Quantitative Measurements of Grain Boundary Sliding in an Ultrafine-Grained Al Alloy by Atomic Force Microscopy

In the current study, quantitative measurements for grain boundary sliding (GBS) in ultrafine-grained (UFG) 5083 Al by atomic force microscopy (AFM) were performed. An ion beam polishing and etching technique was used to reveal grain boundaries in the alloy for AFM characterization. A comparison between the average grain sizes measured from AFM images and those estimated from transmission electron microscopy micrographs and electron backscatter diffraction (EBSD) maps showed excellent agreement. The vertical offset of GBS was measured by comparing predeformation and postdeformation AFM images. By analyzing these measurements, the contribution of GBS to the total tensile strain in 5083 Al was estimated as 25 pct at a strain rate of 10⁻⁴ seconds⁻¹ and a temperature of 473 K (200 °C). It was demonstrated that the relatively low value of the contribution of GBS to the total strain is most likely the result of testing UFG 5083 Al under experimental conditions that favor the dominance of region I (low-stress region) of the sigmoidal behavior characterizing high-strain-rate superplasticity, which was reported previously for the alloy.     

Mechanism of Effects of Rare Earths on Microstructure and Properties at Elevated Temperatures of AZ91 Magnesium Alloy

By using real-space recursion method,the energetics of the undoped and Al and/or RE atoms doped 7（1450）〈0001〉 symmetric tilt grain boundaries（GBs）in AZ91 alloys were investigated.Similar calculations were performed on undoped and doped bulk α Mg for comparison.The results showed that Al atoms segregated at GBs in AZ91 alloys.When RE atoms were added,they also segregated at GBs,and their segregation is stronger than Al atoms＇.Therefore,RE atoms retard the segregation of Al atoms.Calculations of interaction energy indicated that Al atoms repelled each other,and could form ordered phase with host Mg atoms.On the contrary to the case of Al,RE atoms attracted each other,they could not form ordered phase with Mg,but could form clusters.Between RE and Al,there existed attractive interaction,and this attractive interaction was the origin of Al11RE3 precipitation.Precipitation of Al11RE3 particles with high melting point and high thermal stability along GB improves high temperature properties of AZ91 alloys.     

First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part II. Metal (Fe,Al, Cu)-Hydrogen (H) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by hydrogen segregation (trapping) has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy of bcc Fe Σ3(111) and fcc Al(Cu) Σ5(012) symmetrical tilt GBs can be significantly reduced if many hydrogen atoms segregate at the GBs. This indicates that the reduction of the cohesive energy of the GB may cause the hydrogen-induced GB embrittlement in Fe, Al, and Cu. Considering the “mobile” effect of hydrogen during fracture, especially for the Fe system, more hydrogen atoms coming from solid solution state can segregate on the gradually formed two fracture surfaces and reduce further the cohesive energy. We suggest a new idea about the upper and lower critical stresses observed in the constant-load test of hydrogen-induced delayed fracture in high-strength steels; the upper critical stress is determined by the amount (density) of “immobile” hydrogen atoms segregated at the GB before fracture, and the lower critical stress is determined by the total amount (density) of immobile and mobile hydrogen atoms, the latter of which segregate on the two fracture surfaces during fracture.     

A theory for solute impurity diffusion, which considers engel-brewer valences, balancing the fermi energy levels of solvent and solute, and differences in zero point energy

A theory that assumes the Engel-Brewer valence of elements (one for bcc structures, two for cph structures, and three for fcc structures) and considers the effects of balancing the solute and solvent Fermi energy levels and differences in zero point energy between solvent and solute atoms to calculate an “effective” relative valence for solute impurities is presented. The calculated values of relative valence and the experimental values of the differences in diffusional activation energy between solute and solvent atoms, ΔQ, are compared to the values of ΔH ₂ + ΔE calculated from the Lazarus-LeClaire theory for several solute impurities in ten solvent metals. The calculated results agree very well with the experimental values for the large majority of solutes. The theory presented adequately describes solute impurity diffusion in both α-Fe and γ-Fe, Al, Ni, and the noble metals. In particular, the low activation energies for impurity diffusion of the alkali metals (ground state valence of one) in Al (ground state valence of three) are accounted for by the theory. It is shown that the diffusion of the electronegative solute impurities (Cr, Mn, Fe, and Co) in Al is not anomalous when the relative valence is calculated by the proposed theory. The diffusion of electronegative solute impurities in the noble metals, which has been problematic in the past, is also well described by the proposed theory. The proposed theory introduces a simple method of estimating the effective electron densities of solute impurities and illustrates that the Lazarus-LeClaire theory adequately describes solute impurity diffusion in the ten solvent metals studied. It is expected that more accurate calculations of effective electron density for solute impurities would result in even better agreement between experimental and calculated results.     

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.     

Pseudopartial wetting of grain boundaries in severely deformed Al-Zn alloys

After severe plastic deformation by the high-pressure torsion, Al-Zn alloys have three various classes of Al/Al grain boundaries (GBs) wetted with a second zinc-rich phase. Completely wetted Al/Al GBs are coated with the layer of a zinc-rich phase more than 30 nm thick. Partially (incompletely) wetted Al/Al GBs contact particles of the zinc-rich phase with a contact angle >60°, but contain no measurable zinc concentration. Pseudopartially wetted Al/Al GBs also contact Zn particles with a contact angle >60°. However, they have a thin interlayer of the zinc-rich phase with a uniform thickness of 2–4 nm, the presence of which explains the unusually high ductility of Al-Zn alloys after high-pressure torsion.     

Grain Boundaries and Mechanical Properties of Ultrafine-Grained Metals

The concept of grain boundary (GB) engineering of ultrafine-grained (UFG) metals and alloys is developed for enhancement of their properties by tailoring different GBs (low-angle and high-angle ones, special and random, or equilibrium and nonequilibrium) and formation of GB segregations and precipitations by severe plastic deformation (SPD) processing. In this article, using this approach and varying regimes and routes of SPD processing, we show for several light alloys (Al and Ti) the ability to produce UFG materials with different GBs, and this can have a dramatic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various UFG metals and alloys.     

Effect of Aluminum Content on Wear Resistance of Hot-Forged Multiphase Steel

A medium-carbon steel was alloyed with Mn, Cr, Si, and Al to obtain carbide-free bainite steel. The thermomechanics and chemistry of steel were used to produce medium carbon containing four phases: ferrite, pearlite, bainite, and chromium carbide. The morphologies of different phases were characterized and analyzed by using optical and scanning electron microscopes. An abrasive dry sliding wear (pin on ring) of two types of medium-carbon, hot-forged steels containing different aluminum contents was investigated at different pressures and sliding velocities. The sliding duration time was 30 minutes under dry sliding conditions. The wear rate of Alloy 1 and 2 revealed negligible wear rates at low velocity and pressure. On the other hand, the wear rate highly increased to maximum at maximum velocity and pressure for Alloy 1 and 2. Alloy steel 2 of 2 pct Al revealed a maximum wear rate of 720 mg/min compared with 160.8 mg/min for Alloy 1 contains 1 pct Al. Experimental results showed that increased aluminum content is directly proportional to the ferrite volume fraction, which greatly influences the wear resistance performance and mechanical properties of the two types of steel.

     

Intergranular fracture in some precipitation-hardened aluminum alloys at low temperatures

Intergranular fracture at low temperatures from room temperature down to 4.2 K has been studied in some precipitation-hardened aluminum alloys. Microscopic appearance of intergranular facets is revealed to be greatly affected by the microstructure adjacent to the grain boundaries (GBs). When large precipitates on GBs and wide precipitation-free zones (PFZs) are present, coalescence of microvoids initiated at the GB precipitates causes the intergranular fracture with dimples. This fracture process is found to be unaffected by deformation temperature. On the other hand, in the presence of fine precipitates on GBs and narrow PFZs, matrix slip localization exerts significant influence on the fracture behavior. At low temperatures, large stress concentration at GBs leads to intergranular fracture, forming sharp ledges on the fracture surfaces, while at room temperature, the dynamic recovery process is thought to relax such stress concentration, resulting in a transgranular “uctile” rupture. formerly Graduate Student, the Department of Materials Science, University of Tokyo     

Effects of Rolling and Cooling Conditions on Microstructure and Tensile and Charpy Impact Properties of Ultra-Low-Carbon High-Strength Bainitic Steels

Six ultra-low-carbon high-strength bainitic steel plates were fabricated by controlling rolling and cooling conditions, and effects of bainitic microstructure on tensile and Charpy impact properties were investigated. The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites (GBs) and bainitic ferrites (BFs) were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K or 873 K (700 °C or 600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse GBs, while the steels cooled from 873 K (600 °C) contained a considerable amount of BFs having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy (USE). When the steels cooled from 673 K (400 °C) at a cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse GBs or BFs, while the slowly cooled specimens were composed mainly of acicular ferrites (AFs). Since AFs had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had a good combination of strength, ductility, and USE, together with very low energy transition temperature (ETT).     

A New Paradigm for Designing High-Fracture-Energy Steels

The steels used for structural and other applications ideally should have both high strength and high toughness. Most high-strength steels contain substantial carbon content that gives poor weldability and toughness. A theoretical study is presented that was inspired by the early work of Weertman on the effect that single or clusters of solute atoms with slightly different atom sizes have on dislocation configurations in metals. This is of particular interest for metals with high Peierls stress. Misfit centers that are coherent and coplanar in body-centered cubic (bcc) metals can provide sufficient twisting of nearby screw dislocations to reduce the Peierls stress locally and to give improved dislocation mobility and hence better toughness at low temperatures. Therefore, the theory predicts that such nanoscale misfit centers in low-carbon steels can give both precipitation hardening and improved ductility and fracture toughness. To explore the validity of this theory, we measured the Charpy impact fracture energy as a function of temperature for a series of low-carbon Cu-precipitation-strengthened steels. Results show that an addition of 0.94 to 1.49 wt pct Cu and other accompanying elements results in steels with high Charpy impact energies down to cryogenic temperatures (198 K [–75 °C]) with no distinct ductile-to-brittle transition. The addition of 0.1 wt pct Ti results in an additional increase in impact toughness, with Charpy impact fracture energies ranging from 358 J (machine limit) at 248 K (–25 °C) to almost 200 J at 198 K (–75 °C). Extending this concept of using coherent and coplanar misfit centers to decrease the Peierls stress locally to other than bcc iron-based systems suggests an intriguing possibility of developing ductile hexagonal close-packed alloys and intermetallics.     

The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of M _g =1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.     

Effect of Dislocation Mechanisms during Extrusion of Nanostructured Aluminum Powder Alloy

Nanostructured Al-3.0Fe-0.42Cu-0.37Mn powder alloy was deformed by extrusion over a temperature range of 375 °C to 425 °C, a ram speed range of 1 to 30 mm/s, and an extrusion rate of 10:1. Flow stresses and strain rates were calculated from the experimental ram pressures and speeds. The stress–strain-rate–temperature relationship in the extrusion of the nanostructured alloy was found to be similar to that in hot-worked conventional materials. The extrusion, torsion, compression, and creep data of nanostructured and conventional materials, extending over ten orders of magnitude of strain rate and over two orders of magnitude of stress, were correlated by a hyperbolic-sine constitutive equation, because the power and exponential laws lose linearity at high and low stresses, respectively. The hyperbolic-sine equation is widely used to correlate the hot-working behavior of conventional materials. It was concluded that the hot working of nanostructured powders is a thermally activated process in which the rate-controlling mechanism is the climb of edge dislocations. Microstructural changes in the consolidated alloys as a function of the extrusion conditions were investigated. An analysis was made of the dislocation behavior in very small grains of nanostructured metal by transmission electron microscopy (TEM) and we identified a dislocation structure and the different ways it appears in 40- to 100-nm Al-alloy grains. We also discuss the thermally activated propagation of dislocations and their interactions with shear bands/grain boundaries (SBs/GBs), and dislocation loops. Microstructural features including low-angle GBs, high-angle GBs, and equilibrium GBs and subgrain boundaries were observed. Dislocation structures under a deformation condition were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as dislocation tangle zones (DTZs) and dense-dislocation walls (DDWs), and the recovery process is discussed herein.     

Effects of 3d Transition Metal Elements in the B2-FeAl Structure

Based on the Chen–M?bius lattice inversion and a series of pseudopotential total energy curves, a parameter-free method was used to derive Fe(Al)-X (where X = Co, Cr, Cu, Mn, Ni, Sc, Ti, V, and Zn) interatomic potentials to study the effects of 3d transition metal elements substituting Fe or Al atoms in the B2-FeAl structure. Through molecular dynamics, the site preference of each type of defect was determined by comparing total energy calculations. The changes in lattice parameters and bulk modulus associated with the presence of defects in the FeAl matrix were also studied. The results are compared, when available, with experimental data and other theoretical results.     

Electrochemical co-reduction of Y(III) and Al(III) in a fluoride molten salt system and electrolytic preparation of Y–Al intermediate alloys

In this study, a molten salt co-reduction method was proposed for preparing Y–Al intermediate alloys and the electrochemical co-reduction behaviors of Y(III) and Al(III) and the reaction mechanism of intermetallic compound formation were investigated by transient electrochemical techniques. The results show that the reduction of Y(III) at the Mo electrode is a reversible electrochemical process with a single-step transfer of three electrons, which is controlled by the mass transfer rate. The diffusion coefficient of Y(III) in the fluoride salt at a temperature of 1323 K is 5.0238 × 10^?3 cm²/s. Moreover, the thermodynamic properties associated with the formation of Y–Al intermetallic compounds were estimated using a steady-state electrochemical method. Y–Al intermediate alloy containing 92 wt% yttrium was prepared by constant current electrolysis at 1323 K in the LiF–YF₃–AlF₃–Y₂O₃ (6 wt%)–Al₂O₃ (1 wt%) system at a cathodic current density of 8 A/cm² for 2 h. The Y–Al intermediate alloy is mainly composed of α-Y₂Al and Y phases. The development and application of this innovative technology have solved major technical problems, such as a long production process, high energy consumption, and serious segregation of alloy elements at this stage.     

Application of Desirability-Based Multiobjective Optimization Techniques to Study the Wear of Al Self-Lubricating Hybrid Nanocomposites

In this study, novel Al6061–SiC nanocomposites and Al6061–SiC–Gr hybrid nanocomposites were fabricated by ultrasonic cavitation method by adding silicon carbide (SiC) of 0.8 and 1.6% and graphite (Gr) of 0.5 and 1.0% by weight basis for each casting. A Three-level Box–Behnken design of experiment was developed using response surface methodology. Dry sliding wear tests were performed as per the experimental design using a pin-on disc set-up at room temperature. Analysis of variance (ANOVA) was applied to investigate the influence of process parameters viz., load, sliding distance, wt% reinforcement and their interactions on specific wear rate and coefficient of friction. Further, a mathematical model was formulated by applying response surface method in order to estimate the tribology characteristics such as wear and COF of the hybrid nanocomposites. The specific wear rate and coefficient of friction were significantly influenced by % of SiC followed by % of Gr, load and sliding distance. The wear test parameters were optimized for minimizing specific wear rate and COF using desirability function approach. A set of optimum parameter of combination for AMMNC was identified as: SiC 1.36wt%; Gr 0.63 wt%; load 35.65 N and sliding distance 2848 m with specific wear rate of 0.517 g/N-m; coefficient of friction 0.181. The AFM image of Al6061–1.36SiC–0.63Gr hybrid nanocomposite at optimized condition confirmed the improvement in the wear surface smoothness of the hybrid nanocomposite compared to Al6061–SiC nanocomposites.     

On the roles of clusters during intragranular nucleation in the absence of static defects

In some alloy systems, e.g., Cu-Co, prior formation of clusters does not affect the nucleation of precipitates, whereas in others, e.g., Al-Cu-X, where X can be Mg, Ag, Cd, In, Sn, Si, or Ge (singly or in combination), clusters can not only accelerate the nucleation kinetics of the first phase to form but can also change the identity of this phase. When solute atoms in a cluster are strongly bound to one another but none has a high-binding energy to vacancies, clusters will tend to dissolve and to cease forming as a result of the loss of quenched-in vacancies before the incubation time for nucleation has elapsed unless the ambient vacancy concentration is high enough to sustain the clustering process. When solutes are also strongly bound to vacancies, however, clusters can survive through and beyond the incubation time. Two mechanisms are examined through which such clusters can assist nucleation. (a) One or more of the atom species constituting the clusters adsorbs at the interfaces of the nuclei and thereby reduces their interfacial energy. (b) At plate- (or lath-) shaped nuclei formed with an appreciable shear-strain energy, segregation of large atoms to the regions under tension within the local strain field and of small atoms and/or vacancies to local regions under compression can markedly reduce this strain energy. Nucleation taking place in the absence of static defects, such as aggregates, dislocations, internal boundaries, or particles, is clearly homogeneous when clusters do not assist the nucleation process. However, atom probe results have demonstrated the existence of clusters surviving long enough to assist nucleation and in so doing suggest that the definition of “heterogeneous nucleation” be extended to include even these continuously fluctuating heterogeneities.