization. A linear relationship between volume fraction of recrystallized (η + α) grains and the strength is established. The compositional analyses on primary η grains demonstrate that dissolved Al solutes inside η grains precipitate during cold rolling.

Dynamic Recrystallization of Dense Polycrystalline NaCl: Dependence of Grain Size Distribution on Stress and Temperature

Only few models explain the development of a steady state grain size during dynamic recrystallization, and their microphysical basis is poorly understood. In this study, we investigate mechanical and microstructural data on dry and wet NaCl, deformed at a range of stresses and temperatures at elevated pressure, with the aim to evaluate the different models. The results show that dry NaCl continuously work hardens and shows evidence for recrystallization dominated by progressive subgrain rotation, while the wet material shows, at similar conditions, oscillating stressstrain behaviour and recrystallization dominated by grain boundary migration. Taking into account the distribution of grain size, deformation of wet NaCl is best described by flow laws based on composite grain size sensitive (GSS) solution-precipitation creep and grain size insensitive (GSI) dislocation creep. The recrystallized grain size data in wet NaCl can be modeled with the hypothesis that recrystallized grain size organises itself in the boundary between the GSS and GSS creep domains.

04/00685 Formation and dissociation studies for optimizing the uptake of methane by methane hydrates: Link, D. D. et al. Fluid Phase Equilibria, 2003, 211, (1), 1–10

Samples of metamorphosed and deformed granitic rocks were collected from two Alpine complexes with well-constrained metamorphic history: Western Tauern Window and Ötztal-Stubai Crystalline Complex. Zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques, cathodoluminescence (CL) imaging and Raman spectroscopy. The aims were: to describe and interpret complicated secondary textures and microstructures in zircon; based on cross-cutting relationships between secondary microstructures, reconstruct the sequence of processes, affecting zircon crystals; link the evolution of zircon with the history of the host rocks.The results indicate that zircon in the sampled granitic rocks forms growth twins and multi-grain aggregates, which are unusual for this mineral. Moreover, various secondary textures have been found in the sampled zircon, often cross-cutting each other in a single crystal. These include: distorted oscillatory CL zoning with inner zones forming inward-penetrating, CL-bright embayments, which are the evidence of dry recrystallization via annealing/lattice recovery; CL mosaicism with no preservation of growth zoning, but abundant nano- and micro-scale pores and mineral inclusions, which are the evidence of recrystallization by coupled dissolution–reprecipitation and/or leaching; embayed zircon boundaries filled with apatite, monazite, epidote and mylonitic matrix, indicating mineral-fluid reactions resulting in zircon dissolution and fragmentation; overgrowth CL-dark rims, which contain nano-pores and point to transport and precipitation of dissolved zircon matter. We conclude that zircon in our meta-granites is sensitive to metamorphism/deformation events, and was reactive with metamorphic fluids. Additionally, we have found evidence of crystal-plastic deformation in the form of low angle boundaries and bent grain tips, which is a result of shearing and ductile deformation of the host rock. We suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization, prograde metamorphism, peak of amphibolite-facies metamorphism, post-peak cooling and exhumation, formation of ductile shear zones and final cooling to ~ 250 °C.

EPMA and SEM of fuel samples from PWR rods with an average burn-up of around 100 MWd/kgHM

Samples from the high and low power regions of two fuel rods with average burn-ups of 89.5 and 97.8 MWd/kgHM were examined. Electron probe microanalysis (EPMA) was used to measure the radial distributions of Xe, Cs and Nd in the UO 2 fuel matrix, and scanning electron microscopy (SEM) was used to study the change in the UO 2 microstructure across the fuel pellet radius. EPMA showed that a large fraction of the xenon created had been released from the fuel matrix of the high power samples at all radial positions. In these samples, SEM revealed the presence of recrystallised grains at intermediate radial positions where thermal fission gas release had previously occurred. Evidence of recrystallisation was found throughout the pellet cross-sections of the low power samples. It is concluded that recrystallisation of the fuel grains at intermediate radial positions is mainly responsible for the marked increase in fission gas release to the rod free volume at burn-ups above 80 MWd/kgHM.

Microstructural evolution in adiabatic shear bands in Ta and Ta–W alloys

Microstructural evolution of adiabatic shear bands originated due to high strain, high strain rate deformation in Ta and Ta–W alloys has been examined. Tests were performed using a specially designed stepped specimen in a Hopkinson bar. Upon completion of the deformation, the region is cooled to below one half of the temperature achieved during adiabatic heating in less than one millisecond. Microstructural characterization of the shear bands was performed using optical microscopy as well as scanning and transmission electron microscopy. No evidence of recrystallization within the shear bands could be found. This is in contradiction with several recent reports, which claim that recrystallization may take place at these stringent time and temperature conditions. These studies, however, do not take into account the kinetics of boundary refinement processes, which are a distinctive characteristic of a recrystallized microstructure. It will be shown that the absence of recrystallization in Ta and Ta–W adiabatic shear bands can be predicted by a progressive subgrain misorientation (PriSM) recrystallization model, applied successfully in previous studies to predict the microstructure evolution in copper adiabatic shear bands.

Assessment and modelling of isothermal forging of intermetallic compounds Part 4 – NiAl

An alloy based on NiAl has been deformed under isothermal conditions, to investigate the possibility of hot working as a processing route for such materials. Deformation has been shown to occur largely via dynamic recovery, although some evidence of recrystallisation has been noted. Material properties (strain rate sensitivity, activation energy of deformation, etc.) have been calculated and used in a constitutive algorithm to model successfully the forging behaviour of this material under all the conditions examined here.

Gadolinium Oxide Gate Dielectrics for GaN MOSFETs

ABSTRACTSubstrate preparation of GaN, both in-situ and ex-situ, and the growth of gadolinium oxide, Gd2O3, by Gas source molecular beam epitaxy (GSMBE) have been investigated. Ex-situ cleaning techniques included wet chemical etching and UV-ozone treatments to remove surface contaminants and the native oxide. In-situ cleaning consisted of thermal treatment with and without exposure to an electron cyclotron resonance (ECR) oxygen plasma. A GaN (1x3) streaky RHEED pattern was the final product of this surface treatment study. Various growth initiation techniques were explored to produce Gd2O3 films with different microstructures as evidenced by RHEED, TEM, and XRD. Gd2O3 films planarized the initial GaN surface and stoichiometry was maintained over a range of substrate temperatures (300° to 650°C). Single crystal gadolinium oxide films were grown at substrate temperatures of 600-650°C. These films exhibited a breakdown field strength (EBD) of ∼1MV/cm, and showed high leakage current at high forward bias due to defects within the oxide. Single crystal oxide films were found to be thermally stable at annealing temperatures up to 1000°C. Quasi-amorphous films were grown at a substrate temperature of 100°C. These films exhibited a higher E BD of ∼3MV/cm and an interface state density of 3 × 1011 cm−2eV−1. However, the quasi-amorphous films were not thermally stable at 1000°C, showing evidence of re-crystallization in x-ray diffraction (XRD) scans.

Dolomitization of Holocene Shallow-Marine Deposits Mediated by Sulfate Reduction and Methanogenesis in Normal-Salinity Seawater, Northern Belize

ABSTRACT Dolomite constitutes an average of 12% of the Holocene organic-rich sediments over a 15 km2 area of the Cangrejo Shoals mudbank in northern Belize. Although it defines a laterally persistent stratiform body that averages 3 m thick, it is present throughout the 7.6-m-thick sediment section. These transgressive sediments are less than 6400 years old and were deposited in shallow-marine environments of normal salinity. The dolomite is dominantly cement, and average crystal size is 7 m. There are no significant correlations among amount of dolomite vs. sediment texture, mineralogy, porosity, or mole % MgCO3 in associated particulate high-Mg calcite, depth, or location on the shoals. The dolomites are poorly ordered and calcic (39.5-44.5 mole % MgCO3), with low mean Mn (210 ppm) and relatively high mean Sr (1034 ppm) concentrations. There is no evidence of recrystallization or geochemical alteration of the dolomite. 18O values of the dolomites range from 0.5 to 2.8o/ooPDB, and the mean value (2.1o/oo) suggests that the dolomite precipitated from normal-salinity pore water. Dolomite 13C values range from -5.2o/oo to +11.6o/ooPDB (mean seawater 13C = 0.5o/oo), which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water. Dolomitization attending these organodiagenetic reactions apparently was reversible over time, and episodic rather than continual precipitation is indicated. Requisite Mg and Ca were provided by seawater and by some dissolution of host sediments. The most rapid period of dolomitization may have been during early transgression, when relatively high sedimentation rates sustained high levels of organodiagenesis and pore-water alkalinities.

Microstructure and texture evolution in ECAE processed A5056

Equal channel angular extrusion (ECAE) represents an effective means of microstructural refinement through severe plastic deformation. In aluminum based alloys microstructures obtained by this process generally result in interesting and remarkable combinations of mechanical properties, such as for instance enhanced strength together with high levels of ductility. In the present investigation, the evolution of the tensile properties, microstructures and crystallographic textures during ECAE has been studied for an Al-Mg based workhardening type alloy (A5056) by metallographic observation, transmission electron microscopy (TEM) and X-ray diffraction. The mechanical and microstructural characteristics of these alloys have been compared to extrudates of the same material prepared by conventional extrusion. The results show that dynamic recovery and the development of subgrains, mainly separated from each other by low angle grain boundaries, play an important role with respect to the formation of the ultrafine microstructures observed. No evidence for recrystallization and the nucleation of new grains occured up to pressing temperatures of 300°C and very sharp deformation textures were found to develop. Similar to rolling deformation a transition from a pure metal type texture to an alloy type texture in function of the deformation degree occurs. The formation of these ECAE-textures is discussed on the basis of the Taylor theory of polycrystalline deformation.

Extended milling of graphite and activated carbon

Graphite and activated carbon have been ball milled under vacuum for up to 1000 hours. The graphite became amorphous, with no evidence of recrystallisation after heating to 1200°C. There was little change in the structure of the activated carbon. Heating the carbons under argon showed that the two milled samples absorbed substantial gas, over 30% of their mass in some cases. The reactivity of both carbons during combustion increased with milling time, the greatest increase was concomitant with a decrease in graphite crystallinity. After milling for 1000 hours, graphite showed similar combustion characteristics to activated carbon. Although iron was present as a contaminant at appreciable levels, there was no evidence that it had any significant effect on the oxidation of the carbon.

Combination tests on TiAl involving constant strain-rate deformation, annealing and creep

Duplex tests involving constant strain-rate deformation and creep have been performed on polycrystalline single-phase titanium aluminide of nominal atomic composition Ti–52Al. It is found that predeformation (2%) at either room temperature or 800°C speeds up the subsequent primary creep rate and increases the critical strain corresponding to the minimum creep rate, but has little influence on the corresponding time, i.e. there is no obvious change in the time duration of primary creep for the virgin and prestrained specimens. Double creep tests, consisting of creep, annealing and creep, indicate that if the creep test is interrupted for annealing in the primary creep region, the creep rate decreases with time in the subsequent creep. However, if the creep is interrupted in the region of increasing creep strain rate, the creep rate increases with time almost immediately after reloading. Microstructures after various duplex deformations were examined by optical and electron microscopy. Generally, no evidence for recrystallization was observed. These results are interpreted in terms of recovery: the acceleration in creep corresponds to an increasing misorientation across the subgrain boundaries and therefore an increased mobility of these.

Liquid Crystalline Poly(ester amide)s Based on N,N‘-Dimethyldiamines and an Aromatic Ester Triad

A series of thermotropic liquid crystalline poly(ester amide)s was synthesized based on various N,N'-dimethylalkylenediamines and terephthaloyl bis(4-oxybenzoyl chloride), TBOC. Several different synthetic procedures were investigated to synthesize liquid crystalline (LC) polymers having a broad range of molecular weights. The highest molecular weight polymers were obtained by the low-temperature solution condensation of N,N'-bis(trimethylsilyl)diamines and TBOC. Analyses of the polymers by wide angle X-ray diffraction, differential scanning calorimetry, and polarizing optical microscopy showed that the as-prepared crystalline polymers melted to form a nematic,liquid crystalline phase, which showed no evidence of recrystallization upon cooling. The LC polyesteramides with a 6-methylene spacer unit exhibited a monotropic behavior in contrast to the polymers or copolymers with a 12-methylene spacer unit which are enantiotropic. Changes in polymer conformation which occurred on heating in the solid state and in the liquid crystalline state were evaluated by solid state 13 C cross polarization magic angle spinning NMR spectroscopy which showed that the polymers had considerable mobility at temperatures well below the crystalline melting point. The polymers, when precipitated from solution, contained mixtures of syn and anti amide units, but after melting, the syn amide unit was exclusively obtained indicating that it is likely the most thermodynamically stable conformation.

High-temperature ion implantation into quartz

Ion implantation of copper, gallium and silver ions into quartz substrates has been performed. The implantation parameters were identical for each series of implants: 55 keV ion energy and an implanted dose of 6.4*1016 ions cm-2. However, the temperature of the substrate was varied with implants being done at room temperature, 373 K, 498 K, 623 K and 873 K. Rutherford backscattering was used to provide depth profiles for the implanted ions and the substrate elements; the ion profiles were compared with simulations from the TRIM code. The ion profiles were significantly affected by temperatures in all three cases: in particular, bimodal distributions formed at 498 K for the copper and gallium implants and at 623 K for the silver. The ions amorphized the substrate to depths commensurate with the tail of the implant except in the 873 K silver implant. In this case none of the silver was retained and the substrate completely recrystallized. Evidence of recrystallization was also evident in the gallium 873 K implants, which had a smaller amorphization depth than the lower temperature implants.

Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower- T and - P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high- T mélange of metamafic blocks in a matrix of meta-ultramafic rocks. This mélange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both mélanges are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments. Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.

Tensile behavior of the L1 2 compound Al 67Ti 25Cr 8

Temperature-related variations in tensile yield strength and ductility were studied on cast, homogenized and isothermally forged Al 67Ti 25Cr 8. Yield strength dropped discontinously between 623 K and 773 K and then decreased gradually with increasing temperature. Below 623 K, fracture occurred prior to macroscopic yielding. Ductility decreased from 0.2% at 623 K to zero at 773 K, but increased again at higher temperatures. At 1073 K, an elongation of ∼ 19% was obtainable, and ultimate tensile strength and localized necking were observed. Fracture surfaces and deformed microstructures were examined. The 1073 K tensile specimen that exhibited ∼ 19% elongation showed grain boundary serrations and some evidence of recrystallization (likely dynamic) although fracture occurred predominantly via an intergranular mode.