Interface migration during recrystallization: The role of recovery and stored energy gradients

The interface migration rate behavior during recrystallization of a deformed iron single crystal is considered in detail. A spheroidal growth, site-saturated nucleation model of recrystallization is developed for iron which quantitatively takes into account the effect of concurrent recovery in the deformed regions ahead of the advancing recrystallization interfaces. The recovery kinetics model incorporated into the growth model was devised based on the experimental data of Michalak and Paxton.[10] Explanations are advanced for the nonlinear growth of grains during recrystallization of iron. Deformation-induced stored energy gradients are proposed to explain the aspects of nonlinear growth not accounted for by recovery and are introduced into the growth model also. A localized shear band deformation mechanism developed by Harataniet al.[14] is used to rationalize the presence of stored energy gradients in the deformed iron single crystal.     

Microstructural modeling of recrystallization in deformed iron single crystals

Recrystallization kinetics in a (111)[1 0] iron single crystal deformed 70 pet by rolling were characterized experimentally at temperatures between 500°C and 600°C by means of quantitative metallography. A Laplace transform method was applied to the time-dependent global microstructural properties, volume fraction, interface area per unit volume, and the largest intercept-free length (recrystallized grain) to separate nucleation and interface migration effects from the overall recrystallization kinetics. A comprehensive nucleation and growth model was derived from analysis of the microstructural data. The model consisted of the following salient features: (a) nucleation was random and approximately site-saturated with zero incubation time at all temperatures; (b) the recrystallized grains grew three-dimensionally in the shape of prolate spheroids; and (c) interface migration rates were highly anisotropic, the grains growing at an approximately constant rate in one dimension and at a strongly decreasing rate in the other two dimensions. The present findings were compared to a similar earlier study of a deformed iron (111)[ 2] single crystal. The time dependencies of the interface migration rates were rationalized in terms of a deformation-induced, nonuniform distribution of stored energy and an orientation-dependent grain boundary mobility.     

Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based superalloy

The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation. The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the effect of the aging at 850 °C was quite different between orientations favored for the slip system and those favored for the (111) slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored for slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal precipitates inhibited extensive shearing of the γ-γ′ structure by the slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged [001] specimen. In the orientations favored for the (111) slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111} -type slip systems, i.e., the and orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix channel in the single-aged specimens.     

Crystallography of directionally solidified magnesium alloy AZ91

The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP techniques in SEM. The main direction of growth is found to be . The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001), , and -planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction: and .     

Torsion textures produced by dynamic recrystallization in α-iron and two interstitial-free steels

Two interstitial-free (IF) steels and a high-purity α-iron were deformed in torsion over the temperature range of 600 °C to 840 °C, and the textures produced were measured using conventional X-ray techniques. The conditions were chosen so that dynamic recrystallization (DRX) would take place in the ferrite and static recrystallization would be avoided during cooling after deformation. The DRX textures differ from those observed at room temperature and are dominated by the D1 ( )[111], D2( )[111], and E2 ( )[111] components. The D2 becomes increasingly important as the strain is increased, which leads to weakening of the D1 and disappearance of the E2 at large strains. Texture simulations were carried out using a DRX model based on sequential deformation, nucleation, and growth steps. The types of oriented nucleation and selective growth required to reproduce the experimentally observed textures are discussed. The simulations indicate that the low-energy nucleation mechanism plays a dominant role in the formation of bcc DRX textures. The results are also interpreted in terms of the continuous (in situ) and discontinuous mechanisms of dynamic recrystallization.     

The substructure of (252)f martensite formed in an Fe- 8Cr- 1 C alloy

This work deals with a transmission electron microscopy investigation of plate martensite in an Fe-8Cr-lC alloy, which exhibits a typical (252)_f martensite transformation. It was found that macroscopic martensite plates, such as those used for optical microscopy determination of the habit plane and the shape strain, actually consist of many individual small plates having the same crystallographic orientation and habit plane. Individual plates within a macroscopic plate appear to form by auto-catalytic nucleation. The coalescence plane between individual plates is usually parallel to (011)_b = (lll)_f because the growth of the plates is restricted by stacking faults and twins on (lll)_f, formed as a result of accommodation of the shape strain. Deformation of the martensite at the coalescence sites, mainly on the twin system (112)_{b b}, leads to a complex substructure of the macroscopic plates. Stacking faults and twins in the austenite may be inherited in the martensite, which leads to further complexity. Prior to the coalescence, the substructure of individual martensite plates is simple, and consists of a low density of irregularly distributed twins on (112)_{b b} and dislocations having Burgers vector a_b/2 . These defects are probably caused by accommodation deformation. The martensite/austenite interface contains a set of parallel dislocations having a spacing of about 13 A. These dislocations are likely to be screw dislocations with Burgers vector a_b/2 = a_f/2 , which accomplish the complementary shear in the phenomenological crystallographic theory. Formerly with the University of Illinois     

Deformation of a Burgers oriented bimetallic bicrystal of alpha-beta (Ti-13Mn)

The deformation behavior of a Burgers oriented α-β-Ti-13Mn bimetallic bicrystal was studied at two plastic strains, 0.52 and 2.08 pct. Two single crystals, α and β, each corresponding to the orientation of its respective bicrystal component were also investigated. The stress axes were and [1218]_β. The interface planes were and and lay in the x’-z’ plane. The deformation behavior of the a component differed from that of the a single crystal because of plastically induced stresses,T _y’z’ ,T _x’z’ ,T _x’y’ , and σ _x’x’ . Prismatic slip and twinning were found in the single crystal α whereas the bicrystal revealed additionally pyramidal andc + a slip, the latter at the interface. Slip on the front and back surfaces was different and both thec + a and twinning systems acted to maintain compatibility. Slip in the β single crystal and the β bicrystal component were quite similar. However, there were differences in the intensity and amount of primary slip, (231) , on the front and back surfaces. The diminished amount of (231) slip on the back surface was due to plastically induced stresses, and on the front surface the primary slip cross slipped to slip which triggeredc + a slip in α. On the back surface the dominant slip system was which acted in response to the plastically induced stresses. An approximate calculation revealed that the interface deformation zone had about twice the flow stress of the average bicrystal stress. Formerly a Graduate Student in the Department of Physical and Engineering Metallurgy at Polytechnic Institute of New York, Brooklyn, NY     

Interaction of deformation twin and 120 deg-rotational order fault domain boundary in the lamellar structure of two-phase TiAl-based alloys

Interactions between deformation twin and 120 deg-rotational domain boundary were studied by transmission electron microscopy in a two-phase TiAl-based alloy with fully lamellar structure deformed at room temperature. Three types of the interaction were observed, depending on the interaction geometry and crystallography faced by the incident twinning Shockleys. It was found that the incident twinning shear could be accommodated into the barrier domain by a reaction involving emission of 1/2 {111} _B slip in all the three types of interactions presumably since the slip required a small critical resolved shear stress (CRSS) and was always favored by the pile-up stress. Several reaction schemes involving 1/2 {111} _B slip for each type of the interactions were proposed by considering whether the reaction resulted in a reduced elastic energy and if the dissociated dislocations were able to glide away to minimize the total elastic energy associated with a long-range stress field of a pileup of the incident twinning partials. It is suggested that whether a reaction scheme is feasible would depend on behavior of other product dislocation except 1/2 {111} _B .     

Investigation of the annealing texture evolution in hafnium

This article presents a study of the evolution of the annealing texture in hafnium, as measured by electron backscattering diffraction patterns (EBSPs). It was found that the annealing texture of asreceived extruded rod depended on the annealing temperature. After low-temperature recrystallization, the deformation axis was parallel to or and the basal planes were approximately parallel to the deformation axis. These orientations were deduced by the position of the points in the standard stereographic triangle used to produce the inverse pole figure. As the annealing temperature was raised to 1700 °C, the direction parallel to the rolling direction changed to and the grain size increased. It appeared that the increase in grain size occurred by a process of abnormal grain growth, and this abnormal grain growth appeared to be the cause of the change in the texture. Texture was also examined in samples that had been warm rolled to thickness reductions between 10 and 90 pct and then annealed at 1700 °C. In these samples, the main feature of the texture was that the basal plane became parallel to the rolling plane as the amount of rolling increased. The maximum grain size was observed in samples that had been rolled to a reduction in thickness of 50 pct.     

Interdiffusion in Ni-rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that is approximately four times greater than , while and are of the same magnitude. For all concentrations, is two to three times greater than . Both and increase with increasing Al concentration, whereas and show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.     

Orientation relationships between M2C carbide and the austenite matrix in an Fe-Mn-AI-Mo-C alloy

M₂C carbides were observed to precipitate within the austenite matrix of an Fe-24.6Mn-6.6Al-3. IMo-1.0C alloy after quenching from 1200 °C and aging at 750 °C. By means of transmission electron microscopy (TEM) and diffraction techniques, the orientation relationships between M₂C (p) and the austenite (γ) matrix were determined to be: (0001)p//(111)γ, (11– 0)p// (1 0)γ, ( 100)p//(11 )γ. M₂C carbide has been reported by many researchers to precipitate from the ferrite matrix or along austenite/ferrite boundaries in alloy steels containing Mo. However, little information concerning the formation of M₂C in the austenite matrix has been provided. This investigation presents the first evidence for the existence of M₂C carbide wholly within the austenite matrix and its relationship to the austenite. The energy-dispersive spectrometry (EDS) analyses were performed on M₂C carbides, and the results indicate that the solubility of the M₂C carbide for foreign atoms other than Mo is very limited.     

An electron-backscattered diffraction study of the texture evolution in a coarse-grained AZ31 magnesium alloy deformed in tension at elevated temperatures

Electron-backscattered diffraction (EBSD) has been used to investigate the texture evolution during tensile deformation at temperatures between 673 and 773 K of a coarse-grained commercial AZ31 magnesium alloy. A weak (0001) fiber texture was initially present in the hot-rolled magnesium alloy plate. The [0001] directions of the grains spread 0 to 45 deg around the normal direction (ND) of the magnesium alloy plate. This pre-existing weak texture evolved during tensile deformation into a strong texture close to the {0001} 〈1 00〉. The [0001] directions of the grains rotated toward the orientations perpendicular to the tension axis of the samples, indicating that the 〈11 0〉 slip system appeared to be the most active slip system, especially in the early stages of deformation. The EBSD Schmid-factor analysis revealed that, however, with an increase in strain and the rotation of the (0001) slip plane, the {11 2} 〈11 〉 slip system appeared to be more favorable. The {1 00} 〈11 0〉 and {1 01} 〈11 0〉 slip systems remained favored throughout the strains investigated, indicating that {1 00} and {1 01} are two important slip planes for cross slip using the 〈11 0〉 slip vector. It is found that the misorientation across one coarse grain (as high as 38.2 deg) is accommodated by low-angle grain boundaries (LAGBs). The formation of these LAGBs may be an intermediate stage of the coarse grain refinement that occurred during deformation. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

The role of twinning in the cavitation erosion of cobalt single crystals

The deformation characteristics contributing to the superior cavitation erosion properties of HCP cobalt single crystals have been determined. Results indicate that its erosion response is highly orientation sensitive. A homogeneous distribution of and glide occurs in {0110} crystals, whereas slip in the (0001) crystals is much more heterogeneous and consists mainly of dislocations. Continued exposure to cavitation nucleates a large number of twins, predominantly on the and planes in the and (0001) crystals respectively. The former twins are finer and more needle-like than the latter. The crystals are also significantly more erosion resistant than the (0001) crystals. The twin density increases continuously with cavitation exposure until a dense network of twins spans the entire exposed area. This fine-scale twinning is considered responsible for the superior erosion resistance of the metal.     

Coarsening of dispersed intermetallic

Dendritic monocrystals of Al-4.5 wt pct Cu-2 wt pct Mn were directionally solidified at 0.20 m/h under a thermal gradient of 3 × 10³ K/m. Crystal pulling was stopped for various lengths of time prior to quenching the remaining liquid, thus making it possible to evaluate the transformation and coarsening kinetics of dispersed intermetallic phases, Al₆Mn and Al₂₀Cu₂Mn₃, as a function of temperature. Coarsening of Al₆Mn intermetallic particles surrounded by liquid (Type II) follows an average particle size relationship much closer than it does a relationship. This suggests that convection plays no important role in coarsening. For coarsening of Al₆Mn particles in a solid matrix (Type I) the relationship fits the experimental measurements reasonably well. Coarsening kinetics studies were extended to intermetallic particle sizes an order of magnitude finer than those occurring in directionally solidified alloy, in order to derive information required by an on-going project on the effect of intermetallic phase geometry on corrosion behavior. Jt was found that coarsening of Al₂₀Cu₂Mn₃ particles contained in a melt-spun ribbon follows a relationship, as predicted by Kirchner for grain-boundary diffusion-controlled ripening. Finally, coarsening of Al₆Mn particles surrounded by liquid indium film and contained in a plastically deformed matrix follows a relationship, as predicted by LSW theory for liquid diffusion-controlled ripening. Shortening the time required to obtain coarse intermetallic particles during a homogenization treatment is important in deep drawing. Formerly Graduate Student, Department of Metallurgy, University of Connecticut, Storrs, CT 06268     

Multiphase binary diffusion in infinite and semi-infinite media: Part I. On the determination of interdiffusion coefficients

Mathematical relationships describing the multiphase binary diffusions are deduced under the condition that in an infinite medium, and also under the condition that and the fluxes of both the species occur in each other’s opposite direction through the surface in a semi-infinite medium, where are the interdiffusion coefficient and the partial molal volume of componenti in phasej, respectively, and are composition independent. Two graphical evaluation methods for obtaining the interdiffusion coefficients for all the phases present in the diffusion zone have been developed in infinite and semi-infinite media. Concentration-distance curves of both the components for an infinite medium and those curves and also mass change of the couple per unit area of the surface before and after the diffusion anneal for a semi-infinite medium must be prepared to calculate an interdiffusion coefficient for each phase.     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of {10 1}, {10 2}, {11 1} and, {11 2} twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with {11 1} and {11 2} twin boundaries is described here and compared with the results for {10 1} and {10 2} twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. 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.     

Modeling recystallization kinetics in a deformed iron single crystal

The kinetics of recrystallization of a (111) { } single crystal of pure iron deformed 70 pct by rolling were characterized experimentally at temperatures between 450°C and 600°C, using quantitative metallography. The method of Laplace transforms was applied to the overall recrystallization kinetic behavior to separate nucleation from interface migration kinematics. A comprehensive nucleation and growth model was developed to explain all observations quantitatively. the model consisted of the following important features: (a) nucleation sites were distributed randomly; (b) nucleation was site-saturated and occurred with no practical incubation time at all annealing temperatures; (c) recrystallized grains grew three-dimensionally and were spheroidally shaped; (d) all recrystallized grains grew at approximatelly the same rate within experimental error; and (e) interface migration rates were not constant but decreased with time according to at ^−0.38 law at all temperatures. The time dependency of the interface migration rate was rationalized in terms of deformation-induced, nonuniform distribution of stored energy, Recovery processes competing with recrystallization were evident at long annealing times at the two lowest annealing temperatures.     

Orientation dependence of dislocation structures in cyclically deformed Cu-16 At. Pct Al alloy single crystals

The dislocation structures induced by the cyclic deformation of Cu-16 at. pct Al alloy single crystals oriented, typically, as for single slip and [023] and for double slip, were studied by transmission electron microscopy (TEM) and compared with the results of Cu single crystals. Completely unlike the dislocation structures of Cu single crystals of corresponding orientations, the dislocation structures of these oriented Cu-16 at. pct Al alloy single crystals show a typical planar morphology. As the applied-plastic-stain amplitude increases, the dislocation configuration changes, on the whole, from multipolar arrays to dislocation tangles in the primary slip plane and from low-density planar slip bands to well-developed persistent Lüder’s bands (PLBs) in the planes normal to the primary slip plane, respectively. Secondary slips can be clearly observed to activate from very low plastic-strain amplitudes in all three Cu-16 at. pct Al single crystals investigated. Interestingly, the crystallographic orientation has almost no effect on the dislocation structure of Cu-16 at. pct Al single crystals.     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of , , , and twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with and twin boundaries is described here and compared with the results for and twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. 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.