Microstructural Evolution and Mechanical Response of Equal-Channel Angular Extrusion-Processed Al-40Zn-2Cu Alloy

Microstructural evolution, tensile properties, and impact toughness of an aluminum-zinc-copper (Al-40Zn-2Cu) alloy subjected to repetitive equal-channel angular extrusion (ECAE) up to four passes following either route A or route B_C were investigated. The experimental results reveal that the ECAE eliminated as-cast dendritic microstructure along with casting defects such as microporosities almost completely. The ECAE-processed samples consisted of mostly elongated microconstituents via route A and equiaxed microconstituents via route B_C. The high stresses imposed in ECAE lead to the fragmentation of the copper-rich θ phase into smaller particles with significant fragmentation occurring in the first pass and additional breaking in the subsequent passes in both routes. The ECAE processing simultaneously increased both the strength and ductility of the alloy as compared to the as-cast state, regardless of the processing route and number of passes. The deformation behavior of as-cast Al-40Zn-2Cu alloy has changed from brittle to ductile mode after ECAE due to the microstructural refinement, deformation-induced homogenization, and reduction of porosities. The limited impact toughness of as-cast alloy was significantly improved by multipass ECAE, especially in route A.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part II. Microstructural and textural changes

Part I of this study described the changes in order and of structure during cold rolling of a Ni₃Al(B,Zr) alloy. The textural and microstructural changes that occur during deformation are reported in this part. In addition to other features, a high density of shear bands start forming in this alloy from a rather early stage of deformation. The cold rolling texture of the material, which is basically of pure metal type at low strain levels, changes into alloy type after rolling between 35 and 45 pct. The maximum pole density of the alloy type texture is obtained at the {168}〈211〉 location. Transmission electron microscopy (TEM) micrographs show the presence of twins in the material from a deformation level of 35 pct onward, their density increasing with increase in deformation level. As has been proposed earlier, a structural transformation from L1₂ to DO₂₂ appears to take place in the γ′ phase during rolling. This will change the deformation mode from primarily slip to twinning and this could be responsible for the observed textural change with rolling. The γ phase deforms by slip in a manner similar to a fcc material with high stacking fault energy. The final texture of the material actually reflects an aggregate of the components developed in the two phases.     

The effect of temperature and extrusion speed on the consolidation of zirconium-based metallic glass powder using equal-channel angular extrusion

In this study, gas-atomized amorphous Zr_58.5Nb_2.8Cu_15.6Ni_12.8Al_10.3 (Vitreloy 106a) containing 1280 ppmw oxygen was consolidated by equal-channel angular extrusion (ECAE). The powder was vacuum encapsulated in copper cans and subjected to one extrusion pass in the temperature region above the glass transition temperature (T _g) and below the crystallization temperature (T _x). The effects of extrusion temperature and the extrusion rate on microstructure, thermal stability, hardness, and compressive strength are investigated. Compression fracture surfaces were examined to determine the deformation mechanisms. The consolidates in which the time-temperature-transformation (TTT) boundary was not crossed during processing exhibit differential scanning calorimetry (DSC) patterns similar to the initial powder, with a slight decrease in T _x. Compressive strengths of about 1.6 GPa are recorded in the consolidates processed at 30 °C and 40 °C below T _x, which is close to what is observed in cast counterparts. The fracture surfaces exhibit vein patterns covering up to 90 pct of the surface area in some samples, which are characteristic of glassy material fracture. The slight decrease in T _x after consolidation is attributed to thermal-history-dependent short-range order and formation of nanocrystalline islands. The present results show that ECAE is successful in consolidation of metallic glass powder. This processing avenue opens a new opportunity to fabricate bulk metallic glasses (BMGs) with dimensions that may be impossible to achieve by casting methods.     

Effect of Initial Grain Size on the Evolution of {001}?100? Texture in Severely Deformed and Annealed High-Purity Nickel

Development of cube texture ({100}〈001〉) was studied in high-purity Ni (99.97 pct) with widely different starting grain sizes (~28 and 650 μm) following ultrahigh straining (ε _eq  = 6.4) by accumulative roll bonding (ARB) and annealing. The fine-grained starting material (FGSM) develops a much stronger cube texture after different annealing treatments as compared to the coarse-grained starting material (CGSM), despite their very similar bulk deformation texture. A lamellar type deformation structure is observed in both these materials, but the CGSM shows a more fragmented structure and frequent presence of shear bands. The recrystallization texture of the two materials differs right from the onset of recrystallization: cube-oriented grains nucleate and grow in the FGSM in sharp contrast to the nucleation and growth of randomly oriented grains in lamellar as well as shear-banded regions of the CGSM. The observed differences in the evolution of recrystallization texture in the two materials are discussed with regard to the microstructural differences and pertinent theories on the formation of cube texture.     

Formation of Deformation Textures in Face-Centered-Cubic Materials Studied by In-Situ High-Energy X-Ray Diffraction and Self-Consistent Model

The evolution of deformation textures in copper and α brass that are representative of fcc metals with different stacking fault energies (SFEs) during cold rolling is predicted using a self-consistent (SC) model. The material parameters used for describing the micromechanical behavior of each metal are determined from the high-energy X-ray (HEXRD) diffraction data. At small reductions, a reliable prediction of the evolution of the grain orientation distribution that is represented as the continuous increase of the copper and brass components is achieved for both metals when compared with the experimental textures. With increasing deformation, the model could characterize the textures of copper, i.e., the strengthening of the copper component, when dislocation slip is still the dominant mechanism. For α brass at moderate and large reductions, a reliable prediction of its unique feature of texture evolution, i.e., the weakening of the copper component and the strengthening of the brass component, could only be achieved when proper boundary conditions together with some specified slip/twin systems are considered in the continuum micromechanics mainly containing twinning and shear banding. The present investigation suggests that for fcc metals with a low SFE, the mechanism of shear banding is the dominant contribution to the texture development at large deformations.     

Grain-boundary character distribution in recrystallized L1<Subscript>2</Subscript> ordered intermetallic alloys

The grain-boundary character distribution (GBCD) of cold-rolled and, subsequently, recrystallized Co₃Ti and Ni₃(Si,Ti) ordered alloys with an L1₂ structure was studied by the electron backscattered diffraction (EBSD) method, in association with texture. For comparison, the GBCD of recrystallized pure copper and aluminum was also determined. The recrystallization textures of the Co₃Ti alloys as well as the Ni₃(Si,Ti) alloy were significantly weak and different from those of the pure copper and aluminum with a strong cube texture. The GBCD of the Co₃Ti alloys was characterized by a high frequency of Σ3 boundaries. On the other hand, the GBCD of the Ni₃(Si,Ti) alloy was characterized by a lower frequency of Σ3 and higher frequency of random (e.g., Σ>29) boundaries than that of the Co₃Ti alloys. However, the GBCDs of the Co₃Ti and Ni₃(Si,Ti) alloys were similar to each other and also quite similar to those of the pure copper and aluminum, when Σ3 boundaries are excluded from the GBCD. Based on these results, the formation mechanism responsible for the recrystallization textures and the grain-boundary structure and energy of the Co₃Ti and Ni₃(Si,Ti) alloys were discussed, in comparison with those of pure copper and aluminum.     

Low-Cycle fatigue of dispersion-strengthened copper

The cyclic deformation behavior of a dispersion-strengthened copper alloy, GlidCop Al-15, has been studied at plastic strain amplitudes in the range 0.1 pct ≤Δε _p/2 ≤ 0.8 pct. Compared to pure polycrystalline copper, the dispersion-strengthened material exhibits a relatively stable cyclic response as a consequence of the dislocation substructures inherited from prior processing and stabilized by the A1₂O₃ particles. These dislocation structures remain largely unaltered during the course of deformation; hence, they do not reveal any of the features classically associated with copper tested in fatigue. At low amplitudes, the fatigue lifetimes of the dispersion-strengthened copper and the base alloy are similar; however, the former is more susceptible to cracking at stress concentrations because of its substantially greater strength. This similarity in fatigue lifetimes is a consequence of the dispersal of both deformation and damage accumulation by the fine grain size and dislocation/particle interactions in the GlidCop alloy. The operation of these mechanisms is reflected in the fine surface slip markings and rough fracture surface features for this material. Formerly Graduate Research Assistant, University of California, Davis, CA     

Deformation banding and copper-type rolling textures

It is shown by a crystallographic etching technique applicable to copper that deformation banding is an important deformation mode in f.c.c. metals and alloys. In a cold rolled coarse grain copper, deformation banding forms in a three dimensional manner dividing the grain on average into over 600 regions of different orientations. The influence of this important, but long ignored deformation mode, is studied by incorporating it into the Taylor model. The predicted textures from the new model are better than those from other existing models in mainly two respects. Firstly, the DB model predicts the co-existence of the three major f.c.c. rolling texture components, namely {123}〈634〉 or S component, >112>〈111〉 or C and >110>〈112〉 or {f}B{/f}. The existing models are deficient in that they predict either C and S or B, but not their co-existence. The second point is that textures predicted by the existing models are always too sharp compared to the experimental textures. The DB model predicts texture peaks with larger spread and hence more realistic texture sharpness. Another feature of the model is that only two independent slip systems, instead of five, are required to accomodate the imposed shape change, which agrees with experimental observation.     

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti₃Ni₄ precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti₃Ni₄ precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Lüders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Lüders band propagation and result in a more gradual transformation.     

The role of texture in work hardening of copper

The texture of previously-studied copper of grain sizes 3.4, 15 and 150 μ m has been measured, and compared to copper either mechanically processed to “randomize” texture, or cast in an equiaxed condition. Work-hardening behavior was also determined for the latter two types of material. The 3.4 μ m material was found to be very weakly textured, yet its stress-strain curve differed considerably from the Taylor-predicted curve based on a <111> single crystal. More strongly fiber-textured material, such as the 150 μ m material, and also the weakly-textured cast material of 405 μ m grain size, agreed much more closely with the Taylor prediction. It was concluded that any texture effects on work hardening in copper are overwhelmed by grain size effects.     

Effect of deformation route on microstructural development in aluminum processed by equal channel angular extrusion

Aluminum has been deformed by equal channel angular extrusion (ECAE) to obtain submicron-grained structures under different deformation routes. The deformation routes were varied by rotating billets through 0, 90, and 180 deg between each extrusion pass, and were designated as route A, B_C, and C, respectively. Based on quantitative microstructural analysis, the effectiveness of the deformation route is shown to depend upon the different definition used. The order of effectiveness is (a) A > B_C > C for both 90 and 120 deg dies, in terms of the generation of high-angle grain boundaries (HAGBs); (b) B_C > C > A for both 90 and 120 deg dies, in terms of the formation of equiaxed shape of grains; and (c) B_C > A > C for 90 deg die and B_C ∼ A > C for 120 die, in terms of reducing grain size. It is suggested that the generation of HAGBs can be related to the accumulation of nonredundant strain, while the shape and orientation of grains may be linked to the shearing patterns of the deformation route.     

Preparation and Electrorheological Property of Y4O（OH）9（NO3）-NH4NO3 Materials

The new electrorheological (ER) material, a particle material composed of Y4O(OH)9(NO3) and NH4NO3, was obtained.They display better ER performance.The shear stress of the suspension of Y4O(OH)9(NO3)(NH4NO3)2.8 material in dimethyl silicone oil reaches 1469 Pa at an electric field strength (E) of 4.2 kV·mm-1 and the shear rate (γ) of 150 s-1.The relative shear stress, τE/τ0 (τE and τ0 are the shear stresses at E=4.2 and 0 kV·mm-1, respectively), is up to 29, which is 19 times that of pure Y2O3 material.The dielectric and conductive property of the materials play important roles in the modification of the ER effect of the particle materials.The researches on these new ER materials are very useful for obtaining a better understanding on the mechanism of the ER effect and finding an ideal ER material.     

Scanning electron microscope fractography of continuously cast high purity copper after high temperature creep

Fracture surfaces produced by high temperature creep were studied using the scanning electron microscope. The material investigated was continuously cast high purity copper containing a nodal impurity segregation structure at which grain boundary voids are formed during creep. The observed void shape suggests that vacancies are supplied mainly via grain boundaries, and also by enhanced diffusion via segregation nodes; the vacancies seem to originate mainly at internal sources. The known distribution of potential nucleation sites was used to study the efficiency of the segregation structure in nucleating voids under various test conditions. Within the range of conditions employed, three different fracture modes were observed in separate regions of the stress-temperature plane. The regions are sequentially denotedA, B, and C as the temperature is increased at a given stress; they shift to lower temperatures as the stress is increased. In regionA fracture is initiated by extensive cavitation along grain edges (line of junction of three grains); cavitation at the segregation structure seems to be of secondary importance. In regionB formation and growth to coalescence of voids at segregation nodes governs fracture; the change of growth mechanisms with test conditions is discussed. In region C fracture is controlled by plastic instability. A. RUKWIED,Physicist , formerly with Mechanical Properties Section, Metallurgy Division, Institute for Materials Research, National Bureau of Standards, U. S. Department of Commerce, Washington, D. C.     

Hot rolling texture development in CMnCrSi dual-phase steels

The amount of strain below the temperature of nonrecrystallization, T _nr , has an important influence on the phase fractions and the final crystallographic texture of a hot-rolled dual-phase ferrite+martensite CMnCrSi steel. The final texture is influenced by three main microstructural processes: the recrystallization of the austenite, the austenite deformation, and the austenite-to-ferrite transformation. The amount of strain below T _nr plays a major role in the relative amounts of deformed and recrystallized austenite after rolling. Recrystallized and deformed austenite have clearly different texture components and, due to the specific lattice correspondence relations between the parent austenite phase and its transformation products, the resulting ferrite textures are different as well. In addition, austenite deformation textures result from either dislocation glide or the combination of dislocation glide and mechanical twinning, depending on the stacking fault energy (SFE). The texture components in hot-rolled dual-phase steels were studied by means of X-ray diffraction (XRD) measurements and orientation imaging microscopy (OIM). A clear crystallographic orientation difference was observed between the ferrite phase, transformed at temperatures near A _r3 , and the ferritic bainite and martensite phases, formed at lower temperatures. The results suggest that the primary ferrite, nucleated at temperatures close to A _r3 , transformed from the deformed austenite. The low-temperature constituents, bainite and martensite, form in the recrystallized austenite.     

Anelastic behavior of barium-titanate-based ceramic materials

The internal friction (Qsu−1) and Young’s modulus (E) of BaTiO₃-based ceramics were measured vs temperature from −100 °C to 150 °C. Rectangular bars of high-density (96 to 99 pct) ma-terials were driven electrostatically in flexural vibration at a resonance frequency of about 3 kHz, at maximum strain levels of about 10⁻⁶. The curves ofQ ⁻¹(T) andE(T) allow the study of the following three phase transformations: tetragonal to cubic (about 130 °C in pure material), orthorhombic to tetragonal (about 0 °C in pure material), and rhombohedral to orthorhombic (about −80 °C in pure material). Internal friction and modulus data were obtained on pure material and on materials doped with niobium and cobalt to give semiconducting and insulating X7R behavior. Permittivity, dielectric loss, and microstructure data are given and used to aid interpretation of the mechanical measurement data. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee     

Strain-path effects on the evolution of microstructure and texture during the severe-plastic deformation of aluminum

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-μm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 μm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.     

A comparison of ultrasonic and X-ray determinations of texture in thin Cu and Al plates

Ultrasonic techniques for determining the orientation distribution coefficients (ODC’s), which define the preferred orientation of polycrystals, are discussed. The theory is reviewed for thin plates of cubic crystallites for which the texture information is deduced from the velocity anisotropy of guided modes. Experimental ultrasonic and X-ray predictions of the ODC’s of up to an order of 4 are compared for plates of commercially pure electrolytic tough pitch (ETP) copper and aluminum. ForW ₄₂₀ andW ₄₄₀ in both samples andW ₄₀₀ in copper, the predictions agree to |ΔW|∼10^-3. However, considerably greater differences are reported for the predictions ofW ₄₀₀ in aluminum. Interpretation of these comparisons is assisted by a detailed error analysis for the ultrasonic technique and reference to a number of other recent comparisons of ultrasonic and neutron or X-ray predictions of ODC’s Possible applications of the ultrasonic technique during the production and forming of metal sheet are indicated. S. S. LEE, formerly a Graduate Student with Ames Laboratory     

Synchrotron X-Ray Diffraction Study of Texture Evolution in 904L Stainless Steel under Dynamic Shock Compression

The influence of strain rate on development of deformation texture under a dynamic shock compression of a 904L stainless steel was quantitatively investigated using synchrotron X-ray diffraction and crystallographic orientation distribution function (ODF) analysis. The Split-Hopkinson Pressure Bar (SHPB) technique was used to generate a high strain rate of >10³ s⁻¹ for preparing the deformed samples. Starting with an almost random texture in a solution treatment condition, the deformed material developed several typical texture components, such as Goss texture and Brass texture. Compared to the texture components displayed in the state of quasi-static compression deformation, it was found that the high-speed deformation generated much weaker texture components. In combination with the change in microstructures observed by electron backscattering diffraction (EBSD) and the transmission electron microscopy (TEM) technique, the high-energy X-ray diffraction provides a powerful tool for characterizing the strain-rate dependence of grain rotation at each stage of deformation. The deformation heterogeneity evident in our experiment can be explained by a transition of deformation mechanism from the dislocation/twin-dominated mode to a shear-band-dominated one with increasing strain rate.     

Deformation banding and ferrite-type rolling textures

A computer model for predicting rolling textures formed in bcc metals and alloys deforming by slip using the Taylor-Chin framework and including deformation banding is presented. The model is rate insensitive. Deformation banding in both the rolling and transverse planes were investigated and it is shown that the best results are obtained when grain subdivision in the rolling plane only is allowed. By modeling two kinds of deformation banding in the rolling plane, rotations of typical texture components along the γ fiber and the α fiber have been traced to demonstrate the development of individual texture components formed by deformation banding. The influence of latent hardening through a parameter, L _h , on the deformation banding process is included in the model, and it is shown that lower L _h produces peak-type textures, whereas higher values for L _h lead to more deformation banding as expected, and this strengthens the γ fiber.     

Grain Refinement in Pure Aluminum Severely Deformed by Equal Channel Angular Extrusion via Extended Processing Routes

AA1060 pure aluminum billets were processed by eight passes of equal channel angular extrusion (ECAE) using 90 and 120 deg dies via processing routes characterized by an inter-pass billet rotation angle (χ) varying from 0 to 180 deg. The grain refinement efficiencies achieved in the different processing conditions were investigated by comparing misorientation and grain size in the deformed samples measured by electron back-scatter diffraction. The results reveal an overall decrease of grain refinement efficiency with an increase of χ for both dies. This trend corroborates the general observations in various face-centered cubic metals processed using a 120 deg die and can be satisfactorily explained by correlating the relative grain refinement efficiency to the relative significance of newly activated slip systems at pass-to-pass transitions. For ECAE with the 90 deg die, the route-dependency of grain refinement found in the AA1060 samples contradicts some of the observations in the literature, and the main discrepancies are located for routes with χ = 0 to 90 deg. Comparison of the present results with those of pure copper processed under similar conditions further reveals that these discrepancies could be mainly ascribed to differences in the characteristics of the materials, and that it is irrational to simply claim the route with χ = 0 or 90 deg as the optimal route without necessary experimental validations for a specific material.