Grain growth of nanocrystalline cryomilled Fe-Al powders

Nanocrystalline Fe-xAl (x=0, 2.6, or 10 wt pct) powders have been prepared using cryogenic mechanical alloying (cryomilling). The extremely low rate of diffusion of Al in Fe at the process temperature (−196 °C) effectively limits dissolution of Al in the nanocrystalline αFe grains. Thermal stability against grain growth in the cryomilled Fe-Al mixtures is found to be much greater than that of identically prepared pure nanocrystalline Fe. Heat treatment of the cryomilled Fe-Al materials results in a highly inhomogeneous distribution of grain size and microchemistry. The observed thermal stability is evaluated in terms of Zener pinning, solute drag, and chemical ordering mechanisms.     

Mechanical properties, ductility, and grain size of nanocrystalline iron produced by mechanical attrition

The main goal of this investigation is to determine the influence of grain size on the mechanical properties and, specifically, the intrinsic ductility of nanocrystalline (nc) Fe. Ball-milled nc Fe was consolidated into compacts of near theoretical density by uniaxial warm pressing. Compaction parameters and annealing treatments resulted in a range of grain sizes for subsequent mechanical testing. The miniaturized disk bend test, hardness, and the automated ball indentation (ABI) method were used to test nanocrystal (nc) iron in compression and tension. The deformation and fracture morphologies of the tested samples were characterized by light and scanning electron microscopy. The hardness, as a function of the grain size, was described with a Hall-Petch slope, which was smaller than that in coarse-grained Fe. In tension, the material failed in a macroscopically brittle manner, while local ductility in very concentrated shear bands was observed. The compressive characteristics of the nc Fe were similar to those of a perfectly plastic material. The results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

On the Hardness and Strain Rate Sensitivity of Electrodeposited Nanocrystalline Ni–18 wt% Co Alloy Studied by Nanoindentation

Ni–18wt% Co foils made by electrodeposition possesses an average grain size, computed using diffraction contrast in TEM, of about 30 nm. Vickers microindentation and depth-sensing nanoindentation have been adapted to assess the deformation parameters such as hardness, strain rate sensitivity (SRS) and activation volume. These foils with single-phase fcc-structured solid solution exhibit the hardness values of 4.5 ± 0.1 GPa and 6.2 ± 0.2 GPa measured by microindentation and nanoindentation, respectively. The dependence between hardness and applied load in the present solid solution cannot be attributed to the indentation size effect, but to the changes in internal friction and/or reduction in stacking fault energy that may have resulted from the Co additions. An elastic modulus of 193 ± 3 has been realized in these foils. Performing nanoindentation at various loading rates and subsequent analysis has resulted in a SRS of 0.017 and an activation volume of 7.6 b3. These values suggest that in these nanocrystalline Ni–18Co foils made by electrodeposition, interfaces such as grain boundaries, triple junctions and quadruple junctions play a governing role in dictating the deformation kinetics. A model reported in the literature has been successfully modified to reasonably explain the dependence of SRS on grain size for various Ni-based alloys including the one reported in the present study. However, the model fails to follow the established dependence for the materials with grain size below 10 nm as the deformation mechanisms at these extremely finer length scales (below 10 nm) are expected to be totally different from considerations applicable for the present alloy with a grain size of 30 nm.     

Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review

The extremely fine grain size of nanocrystalline (nc) metallic alloys results in significantly different mechanical, electrochemical and oxidation properties as compared to the coarse-grained alloys of the same composition. Although the synthesis and attractive mechanical properties of nanocrystalline materials have been investigated and reviewed in great depth, the high temperature oxidation and electrochemical corrosion of these materials has received limited attention. The difference in the active dissolution and passivation behavior of nc alloys as compared to their microcrystalline counterparts varies for each alloy system. However, a unified theory explaining these phenomena still eludes us. In this context, this article reviews the progress in the electrochemical corrosion behavior of different nanocrystalline alloys, and hence, develops a better understanding of the effect of grain size, composition, interfacial phenomena and selective dissolution on corrosion of nanocrystalline alloys. This review also presents the role of nanometric grain size and the associated increase in grain boundary diffusion on the high temperature oxidation of nc alloys. The attractive possibility of enhanced oxidation resistance at lower alloying additions as compared to the coarse-grained alloys has been discussed. Although the primary focus of the article is on ferrous alloy systems, however, the lead studies on the role of ultrafine grain size in oxidation/corrosion behavior of other alloys systems have also been reviewed.     

Quantitative characterization of the three-dimensional microstructure of polycrystalline Al-Sn using X-ray microtomography

Characterizing the three-dimensional topology of grain-boundary networks in polycrystalline materials is a crucial step in the modeling of properties that depend on the sample microstructure. Using absorption-contrast X-ray microtomography, we have carried out a large-scale microstructural characterization of polycrystalline Al doped with 2 at. pct Sn, which is immiscible in Al in the solid state. The segregation of Sn to the grain boundaries imparts a strong contrast in X-ray attenuation that can be reconstructed tomographically; however, the nonuniformity of the segregation process presents a formidable challenge to the automated segmentation of the reconstructions. By employing an iterative grain-finding algorithm followed by a novel grain-boundary-network optimization routine (based on a phase-field simulation of grain growth), we were able to extract reliable values for the size and topology of nearly 5000 individual grains in polycrystalline Al-Sn. The distributions and averages of these data deviate significantly from the corresponding microstructural parameters of the three-dimensional (3-D) Poisson-Voronoi tessellation often used to model polycrystalline samples. Much better agreement was observed with microstructures generated by the computer simulation of three-dimensional grain growth. This article is based on a presentation made at the symposium entitled “Characterization and Representation of Material Microstructures in 3-D” during the TMS Fall meeting, October 6–10, 2002, in Columbus, Ohio.     

Developing Superplastic Ductilities in Ultrafine-Grained Metals

Conventional superplasticity is generally achieved in metals having grain sizes in the range of ∼2 to 5 μm, but processing by equal-channel angular pressing (ECAP) provides the opportunity of introducing exceptional grain refinement and producing materials with ultrafine grain sizes in the submicrometer range. These materials have the potential for exhibiting excellent superplastic properties when tested in tension at elevated temperatures and examples are presented for representative aluminum and magnesium alloys. When these ultrafine-grained materials deform in superplasticity, internal cavities develop as in conventional superplastic alloys. An example is presented for an aluminum-based alloy, and it is shown that the cavity growth processes are also similar to those in conventional alloys. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: From Basics to Application”, which occurred September 25–27, 2006, in Kloster Irsee, Germany.     

Fiber-Matrix interactions in brittle matrix composites

Brittle matrix composites, including carbon-carbon (C-C) and ceramic matrix, offer a new dimension in the area of high-temperature structural materials. Fiber-matrix interactions determine the mechanism of the load transfer between the fiber and matrix and resulting mechanical properties. Composites studied in this work include a C-C composite densified with a chemical vapor infiltration (CVI) pyrolytic carbon, silicon carbide fiber-silicon carbide matrix composite, and carbon fiber-silicon carbide matrix composites densified by the CVI technique. The type of the interfacial carbon in C-C composites was found to control their mechanical properties. The presence of the compressive stress exerted by the matrix on the carbon fibers was attributed to an increase in flexural strength. The transverse matrix cracking in C/SiC composites was believed to cause a lowering in the flexural strength value. Brittle fracture behavior of SiC/SiC composites was correlated with the presence of an amorphous silica layer at the fiber-matrix interface. This invited paper is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presented as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.     

Microstructure and tensile properties of compacted,mechanically alloyed,nanocrystalline Fe-AI

Data on mechanical properties of nanocrystalline materials have been limited, due in part to the difficulty in producing consolidated nanocrystalline materials of sufficient quantity for characterization and evaluation. A second problem is consolidation and retention of the nanostructure. A vacuum hot-pressing consolidation program has been developed to produce full-dense compacts from attrition milled, mechanically alloyed, nanograin micron-size particles of Fe-2 wt pet Al powder. The resulting compacts were of sufficient size to allow evaluation of microstructure, density, hardness, and tensile properties. The compacted microstructure was a composite of pure iron submicrograins and Fe-A1 nanograin clusters. Tensile strength was found to be proportional to the sample’s density squared. For full-dense compacts, tensile strength of nanocrystalline compacts approaching 1 GPa was obtained.     

Strength and ductility under dynamic loading in fine- grained IN905XL aluminum alloy

Three IN905XL aluminum alloys with fine grain (1 μm), intermediate grain (3 μm), and coarse grain (5 μm) have been developed by a combination of mechanical alloying (MA) and conventional extrusion in order to investigate their mechanical properties at dynamic strain rates of 1 × 10³ and 2 × 10³ s⁻¹ and a quasi-static strain rate of 10^-3 s⁻¹. Flow stresses are found to increase with decreasing grain size for all the strain rates tested. Negative strain-rate sensitivity of flow stress is observed up to 1 × 10³ s⁻¹ in both intermediate- and coarse-grained IN905XL. At the highest strain rate of 2 × 10³ s⁻¹ however, all samples showed a positive strain-rate sensitivity of strength. Total elongation at high strain rates is generally larger than that at low strain rates. Total elongation also decreases with grain size for all the strain rates. This decrease in elongation results from an initiation of microcracks at interfaces between the matrix and particles finely dispersed near grain boundary regions, introduced during MA processing; then, this initiation leads elongation of alloys to small limited values. Formerly with the Department of Mechanical Systems Engineering, University of Osaka Prefecture. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

The mechanical properties of grain refined β- cuaini strain-memory alloys

A study has been made of the effect of grain refinement on the mechanical and the strain-memory properties of β-CuAlNi alloys. Addition of 0.5 pct Ti to CuAINi decreased the grain growth rate of the beta phase significantly. This appeared to be due mainly to the small fraction of the titanium in solid solution in the β-CuAlNi. By controlled annealing, a grain size as small as 15 μrn could be obtained, though some second phase γ₂ was present due to incomplete precipitate dissolution. Stress-strain curves for most specimens in both the strain-memory and pseudoelastic states showed a three-stage characteristic with a region of lower slope between two regions of higher modulus. It was found that σ₁, (the transition stress between stages 1 and 2) and (dσ/dε@#@) (the slope of stage 2) increased with grain size according to a (g.s.)^-1/2 relationship. The ultimate tensile strength and strain to fracture also followed a similar Hall-Petch relationship. The alloys showed higher strength in the martensitic state than in the pseudoelastic one. The presence of second-phase particles had no significant effect on the mechanical properties and martensite deformation behavior. Fracture strains as high as 7 pct were obtained at the finest grain sizes. It was found that the strain-memory and pseudoelastic recovery properties were not affected significantly by decreasing grain size and the presence of second phase particles. Maximum recovery strains of 6.5 pct were obtained in fine grain samples. Formerly Graduate Student, Department of Metallurgical Engineering, University of British Columbia     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

Recent trends and developments with rapidly solidified materials

Excellent progress is being made in demonstrating the significant improvements in metallic structures and properties through rapid solidification (RS) and rapid solid-state quenching. A number of atomization techniques are able to achieve solidification rates of the order of 10⁵ K/s at economical production rates, yielding excellent refined microstructures with resultant outstanding mechanical, physical, and chemical properties. Powder production rates need to be significantly improved, and several techniques offer realistic promises in this area. The spray deposition processes, which bypass the powder production phase, achieve excellent solidification and solid-state cooling rates and offer high tonnage rates for preforms with outstanding hot, warm, and cold working response and with excellent physical and mechanical properties. This invited critical overview paper is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presented as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

Fracture Resistance of Nanocrystalline Ni

Atomic level simulations are used to study crack propagation mechanisms in nanocrystalline Ni. Digital samples with a mean grain size of 5 and 8 nm containing 125 grains were used. For both grain sizes, the mechanism of crack propagation involves the formation of nanocracks along grain boundaries in the vicinity of the main crack. Crack resistance curves for the two grain sizes indicate that the smaller grain sizes are more ductile, requiring higher stress intensities for crack propagation. This result is consistent with softer behavior for smaller grain sizes in the inverse Hall–Petch regime, where deformation is accommodated by grain boundary mechanisms. The present simulations specifically show that grain boundary sliding also plays an important role in crack blunting observed in these materials. In many cases, the crack is arrested as it encounters grain boundaries in its path, showing increased resistance to propagation. Increased ductility for smaller grain sizes in this regime indicates that there is a minimum in ductility as a function of grain size in these materials, located around the 10- to 12-nm grain size. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Nanocrystalline metals prepared by high-energy ball milling

This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries. E. Hellstern, formerly Research Associate, California Institute of Technology This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Statistically representative three-dimensional microstructures based on orthogonal observation sections

Techniques are described that have been used to create a statistically representative three-dimensional model microstructure for input into computer simulations using the geometric and crystallographic observations from two orthogonal sections through an aluminum polycrystal. Orientation maps collected on the observation planes are used to characterize the sizes, shapes, and orientations of grains. Using a voxel-based tessellation technique, a microstructure is generated with grains whose size and shape are constructed to conform to those measured experimentally. Orientations are then overlaid on the grain structure such that distribution of grain orientations and the nearest-neighbor relationships, specified by the distribution of relative misorientations across grain boundaries, match the experimentally measured distributions. The techniques are applicable to polycrystalline materials with sufficiently compact grain shapes and can also be used to controllably generate a wide variety of hypothetical microstructures for initial states in computer simulations. This article is based on a presentation made at the symposium “Characterization and Representation of Material Microstructures in 3-D” held October 8–10, 2002, in Columbus, OH, under the auspices of ASM International’s Phase Transformations committee.     

An evaluation of fiber-reinforced titanium matrix composites for advanced high-temperature aerospace applications

The current capabilities of continuous silicon-carbide fiber-reinforced titanium matrix composites (TMCs) are reviewed with respect to application needs and compared to the capabilities of conventional high-temperature monolithic alloys and aluminides. In particular, the properties of a firstgeneration titanium aluminide composite, SCS-6/Ti-24Al-11Nb, and a second-generation metastable beta alloy composite, SCS-6/TIMETAL 21S, are compared with the nickel-base superalloy IN100, the high-temperature titanium alloy Ti-1100, and a relatively new titanium aluminide alloy. Emphasis is given to life-limiting cyclic and monotonie properties and to the influence of time-dependent deformation and environmental effects on these properties. The composite materials offer a wide range of performance capabilities, depending on laminate architecture. In many instances, unidirectional composites exhibit outstanding properties, although the same materials loaded transverse to the fiber direction typically exhibit very poor properties, primarily due to the weak fiber/matrix interface. Depending on the specific mechanical property under consideration, composite cross-ply laminates often show no improvement over the capability of conventional monolithic materials. Thus, it is essential that these composite materials be tailored to achieve a balance of properties suitable to the specific application needs if these materials are to be attractive candidates to replace more conventional materials. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Determination on the Effect of Ti Addition on the Microstructural,Mechanical and Wear Behavior of Cu–6Sn Alloy in as—Cast Condition

The objective of this research is to fabricate a ternary alloy (Cu–Sn–Ti), incorporating titanium into bronze with varying weight percentage of titanium (0.5 wt%, 1 wt%) to investigate its impact on microstructural and mechanical properties and wear behavior and to collate these results with those of conventional bronze (Cu–6Sn). The microstructure of the alloys was observed using a metallurgical microscope, and results exhibit a finer grain refinement in the dendritic structure, which causes an improvement in mechanical properties. The mechanical properties were tested (tensile strength, hardness), and they showed an increment in values corresponding to the increase in the weight percentage of titanium. However, owing to the formation of an inclusion (blowhole), there was a reduction in the tensile strength for Cu–6Sn.0.5Ti. The wear analysis was also carried out using a pin-on-disk tribometer with selected parameters of load (10–30 N), sliding distance (1000 m) and sliding velocity (1–3 m/s), and it was noted that there was an increase in the wear rate with an increase in load and distance for all combinations of parameters. There was also an improvement in the wear resistance with an increase in the weight percentage of Ti, in comparison with the conventional base alloy.     

Understanding creep in nanocrystalline materials

Only a limited number of creep investigations have been carried out on nanocrystalline materials to-date. These studies have remained largely inconclusive in establishing the mechanisms of creep in nanocrystalline materials. The stress exponent and activation energy values obtained from nanocrystalline materials do not correlate well with conventional well established creep models. Furthermore discrepancies between experimentally determined deformation rates and theoretical predictions suggest that an entirely new mechanism of creep could be operational in these exotic materials. In this work we aim to develop an understanding of the creep behavior of nanocrystalline materials by considering a stress assisted grain growth mechanism that has been recently identified in these materials. In turn a model has been developed that provides a quantitative understanding of some of the observations made in creep literature.     

Fatigue and Compression Characteristics of Al 6061–B 4 c Composite Materials

Aluminum matrix composites reinforced with boron carbide are a kind of materials that are widely used because of high strength, low density, and improved tribological properties. In this study, mechanical properties of Al 6061–B4C composites reinforced with B4C of three different particle sizes were investigated. In the Al 6061–B4C composite materials, produced by the powder metallurgy methods (extrusion of billets obtained by sintering at temperature of 550°C under pressure of 450 MPa), the change of mechanical properties such as hardness, compressive strength, and fatigue life, related to B4C particle size and the applied heat treatment mode (aging at 180°C for 5 h), were investigated. The hardness of the materials is increased with B4C grain size and the heat treatment. After the heat treatment, the fatigue life of Al 6061–B4C (3 μm) material increases slightly, while that of the composite materials decreases with larger size of B4C reinforcement. The fatigue life of the composite materials reinforced with a larger grain size B4C is reduced by heat treatment. While the compression test data of untreated composite materials were similar to each other, the heat treatment increased these values in all samples. The highest increase in the compression strength was observed in the composite reinforced with 17 μm sized B4C. The addition of graphite reduces the deformation ability of the composites.