Microstructural evolution and deformation of cryomilled nanocrystalline Al-Ti-Cu Alloy

The microstructural evolution during processing and tensile deformation of a nanocrystalline Al-Ti-Cu alloy was investigated using transmission and scanning electron microscopy. Grain refinement was achieved by cryomilling of elemental powders, and powders were consolidated by hot isostatic pressing (“HIPing”) followed by extrusion to produce bulk nanocrystalline Al-Ti-Cu alloys. In an effort to enhance ductility and toughness of nanocrystalline metals, multiscale structures were produced that consisted of nanocrystalline grains and elongated coarse-grain (CG) bands of pure aluminum. Examination of bulk tensile fracture samples revealed unusual failure mechanisms and interactions between the CG bands and nanocrystalline regions. The ductile CG bands underwent extensive plastic deformation prior to fracture, while nanocrystalline regions exhibited nucleation and growth of voids and microcracks. Cracks tended to propagate from nanocrystalline regions to the CG bands, where they were effectively arrested by a combination of crack blunting and crack bridging. These processes were instrumental in enhancing the toughness and ductility of the nanocrystalline alloy.     

Investigation of recrystallization and grain growth of copper and gold bonding wires

Copper bonding wires were characterized using electron backscatter diffraction (EBSD). During drawing, shear components are mainly located under the surface and 〈111〉 and 〈100〉 fiber texture components develop with similar volume fractions. Grain average misorientation (GAM) and scalar orientation spread (SOS) of the 〈100〉 component are lower than those of the 〈111〉 or other orientations. Also, 〈100〉 components grow into other texture orientations during recrystallization. Copper wires experience three stages of microstructure change during annealing. The first stage is subgrain growth to keep elongated grain shapes overall and to be varied in aspect ratio. The grain sizes of the 〈111〉 and 〈100〉 components increase. The volume fraction of the 〈100〉 component increases, whereas that of the 〈111〉 decreases. The second stage is recrystallization, during which equiaxed grains appear and coexist with elongated ones. The third stage is grain growth, which eliminates the elongated grains. The 〈111〉 and 〈100〉 grains compete with each other, and the 〈111〉 grains grow faster than the 〈100〉 grains during the third stage. Comparison of recrystallization and grain growth processes in copper and gold wires reveals many common microstructural features.     

About the shape of eutectic grains solidifying in a thermal gradient

In eutectic alloys solidified in a thermal gradient, it has been observed that the final shape of the grains nucleated in the bulk of the liquid is more elongated in the direction opposite to that of the heat flow. This experimental result appears to be in contradiction with that expected for the growth of an isolated grain, since the portion of the interface located in the downstream heat flow direction is the most undercooled and thus has the highest growth rate. However, when considering a family of grains which continuously nucleate in the bulk of the liquid, it is shown that the impingement of the grains limits their growth in the downstream heat flow direction and thus explains their final shape. In order to investigate this phenomenon, the differential equation which governs the growth of an isolated eutectic grain in a thermal gradient has been derived and solved analytically for the two extreme positions of the interface along the heat flow direction. Using these relationships, the asymmetry factor of the grains has been deduced as a function of the solidification parameters. The overall shape of an isolated grain has also been predicted using numerical integration. Finally, these results are integrated into a stochastic model of grain structure formation and the simulated microstructure is compared with experimental micrographs previously obtained for hypereutectic aluminium-silicon alloys remelted by laser.     

The role of orientation pinning in statically recrystallized oxygen-free high-conductivity copper wire

The role of orientation pinning by neighboring grains on migrating boundaries in a statically recrystallized oxygen-free high-conductivity (OFHC) copper was investigated. Two specimens of heavily drawn OFHC copper wires deformed to true strains of 2.31 and 3.56 were annealed at 170 °C and local orientations were mapped by means of the automated electron backscattered diffraction technique. Inverse pole figures, misorientation distribution functions, and grain boundary misorientations were calculated from local orientation data. In spite of annealing, the microstructure of the low-strain specimen was characterized by elongated grains, similar to the as-deformed structure, whereas the microstructure of the high-strain specimen showed a high fraction of well-defined recrystallized grains. The recrystallized grains consisted of type A grains, which mostly grew laterally with {hkl}〈100〉 orientations, and type B grains, which generally grew axially with {hkl}〈111〉 orientations. Type A grains were larger and of higher frequency than type B grains. The large size of Type A grains was attributed to the high frequency of the mobile boundaries with misorientations in the 40 to 50 deg range. Boundaries that were misoriented at 60 deg 〈111〉 (Σ3) were found to exert the greatest pinning effect on the growing grains. This caused recrystallized grains to grow either laterally or axially, and sometime led to “branching.” A detailed analysis of the influence of the next neighbor misorientations in the perimeter of the recrystallized grains is presented.     

Microstructural characterization of a rapidly solidified ultrahigh strength Al94.5Cr3Co1.5Ce1 alloy

The microstructure of a rapidly solidified Al_94.5Cr₃Co_1.5Ce₁ alloy has been examined in detail by means of high resolution transmission electron microscopy (HRTEM) and atom probe field ion microscopy (APFIM). In the as-quenched microstructure, nanoscale particles of a solute-enriched amorphous phase and an Al-Cr compound are dispersed in randomly oriented fine grains of α-Al (∼200 nm). The interface between the Al grains and the amorphous particles is not smooth but irregular with atomic protrusions and concavities, suggesting that interfacial instability occurs during the solidification process. Nanoscale amorphous particles are formed as a result of solute trapping within the rapidly grown Al grains. After annealing at 400 °C for 15 minutes grain growth occurs, and the interface of the Al grains is smoothed. The amorphous region trapped within the grains is crystallized to an Al-Cr compound, but no icosahedral phase has been confirmed. The APFIM results have revealed that Cr and Ce atoms have a similar partitioning behavior, i.e., they are rejected from the α-Al phase and partitioned into the trapped amorphous regions. On the other hand, Co atoms are not partitioned between the two phases in the as-quenched state but are partitioned into the α-Al grains in the annealed alloys being rejected from the Al compounds and finally form Al-Co compounds. Based on these microstructural characterization results, the origins of high strength of this alloy are discussed.     

Investigation of “shock-induced” and “shock-assisted” chemical reactions in Mo + 2Si powder mixtures

Chemical reactions occurring in Mo + 2Si powder mixtures under “shock-induced” (during the high-pressure shock state) and “shock-assisted” (due to bulk temperature increases subsequent to unloading from the shock state) conditions were investigated using shock recovery experiments performed under a range of loading conditions. Cylindrical implosion geometry experiments showed fully reacted mixed-phase eutectic microstructure (MoSi₂ and Mo₅Si₃) in axial regions and a partially reacted region (containing MoSi₂ spherules surrounding molybdenum particles in melted and resolidified silicon matrix) in outer peripheral areas of the compact cross sections. Planar-pressure geometry experiments showed a single-phase MoSi₂ microstructure in almost the entire compact. Calculations of the peak shock pressure and maximum mean bulk temperature in the different regions of the compacts and their correlation with the observed microstructures suggest that the formation of the mixed-phase and partially reacted products in the implosion geometry samples is due to “thermally initiated” liquid-liquid or solid-liquid reactions. In contrast, formation of the single-phase product in the planar-pressure geometry experiments is due to solid-state “pressure-initiated” reactions. The “thermally initiated” reactions are a result of large increases in shock-generated bulk temperatures, produced in time scales of thermal equilibration following unloading from the high-pressure state; hence, these are referred to as “shock-assisted” chemical reactions. However, the pressure-initiated reactions occur during the rise to the peak pressure and in time scales of pressure equilibration; hence, these are referred to as “shock-induced” chemical reactions.     

Evolution of microstructure and texture during casting of AISI 304 stainless steel strip

The solidification behavior of AISI 304 stainless steel strip was studied using a melt/substrate contact apparatus, whereby a copper substrate embedded in a moving paddle is rapidly immersed into a steel melt to produce thin (∼1-mm gage) as-cast coupons. For cases where other casting conditions were kept constant, the effect of substrate topography and melt superheat on the development of microstructure and texture during solidification was studied using electron backscatter diffraction (EBSD) and optical microscopy. It was found that nucleation and growth of grains during solidification were influenced both by substrate topography and melt superheat. A ridged substrate produced a high density of randomly oriented grains at the chill surface with the preferred growth of 〈001〉-oriented grains perpendicular to the substrate wall producing a coarse columnar grain structure exhibiting a strong 〈001〉 fiber texture at the strip center. In contrast, a smooth substrate resulted in a lower nucleation density to produce a very coarse-grained columnar microstructure with moderate and essentially constant 〈001〉 fiber texture throughout the strip thickness. By the manipulation of casting parameters, it is possible to produce strip-cast austenitic stainless steel with a particular microstructure and texture.     

Warm Deformation of Low Carbon Steel Using Forward Extrusion-Equal Channel Angular Pressing Technique

A combination of extrusion and equal channel angular pressing (ECAP) was used to deform a plain low carbon steel. This process consists of two successive deformations by extrusion and ECAP in a single die (Ex-ECAP). Cylindrical samples were heated to predefined temperatures (650 and 850 ℃) and then pressed through a die channel with crosshead speed of 10 mm/s. Microstructure and resultant mechanical properties of processed material were studied. The results showed that pressing temperature has a significant effect on the resultant microstructure. While at 650 ℃, the cold worked structure with elongated ferrite grains were obtained, and at 850 ℃ the microstructure consisted of elongated ferrite grains and very fine grains at their boundaries as a consequence of continuous dynamic recrystallization (CDRX) of ferrite phase. Also at 850 ℃, a particular microstructure consisted of cold worked ferrite and static recrystallized grains on shear bands was obtained.     

The effects of Cr additions to binary TiAl-base alloys

The effects of Cr additions to y-base alloys have been investigated, using bulk materials consolidated from rapid solidification-processed ribbons. The composition ranges studied were 0 to 4 at. pet Cr and 44 to 54 at. pet Al. It was found that Cr additions do not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean duplex alloys which contain grains of single-phase γ and grains of lamellar γ/α₂. Other Cr effects on microstructure, phase stability, site occupancy, and deformation sub-structures were characterized and correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr in duplex alloys is partially due to the tendency of Cr to occupy Al lattice sites. Ductilization is also partially due to the ability of Cr to modify the Al partitioning and, therefore, the thermal stability of transformed α₂ laths.     

Effects of Applied Strain on Interface Microstructure and Interdiffusion in the Diffusion Couples

The effects of applied strain on the interface microstructure and atomic interdiffusion in the binary alloy diffusion couples were studied using the phase-field model. In the two-phase diffusion couples, the single-phase regions are formed beside the interface without applied strain, and the width of single-phase regions enlarges as temperature increases. When the strain is applied, the phases are elongated and they are across the initial interface, which makes the diffusion couples to syncretize as the temperature increases or concentration difference decreases. In the diffusion couples formed by single and two phases, the larger composition difference results in the larger movement distance of interface, the atomic diffusion direction is determined by the initial composition difference. Under the applied strain, the elongated two phases are also across the initial interface with the small concentration difference. However, when the concentration difference is large, the two-phase region is recessional as the single-phase region moves forward. When the applied strain makes the morphology parallel to the initial interface of the diffusion couple, the single-phase regions are formed beside the interface.     

Effects of sulfur impurity on the scale adhesion behavior of a desulfurized Ni-based superalloy aluminized by chemical vapor deposition

The surface of a single-crystal Ni-based superalloy, which contained a bulk sulfur content of ∼0.4 ppmw, was aluminized in a hot-wall chemical vapor deposition (CVD) reactor, using AlCl₃ and H₂ as gaseous precursors, at 1100 °C. The chemical composition and microstructure of the resulting aluminide coating were characterized with particular emphasis on sulfur incorporation as an impurity during aluminizing. Depth profiling by glow-discharge mass spectroscopy (GDMS) was used as a qualitative means of assessing the level of sulfur in the coating structure. Sulfur contamination, which was initially observed at the coating surface and the substrate-coating interface, could be reduced by some minor reactor modifications. With the reduced sulfur content, scale adhesion on the surface of the aluminide grains of the coating was significantly improved during cyclic oxidation, whereas scale spallation at the coating’s grain boundaries became more apparent.     

Room-temperature mechanical behavior of cryomilled al alloys

Room-temperature mechanical properties of cryomilled Al-7.5 pct Mg and Al 5083 alloys are discussed in the context of a duplex microstructure, which arises during processing. After consolidation via hot isostatic pressing (“hipping”), coarse-grained regions are formed in former interparticle void volumes, and these regions become elongated during extrusion. Comparison of tensile and compression testing results on both “as-hipped” and extruded materials shows that tension-compression asymmetry is the result of these coarse-grained regions and not necessarily a fundamental property of ultrafine grained Al. The strength of the extruded materials is consistent with the Hall-Petch model of strengthening by grain size refinement, but the hipped material deviates from this trend, with a lower strength despite finer average grain size. This can also be attributed to the presence of coarse-grained regions, which substract from the strength in a predictable manner and also enhance the ability of the cryomilled material to work harden.     

Development of a necklace microstructure during isothermal deformation and its properties relative to uniform microstructures

A necklace microstructure containing a controlled amount of fine (i.e., 4 to 6 μm) grains along the boundaries of larger (i.e., 30 to 40 ώm) warm-worked grains is intended to achieve a balance in mechanical properties for applications such as gas turbine disks. The development of this duplex grain microstructure critically depends on the starting microstructure, the strain and strain rate of the deformation process, and the subsequent heat-treatment conditions. These aspects of microstructural development were studied using isothermal compression tests on powder metallurgy (PM) ASTROLOY consolidated by “hipping” or extrusion. Tensile, creep, and stressrupture (S/R) properties of the necklace microstructure were also evaluated and compared with those of uniform grain microstructures. Formerly Senior Scientist, Research and Development Department, Cameron Forge Company     

Microscale and Mesoscale Crystallographic Textures of Nanocrystalline Ni-Based Electrodeposits

The microscale and mesoscale crystallographic textures observed in nanocrystalline Ni, Ni-20 pct Fe, and Ni-50 pct Fe electrodeposits are described. The nanosized grains are arranged in coarse mesoscale colonies. In the as-deposited state, the bulk texture of the Ni-20 pct Fe alloy displays a dominant 〈001〉 fiber parallel to the macroscopic deposition direction (DD). The grains are elongated along the 〈001〉 crystal lattice direction, which is mostly parallel to the local DD, producing a well-defined 〈001〉//DD fiber microtexture on a local scale. The grain misorientation histograms show some content of low-angle boundaries, frequently associated with the presence of grain clusters, and are dominated by high-angle boundaries with a possible enhanced frequency of ∑5 and ∑7 coincident site lattice boundaries and a significant content of ∑3 twin boundaries. For all three alloys, the coarsened grains, obtained by annealing, within the mesoscale colonies show a fiber mesotexture characterized by a 〈111〉 axis approximately perpendicular to the colony hemispherical growth surface (parallel to the local DD). It is surmised that a similar “cobblestone” mesotexture with a 〈001〉 fiber axis already exists in the as-received state, as previously proposed by a number of the present authors and as supported by the results on the Ni-20 pct Fe alloy presented here.     

The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

The microstructure of rolled and annealed tungsten rod

This paper reports a study of the microstructural changes that occur when potassium-doped tungsten ingots are rolled at elevated temperatures. The effect of annealing on the microstructure of the rolled material is also considered. All samples were rolled on a Kocks mill. At low levels of deformation, the grain boundaries are primarily high-angle boundaries, and many grains are dislocation free. Both of these features probably result from dynamic recrystallization during rolling. As deformation increases, the grains become more elongated, and more low-angle boundaries are found within the material. Also, the potassium gets drawn into narrower and longer tubes. When these rolled rods are annealed at temperatures between 1275 ‡C and 1950 ‡C, several changes occur in the microstructure. The material undergoes abnormal grain growth. The temperature at which this occurs depends on the length of the anneal, the amount of de-formation the rod has received, and the spatial location in the rod. This spatial distribution most likely results from strain gradients that exist in the rolled rod. The abnormal grain growth is accompanied by a decrease in hardness. The potassium-containing tubes in the rod also break up into bubbles during annealing. The temperature at which this breakup occurs again depends on the length of the anneal and the amount of deformation.     

Microstructure Evolution during Friction Stir Welding of Mill-Annealed Ti-6Al-4V

In this study, mill-annealed Ti-6Al-4V plates were successfully friction stir welded over a wide range of processing parameters using a tungsten-1 pct La₂O₃ tool. Two K-type thermocouples embedded in the tool indicated that approximately 25 pct of the heat generated during welding was transferred out of the workpiece and into the tool. The thermocouple data, combined with observations of the microstructure, indicated that the stir zone of all welds exceeded the β transus. The microstructure and texture of two representative welds made just above and high above the β transus were investigated with scanning electron microscopy and electron backscatter diffraction (EBSD). The β phase orientations were reconstructed with a fully automated technique from the as-collected α phase data through knowledge of the Burgers orientation relationship. The results suggest that the fine β grains in the stir zone are formed from the base material ahead of the advancing tool by dissolution of secondary and primary α phase, and there is no further recrystallization. These grains subsequently deform by slip and rotate toward the orientations that are most stable with respect to the shear deformation induced by the tool. In the highest temperature weld, diffusion tool wear in the form of periodically spaced bands provided an internal marker of the tool/workpiece interface during welding. The flow patterns evident within the tungsten-enriched bands suggest that flow is considerably more chaotic on the advancing side than in the central stir zone.     

Calculation of the interfacial energies between α and γ iron and equilibrium particle shape

The interfacial energies between α and γ iron were calculated for Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (N-W), and one orientation relationship (OR), which may represent irrational OR, with varying interface orientation. The relaxation of atomic structure in the vicinity of the interface was performed by the Monte Carlo method using an embedded atom method (EAM) potential. Whereas the polar plot of calculated interfacial energies exhibited small and large cusps for the K-S and N-W ORs, an almost equiaxed energy surface with shallow cusps was obtained for the irrational ORs. The equilibrium shape of an α particle, N-W oriented with the γ matrix, was a thick rectangular plate with broad facets containing monatomic ledges. In contrast, the equilibrium shape of a particle, K-S oriented with the γ matrix, was elongated nearly in the closed-packed directions with {112}_γ type and another broad facet that achieves relatively good matching. The volume of these shapes in the Wulff space tends to be smaller for K-S than that of the N-W OR. The equilibrium shape of the grain boundary α particle at the γ grain boundary was calculated under the assumption that the α particle has K-S OR with one of the two γ grains using the modified Wulff construction proposed earlier. The variation of the shape of the α particle with γ grain boundaries at an early stage of precipitation may primarily be ascribed to the change in the variant of the smallest nucleation activation energy and less prominently to the change in the γ grain boundary energy. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.     

Early stages of recrystallization in nickel

This paper reports on an investigation into the early stages of recrystallization in nickel from the standpoint of the misorientation between newly formed recrystallized grains and the surrounding deformed microstructure. For the case of recrystallization “nuclei”(i.e., <l-μm diameter), half the sampled interfaces were close to low-gS coincident site lattice (CSL) misorientations, particularly gS = 3. Established new grains(i.e., >l-μm diameter) tended to be either not low-gS CSLs or less close to a low-gS CSL than for the “nucleus” case. The interpretation of these data is that the very earliest stage of recrystallization is facilitated at a boundary which is more mobile than average, often a characteristic of a low-gS CSL. After nucleation of recrystallization has been established, the important criterion for the recrystallization interface is a large capacity for dislocation absorption, which is associated with general boundaries rather than CSLs. The number of boundaries bordering the new grains and their geometrical characteristics were consistent with a model which relates sides per grain to grain boundary energy.