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 .     

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.     

β brass bicrystal stress-strain relations

The stress-strain relations of three isoaxial and one nonisoaxial bicrystal whose grain boundaries are parallel to the stress axis have been studied. From one set of isoaxial bicrystals it has been possible, for a given strain, to determine the average stress in the grain boundary deformation zone from the relationship where σ _T is the applied stress, σ _b is the single crystal flow stress for the given strain, andV _gb is the volume fraction of grain boundary deformation zone. was determined from measured values of σ _T , σ _b , andV _gb . From these data and it was possible to extrapolate to the grain boundary to obtain σ _gb , the grain boundary stress. From the nonisoaxial bicrystal series containing a hard and a soft component, it was possible to determine at a given strain the stresses in each component and therefore the stress-strain relations from the relationship where and are the average stress in the hard and soft components respectively, andV _H andV _S are the corresponding volume fractions. The remaining two isoaxial bicrystal series were used to evaluate strengthening effects of bicrystal boundaries. YII-DER CHUANG, formerly Graduate Student, Department of Metallurgy and Materials Sciences, School of Engineering, New York University, Bronx, N. Y. 10453 This paper is based on a thesis submitted by Yii-der Chuang in partial fulfillment of the requirements for the Ph.D. degree in metallurgy at New York University, New York, N. Y.     

Modulated microstructures inβ Cu-Zn-Al

β Cu-Zn-Al samples with different surface orientations have been examined by electron microscopy and diffraction. The nature of the modulated microstructure or tweed and the accompanying diffuse streaking has been studied. Tweed in samples without surface martensite is confirmed to be a subsurface phenomenon, and the diffuse streaking is attributed to both dynamic and static {110} displacement waves. The image contrast in samples with surface martensite is shown to be determined essentially by the surface martensite features and probably tweed in the surface martensite. Possible reasons for the observed tweed are discussed. Formerly at Division Metales, Centro Atómico Bariloche, Argentina.     

Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures

The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {10 0} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal sli { 2 } is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873 K. The {10 2}, {11 1}, and {11 2} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.     

Elastic interaction stresses: Part II. The influence of orientation on stresses generated in iso-axial bicrystals

The effects of four types of incompability on stresses generated in seven iso-axial bicrystals of 70-30 alpha brass subjected to tension were examined by finite element methods. The distribution of applied stress and resolved shear stresses was determined. The applied stress, { }, was higher at the interior than at the surface, while the resolved shear stresses were generally higher at the surface. For both applied { } and resolved shear stresses, the largest stresses occurred at the bicrystal boundaries. The effect of end constraints on a [213] oriented single crystal was found to create nonuniform stresses and strains, and an explanation for this behavior was proposed. The effect of orientation on the magnitude of grain boundary resolved shear stress, { }, and the relative extent (volume fraction) of enhanced grain boundary resolved shear stress,V _gb, were determined. The highest values of { } andV _gb were reached in different portions of the stereographic triangle. The elastic interactions producing this behavior were too complex to permit a simple interpretation. TZI-KANG CHEN formerly Graduate Student at Polytechnic University     

Heterogeneous nucleation of pb particles embedded in a Zn matrix

Zinc-10 and 20 wt pct Pb alloys have been rapidly solidified by melt spinning to obtain a very fine scale dispersion of nanometer-sized Pb particles embedded in Zn matrix. The microstructure and crystallography of the Pb particles have been studied using transmission electron microscopy (TEM). Each embedded Pb particle is a single crystal, with a truncated hexagonal biprism shape with the 6/mmm Zn matrix point group symmetry surrounded by and facets. The Pb particles solidify with a well-defined orientation relationship with the Zn matrix of . The melting and solidification behavior of the Pb particle have been studied using differential scanning calorimetry (DSC). The Pb particles solidify with an undercooling of approximately 30 K, by heterogeneous nucleation on the {0001} facets of the surrounding Zn matrix, with an apparent contact angle of 23 deg.     

Interdiffusion in ternary Fe-Cr-AI alloys

Interdiffusion coefficient matrices, have been experimentally measured at 31 points in the αδFe phase field of the Fe-Cr-AI ternary equilibrium phase diagram at 900 °C (1173 K). Analyses of the computed matrices were carried out by subjecting each measured -Dⁿ _ij to two consistency tests comprised of Onsager’s and Kirkaldy’s relations. Good consistency with these relationships was obtained in the central region of the ternary phase field, but there were significant deviations near the phase boundaries. These may beattributable to systematic uncertainties in the analytical procedure. The results indicate that falls quickly with increasing C_Cr, with C_Al > 0.2, but is not a strong function of C_Al. Conversely, is not a strong function of C_Al but decreases with increasing C_Cr. formerly with the Lawrence Berkeley Laboratories of the University of California, Berkeley At the time of this research, Dr. Stringer was on sabbatical leave at the Lawrence Berkeley Laboratories, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720.     

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 .     

Load relaxation in aluminum: I. Theory of plastic deformation. II. Plastic equation of state

According to the theory of thermally-activated deformation, the plastic strain rate equality will hold in a load relaxation experiment, wheret = 0 is de-fined as the time at which the crosshead stops. In this theory, plastic flow is intrinsically time dependent and its rate is controlled by interaction of glide dislocations with thermal obstacles(e.g. forest dislocations). The strain rate equation is of the form and att = 0 none of these variables changes instantaneously. Measurements reported here for [111] aluminum single crystals indicate that this prediction is wrong. The ratio is near zero at low stress and approaches unity only at high stress. This result is predicted if plastic strain itself is time-independent (athermal), as in the author’s recent theory. Time-dependent strain is then the result of thermal changes in structure, namely loss (recovery) and rearrangement of obstacle dislocations. Experi-ments were also done to test further the essential hypothesis of Hart’s recent formula-tion of an equation of state for plastic deformation-namely that each distinct σ-@#@ curve derived from load relaxation data corresponds to a unique “hardness” state and that re-covery does not occur. Significant differences were observed in the 77 K strsss-strain curves for 295 K relaxed and unrelaxed samples which indicate that substantial loss and some rearrangement of dislocations has occurred during the relaxation. It is concluded from both experiments that load relaxation in aluminum is a manifestation of recovery creep and cannot be taken as evidence for a plastic equation of state.     

The development of the deformation texture in zirconium during rolling in sequential passes

Slip and twinning systems activated by rolling in sequential passes were observed on a coarse grained zirconium polycrystal. At least five independent deformation modes are activated; slip and twinning systems occur simultaneously. For low degrees of deforma-tion the main slip system is prism slip , even when the orientation is not favorable. The lattice rotations caused by slip proceed gradually with increasing defor-mation; they are relatively small although the strain achieved can be large. On the other hand, twinning causes spontaneous, large-scale lattice rotations, even for low degrees of deformation. The type of twinning depends largely on the crystallographic orientation of the matrix. For basal pole orientations of the undeformed grains in the area 0 to 50 deg from the normal direction twinning becomes preferentially operative. For basal pole orientations of the undeformed grains in the area 50 to 90 deg from the normal direction, however, twinning becomes preferentially operative. In both orientation areas as a complementary system twinning is operative. For all deformation sys-tems their operation is independent of the azimutal position of the basal pole in these areas. The lattice rotations alter the orientation of the crystallites in such a way that the basal poles all become aligned more or less in the direction of the deforming compres-sive force. For higher degrees of deformation pyramidal slip with a (c + a) type Burgers Vector can explain why this preferred orientation is maintained as final position, which for zirconium shows a split of basal poles of ±30 to 50 deg towards the transverse direc-tion. The method of following the complicated interactions between different slip and twinning systems in a stepwise deformed, coarse grained sheet by 1) trace analysis of the deformation modes, 2) by correspondingly derived lattice rotations, and 3) by texture measurements leads to an explanation of the texture development in zirconium. It is dis-cussed on the basis of basal pole rotations.     

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.     

Design and development of hot

As the first step to design and develop the new hot corrosion-resistant nickel-base single-crystal superalloys by thed-electrons alloy design theory (or the New PHACOMP), an evaluation of the phase stability of the modified hot corrosion-resistant IN738LC nickel-base superalloys (Ni-16Cr-9.5Al-4.0Ti-8.0-Co-0.55Nb-0.06Zr-0.05B-0.47C-(0~3)Ta-(0~3)W-(0~3)Mo (in atomic percent)) was conducted with the alloy design theory. The critical conditions for suppressing the precipitation of the brittle σ phase can be described by the electronic parameters and 0.93 which can be applied to thed-electrons alloy design of hot corrosion resistant nickel-base superalloys with high Cr contents. Formerly with the Institute of Formerly with the Institute of     

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.     

Competition among basal,prism, and pyramidal slip modes in hcp metals

The competition of slip among , and slip modes of hcp metals has been analyzed geometrically in terms of a critical resolved shear stress, CRSS, criterion. Under the action of an applied stress slip systems of one or more modes may be activated depending on the value of the CRSS and on the orientation of the slip systems with respect to the applied stress. If the CRSS of a given slip mode should exceed a limiting value relative to the CRSS of the other modes, however, the given mode becomes inoperative even under the most favorably stressed conditions. It is found by an examination of the yield loci that basal slip is inoperative if α₂ < cos θ; prism slip is inoperative if α₂ < α₁ sin θ; and pyramidal slip is inoperative if α₂ > cos θ + α₁ sin θ where and are, respectively, the ratios of CRSS for prism and pyramidal slips relative to basal slip, and ϕ is the angle between the (0001) and normals. Since the value of ϕ is a function ofc/a, the limiting values of α₁ and α₁ depend on thec/a ratio of the crystal structure.     

Martensitic transformation in Zr−Ti alloys

Martensitic transformation in a series of Zr−Ti alloys has been studied by transmission electron microscopy. A transition in morphology and substructure was observed with increasing additions of titanium. The 5 wt pct Ti alloy was found to be predominantly dislocated lath martensite while the 10 wt pct Ti alloy showed mainly a twinned plate morphology. Twins within the martensite plates could be classified into two categories, namely, thick twins and thin twins mainly on planes and, to a lesser extent, on and planes. In the case of the thick twins, the specific variant of the twin plane was always found to correspond to a plane of the {110}_bcc type, which is a mirror plane in the parent bcc crystal. When the specific variant of the composition plane was not parallel to a mirror plane, the twins were observed to be very thin. Evidence of slip within individual twin bands pointed to the operation of a multiple shear inhomogeneous deformation. Observations concerning deformation twins due to impingement effect are also discussed in this paper.     

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     

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 twinning and texture variation in Zr-2.5 pct Nb alloy during rolling

Recently, it was proven that delayed hydride cracking (DHC) is accompanied by deformation twinning through the texture analysis of a fractured surface. Thus, in order to understand the operation of deformation twinning, the texture variations by rolling were investigated using Zr-2.5 pct Nb alloy with {11 0} 〈10 0〉 texture. It was observed that deformation twinning was operated predominantly in the range of a 5 to 15 pct strain. The basal poles were rotated in the normal direction of a rolling plane with the strain, and the (0002) texture was fully reversed after 15 pct strain. This finding was established to be due to the operation of the {10 2} and {11 1} twinning systems through the analysis of the inverse pole figure. It appeared that the degree and easiness of the twinning operation was affected by changing the direction of compression during rolling with respect to the initial {11 0} 〈10 0〉 texture. The contribution of deformation twinning to strain was quantitatively calculated using the change in the basal pole components. 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.     

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.