Microstructure evolution and mechanical behavior of bulk copper obtained by consolidation of micro- and nanopowders using equal-channel angular extrusion

The consolidation of copper micro- and nanoparticles (325 mesh, 130 nm, and 100 nm) was performed using room-temperature equal-channel angular extrusion (ECAE). The effects of extrusion route, number of passes, and extrusion rate on consolidation performance were evaluated. The evolution of the microstructure and the mechanical behavior of the consolidates were investigated and related to the processing route. Possible deformation mechanisms are proposed and compared to those in ECAE-processed bulk Cu. A combined high ultimate tensile stress (470 MPa) and ductility (∼20 pct tensile fracture strain) with near-elasto-plastic behavior was observed in consolidated 325-mesh Cu powder. On the other hand, early plastic instability took place, leading to a continuous softening in flow stress of bulk ECAE-processed copper. Increases in both strength and ductility were evident with an increasing number of passes in the bulk samples, which appears to be inconsistent with grain-boundary-moderated deformation mechanisms for a microstructure with an average grain size of 300 to 500 nm. Instead, this increase is attributed to microstructural refinement and to dynamic recovery and bimodal grain-size distribution. Near-perfect elastoplasticity in consolidated 325-mesh Cu powder is explained by a combined effect of strain hardening accommodated by large grains in the bimodal structure and softening caused by recovery mechanisms. Compressive strengths as high as 760 MPa were achieved in consolidated 130-nm copper powder. Although premature failure occurred during tensile loading in 130-nm consolidated powder, the fracture strength was still about 730 MPa. The present study shows that ECAE consolidation of nanoparticles opens a new possibility for the study of mechanical behavior of bulk nanocrystalline (NC) materials, as well as offering a new class of bulk materials for practical engineering applications.     

High copper content bulk glass formation in bimetallic Cu-Hf system

We report in this article that strip-shaped amorphous samples with thicknesses from 0.5 to 2 mm were successfully synthesized for binary Cu-Hf alloys containing 60 to 68 at. pct Cu by the traditional copper mold casting method. The best glass former Cu₆₆Hf₃₄ with casting thickness up to 2 mm has an undercooled liquid region (ΔT=T _x −T _g , where T _g is the glass transition temperature and T _x is the onset temperature of the first crystallization event) of 51 K, which is somewhat narrow compared with other neighboring alloys in the same system. The bulk glassy Cu₆₆Hf₃₄ alloy exhibits Vicker’s Hardness (H _v ) ∼779 kg/mm², Young’s modulus ∼108 GPa, fracture strength ∼2.1 GPa, and an almost constant elastic elongation ∼1.8 pct upon compression. The discovery of Cu₆₆Hf₃₄ as a bulk glass confirms the existence of rather simple bulk-glass-forming metallic systems. Moreover, the present Cu-Hf alloys may be the highest copper content bulk metallic glasses reported to date, to the best of our knowledge.     

Effect of Au Content on Thermal Stability and Mechanical Properties of Au-Cu-Ag-Si Bulk Metallic Glasses

The thermal stability, glass-forming ability (GFA), and mechanical and electrical properties of Au-based Au _x Si₁₇Cu_75.5–x Ag_7.5 (x = 40 to 75.5 at. pct) metallic glasses were investigated. The glass transition temperature (T _g ) and crystallization temperature (T _x ) decreased with increasing Au content. The ultralow T _g values below 373 K (100 °C) were obtained for alloys with x = 55 to 75.5. The alloys with x = 45 to 70 exhibited a high stabilization of supercooled liquid and a high GFA, and the supercooled liquid region and critical sample diameter for glass formation were in the range of 31 K to 50 K and 2 to 5 mm, respectively. The compressive fracture strength (σ _c,f ), Young’s modulus (E), and Vicker’s hardness (H _v ) of the bulk metallic glasses (BMGs) decreased with increasing Au content. A linear correlation between Au concentration and the characteristic temperature, i.e., T _g and T _x , and mechanical properties, i.e., σ _c,f , E, and H _v , as well as electrical resistivity can be found in the BMGs, which will be helpful for the composition design of the desirable Au-based BMGs with tunable physical properties.     

Effect of particle size and volume fraction on hot extrusion reaction synthesis of SiC particle reinforced NiAl

Hot extrusion reaction synthesis (HERS) was used to fabricate nickel aluminide/SiC _p composites from elemental powder mixtures of nickel, aluminum, and silicon carbide. The effect of extrusion temperature, silicon carbide particle size, and volume content on the developed microstructures and on the peak extrusion pressure was investigated using a miniature extrusion rig. Matrix microcracking and loss of aluminum were observed in the final microstructures. The large surface area to volume ratio of the miniature extrudate “wires” in conjunction with a shorter reaction time at temperature reduced the reaction between the matrix and the SiC reinforcements. Although extrusion should have eliminated reaction synthesis related porosity, considerable levels of porosity were still generally present in the final extrudates, because all the elemental powder extrusions reacted after emerging from the die exit instead of before, thus by-passing the consolidation stage of extrusion. A novel two-stage extrusion method has been identified to overcome this problem.     

Finite-element modeling of nonisothermal equal-channel angular extrusion

Deformation during conventional (nonisothermal) hot working of metals via equal-channel angular extrusion (ECAE) was investigated using two-dimensional (2-D) and three-dimensional (3-D) finite-element modeling (FEM) analysis. The effects of material flow properties, die-workpiece heat-transfer and friction conditions, and die design on metal flow were examined. Friction and die design were shown to be the most important parameters governing the formation of dead-metal zones during extrusion. On the other hand, thermal gradients induced by die chill and deformation heating were found to exacerbate the extent of flow localization that arises due to material-flow softening alone. The FEM predictions were validated by ECAE experiments on a Ti-6Al-4V alloy with a colony alpha microstructure. Preforms exhibited minor edge cracking and mild flow localization during extrusion at 985 °C, but severe shear localization and fracture during extrusion at 900 °C. The 2-D FEM simulations predicted deformation detail, including shear localization, that was in good agreement with the experimental results, but 3-D FEM simulations were required to realistically predict die chill. A combined approach, in which thermal data were extracted from 3-D simulations and inserted into 2-D simulations, produced load-stroke and fracture predictions in general agreement with measured values.     

Effects of tungsten fiber on failure mode of zr-based bulk metallic glassy composite

The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 metallic glassT _g, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearT _g are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.     

Synthesis of new bulk metallic glassy Ti60Al15Cu10W10Ni5 alloy by hot-pressing the mechanically alloyed powders at the supercooled liquid region

The mechanical alloying method has been used to fabricate multicomponent Ti₆₀Al₁₅Cu₁₀W₁₀Ni₅ glassy alloy powders, using a low-energy ball-milling technique. The glassy powders that were obtained after 720 ks of milling have a sphere-like morphology with an average particle size of 0.38 μm in diameter. This new glassy alloy exhibits a glass transition temperature (T _g) at 733 K. It crystallizes at a crystallization temperature (T _x ) of 804 K through a single sharp exothermic peak, with an enthalpy change of crystallization (ΔH _x ) of −5.20 kJ/mol. The supercooled liquid region before crystallization ΔT _x of the obtained glassy powders shows a large value (71 K). The reduced glass transition temperature (ratio betweenT _g and liquidus temperatures,T ₁(T _G /T ₁) was found to be 0.46. The synthetic glassy powders were uniaxial hot-pressed into consolidated round objects with large dimensions (20 mm in diameter × 30 mm in height) in an argon gas atmosphere at several temperatures with a pressure of 936 MPa. The samples that were consolidated at the temperature range of 755 to 775 K (within the ΔT _x region) are fully dense (∼99.85 pct) and maintain the chemically homogeneous glassy structure. These hot-pressed glassy samples exhibit excellent mechanical properties for Ti-base metallic glasses. They have high Vickers microhardness values, in the range between 8.0 and 8.2 GPa. They also show high fracture strength (2.28 GPa) with an extraordinarily high Young’s modulus of 153 GPa. Neither yielding stress nor plastic strain could be detected for this glassy alloy, which shows an elastic strain of 1.39 pct.     

Amorphous Zr-Based Foams with Aligned,Elongated Pores

Interpenetrating phase composites are created by warm equal channel angular extrusion (ECAE) of blended powders of amorphous Zr_58.5Nb_2.8Cu_15.6Ni_12.8Al_10.3 (Vit106a) and a crystalline ductile metal (Cu, Ni, or W). Subsequent dissolution of the continuous metallic phase results in amorphous Vit106a foams with ~40 pct aligned, elongated pores. The extent of Vit106a powder densification in the composites improves with the strength of the crystalline metallic powder, from low for Cu to high for W, with a concomitant improvement in foam compressive strength, ductility, and energy absorption.     

Fatigue and fracture behavior of bulk metallic glass

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅ bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformation, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.     

Effects of Changes in Chemistry and Testing Temperature on Mechanical Behavior of Al-Based Amorphous Alloy Ribbons

The effects of changes in composition, testing temperature, strain rate, and thermal exposure on the flow behavior of a series of Al-Gd-Ni-X amorphous alloy ribbons have been determined via hot microhardness and tension testing. It is shown that the addition of Fe, Co, and Fe/Co combination into these materials increases the strength, T_g, and T _x1 in addition to the activation energy for crystallization, whereas the window between T_g and T _x1 remains similar. The uniaxial tensile tests show these ribbons exhibit a high strength, around 1 GPa, at room temperature (RT), and the results also show that these ribbons maintain their strength, nearly 45 pct of their RT value, at temperatures near T_g. Scanning electron microscopy images of fracture surfaces obtained from tests conducted near T_g illustrate ductile rupture and homogeneous flow behavior near the fracture tip.     

Hot working of Ti-6Al-4V via equal channel angular extrusion

The deformation behavior of Ti-6Al-4V during high-temperature equal channel angular extrusion (ECAE) with or without an initial increment of upset deformation was determined for billets with either a lamellar or an equiaxed alpha preform microstructure. For conventional ECAE (i.e., deformation by simple shear alone), flow localization and fracture occurred at temperatures between 900 °C and 985 °C. In contrast, billets deformed at temperatures between 845 °C and 985 °C using an initial increment of upset deformation immediately followed by the simple shear deformation of ECAE exhibited uniform flow with no significant cracking or fracture. A simple flow-localization criterion was used to explain the influence of preupsetting on the suppression of localization in billets with the lamellar microstructure. The suppression of flow localization for the equiaxed microstructure and the elimination of edge cracking for both types of microstructures were explained in terms of heat transfer (die chill) and workpiece geometry. Further evidence of the relative importance of microstructural and thermal effects was extracted from the results of two-pass extrusions, the first with upsetting and the second without upsetting.     

Fabrication and characterizations of new glassy Co<Subscript>71</Subscript>Ti<Subscript>24</Subscript>B<Subscript>5</Subscript> alloy powders and subsequent hot pressing into a fully dense bulk glass

A single glassy phase of Co₇₁Ti₂₄B₅ alloy has been synthesized by high-energy ball milling the elemental powders at room temperature, using the mechanical alloying method. The synthetic glassy powder obtained after 130 ks of ball milling exhibits good soft magnetic properties with a polarization magnetization and coercivity values of 1.01 T and 2.86 kA/m, respectively. This ternary glassy alloy in which its glass transition temperature (T _g ) lies at a rather high temperature (805 K) crystallizes at 868 K through a single sharp exothermic peak with an enthalpy change of crystallization (ΔH _x ) of −3.28 kJ/mol. The supercooled liquid region before crystallization, ΔT _x of the synthesized glassy powders shows a large value (63 K) for a ternary system. The reduced glass transition temperature (ratio between T _g and liquidus temperatures, T _l (T _g /T _l )) was found to be 0.55. The end product of the glassy powder (130 ks) was compacted in an argon gas atmosphere at 835 K with a pressure of 780 MPa, using the hot-pressing technique. The consolidated sample is fully dense (∼99.5 pct) and maintains its chemically homogeneous glassy structure. The measured polarization magnetization and coercivity values of as-consolidated powders are measured and found to be 0.96 T and 2.92 kA/m, respectively. The Vickers microhardness of the bulk glassy Co₇₁Ti₂₄B₅ sample is measured and found to be in the range between 7.32 and 7.46 GPa.     

Evolution of crystallographic texture during equal channel angular extrusion of copper: The role of material variables

The evolution of crystallographic texture during equal channel angular extrusion (ECAE) using route A has been investigated experimentally as well as by simulations for three types of materials: pure, commercially pure, and impure (cast) copper. The ECAE texture of copper can be compared with simple shear textures. However, there are deviations in terms of location of the respective components. These differences can be nearly reproduced using a recent flow line approach for ECAE deformation (L.S. Tóth, R. Arruffat-Massion, L. Germain, S.C. Baik, and S. Suwas: Acta Mater., 2004, vol. 52, pp. 1885–98) with the help of the viscoplastic self-consistent polycrystal model. The main texture components common to all three materials are A_1E and B_E/B_E; the latter ones are significantly stronger in the cast material. The effect of further deformation on texture modification depends on material variables, such as purity, initial microstructure, and texture.     

Bulk amorphous metallic alloys: Synthesis by fluxing techniques and properties

Bulk amorphous alloys having dimensions of at least 1 cm in diameter have been prepared in the Pd-Ni-P, Pd-Cu-P, Pd-Cu-Ni-P, and Pd-Ni-Fe-P systems using a fluxing and water-quenching technique. The compositions for bulk glass formation have been determined in these systems. For these bulk metallic glasses, the difference between the crystallization temperature (T _x) and the glass transition temperature (T _g, ΔT=T _x−T _g) ranges from 60 to 110 K. These large values of ΔT open the possibility for the fabrication of amorphous near-net-shaped components using techniques such as injection molding. The thermal, elastic, and magnetic properties of these alloys have been studied, and we have found that bulk amorphous Pd₄₀Ni_22.5Fe_17.5P₂₀ has spin glass behavior for temperatures below 30 K. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Effects of Microalloying on Glass Forming Ability and Thermodynamic Fragility of Cu-Pr-Based Amorphous Alloys

The effects of microalloying of Ti and B on the glass formation of Cu60Pr30Ni10Al10-2xTixBx（x = 0, 0.05% （atom fraction）） amorphous alloys was investigated using differential scanning calorimetry （DSC） and X-ray diffraction （XRD）. XRD analysis showed that mieroalloying with 0.05% Ti and 0.05% B improved the glass forming ability （GFA）. The smaller difference in the Gibbs free energy between the liquid and crystalline states at the glass transition temperature （△G1-X（Tg）） and the smaller thermodynamic fragility index （△Sf/Tm, where ASf is the entropy of fusion, and Tm is the melting temperature） after mieroalloying correlated with the higher GFA.     

Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425 °C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a single-pass extrusion, including evidence of 〈c〉 and 〈c+a〉 dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation. 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.     

Effect of Aluminium Addition on Glass Forming Ability of Nd55-x Al10＋x Fe15 Co20 （x = 0, 5, 10） Alloys

Nd55-x Al10＋x Fe15 （x =0, 5, 10） bulk glass-forming alloys with distinct glass transition in differential scanning calorimetry （DSC） traces were obtained by suction casting, The glass forming ability （GFA） of the alloys was investigated. It was found that the reduced glass transition temperature （Trg） and the parameter γ of the alloys increased with the increasing concentration of Al. The glass formation enthalpy of the alloys was calculated based on Miedema＇s model, and it was suggested that the GFA of the alloys could be enhanced by the decrease of the glass formation enthalpy with Al additions.     

Severe plastic deformation of nickel-coated aluminum precursor powders at elevated temperatures

Two nickel (Ni)-coated aluminum (AI) powders were consolidated to full or near-full density by using the equal-channel angular extrusion (ECAE) technique. Mixtures (in at. pct) of 78Al-22Ni (63Al-37Ni in wt pct), hereafter termed Al-22Ni, and of 39Al-61Ni (23Al-77Ni in wt pct), hereafter termed Al-61Ni, were placed in square-shaped copper (Cu) or Ni casings, sealed, and heated to a uniform temperature. Subsequently, the billets were dropped into the ECAE die and were single-pass extruded. Preheating temperatures ranged from ambient temperature to 700 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness measurements were used to examine the resultant intermetallics. The onset and nature of the transformation from the precursors into the products were further studied by differential thermal analysis. It was found that the preheating temperature affected the transformation of the Ni-coated Al powder into a nickel-aluminide intermetallic. Specifically, samples of Al-22Ni and Al-61Ni, preheated and consolidated below the nominal reaction temperature of Ni and Al, consisted of a cellular Al structure interposed with a Ni boundary layer. The samples preheated and consolidated above this temperature consisted of Al₃Ni and Al₃Ni₂, AlNi, and AlNi₃, respectively. In this case, ECAE caused the multiphase structure of the intermetallics to homogenize and disperse; this was more so for 78Al-22Ni than for 39Al-61Ni. It was also found that the use of a Ni vs Cu casing material greatly improved the densification of the powders. Unlike that found with the Cu casing, full densification using a Ni casing was achieved in a single pass.     

Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425°C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a singlepass extrusion, including evidence of <c> and <c+a> dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation. S.R. AGNEW, formerly with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115 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.     

Hot Extrusion of Nanostructured Al-Powder Alloys: Grain Growth Control and the Effect of Process Parameters on Their Microstructure and Mechanical Properties

Two nanostructured aluminum powder alloys (supersaturated Al4.5Cu prepared by mechanical alloying, and Al3.0Fe0.42Cu0.37Mn rich in precipitates and prepared by rapid solidification via gas atomization) were consolidated into bulk material under various processing conditions via hot extrusion. The microstructural modifications and mechanical properties of the consolidated alloys as a function of the extrusion conditions were investigated and are discussed here. The effect of pre-existing precipitates from nonsupersaturated alloy is shown to be more effective for controlling grain growth during consolidation. The increase in the extrusion load, with a concomitant increase in the extrusion rate and decrease in temperature, is shown to lead to microstructural modifications. The differences in mechanical properties measured by compressive tests are also discussed in association with the extrusion parameters. Furthermore, suggestions are given for rationalizing the extrusion rate and temperature for the consolidation of nanostructured aluminum powder alloys via hot extrusion.