Using friction stir processing to fabricate MgAlZn intermetallic alloys

Friction stir processing (FSP) is applied in mixing elemental thin sheets of Mg, Al, and Zn in various portions, resulting in hard intermetallic alloys with Vicker’s hardness in excess of 350. The Mg₃Al₂Zn₃ τ phase formed at 360 °C during FSP, coupled with some other binary or ternary phases of nano size, accounts for the high hardness.     

Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets

Pure nickel sheets are subjected to severe plastic deformation by constrained groove pressing technique at room temperature thereby imparting an effective plastic strain of 3.48. The evolution of mechanical behaviour with increasing number of passes revealed intensive increase in strength properties after first pass; however marginal increase is observed subsequently. Gain in ductility which is attributed to dislocation recovery is observed after third pass along with marginal drop in strength. Microstructural evolution during groove pressing of sheets is characterized by X-ray diffraction profile analysis using Williamson–Hall method. Besides the observation of strong shear texture in constrained groove pressed sheets, improvement in strain isotropy with increased straining is revealed from Williamson–Hall plots. The sub-grain/cell size estimated by analysing the diffraction profile of deformed sheets is found to be ～1390 nm after processing up to three passes. Low grain refinement efficiency observed in this process compared to other severe plastic deformation techniques at similar strain conditions is explained by the deformation characteristics and loading behaviour experienced by the sheets during constrained groove pressing.     

Microstructure and superplasticity of AZ31 sheet fabricated by differential speed rolling

Microstructure evolution and superplasticity in AZ31 alloy by differential speed rolling (DSR) with a high-speed ratio (=3) between the upper and lower rolls was examined. With increase in thickness reduction by DSR at 473 K, degree of grain refinement and microstructure homogeneity increased. The microstructure obtained by a single rolling pass for a 70% reduction in thickness consisted of recrystallized grains with a mean size of 2 μm, and fraction of HAGBs and average misorientation angle determined by the EBSD analysis were 0.47 and 23.21°, respectively. The DSR AZ31 alloy exhibited enhanced superplasticity as compared with the conventionally processed AZ31. A maximum elongation of 830% was obtained at 2 × 10⁻⁴ s⁻¹ and 673 K. The strain hardening exponent measured at 2 × 10⁻⁴ s⁻¹ and 673 K was as high as 0.71, which could be related to accelerated grain growth in the highly refined microstructure during superplastic flow.     

Microstructure and mechanical properties of AM31 magnesium alloys processed by differential speed rolling

Ingot casted AM31 alloys were rolled at a warm temperature of 350 °C and subsequently rolled at 300 °C using equal speed rolling (ESR) and differential speed rolling (DSR) with speed ratios of top roll to bottom roll, 1.2 and 1.5, respectively. Microstructures, textures and mechanical properties of the as-rolled AM31 sheets were examined. Ductility was improved by DSR due to inclination of basal poles and weakened texture. The sheets produced by DSR with a speed ratio of 1.2 showed highest strength and ductility at room temperature, which can be attributed to homogeneous fine grain distribution and tilted basal texture.     

Microstructural changes and estimated strengthening contributions in a gamma alloy Ti–45Al–5Nb pack-rolled sheet

Thin sheets made of a gamma-titanium aluminide alloy, Ti–45Al–5Nb, produced by a pack-rolling process, were evaluated for microstructure variation and evolution taking place during aging and annealing treatments. The as-received sheet material was characterized by remarkably high yield strength, 810 MPa, and a complex bimodal microstructure. The microstructure consisted of a matrix of twinned gamma-phase grains and fine-lath lamellar grain microconstituent, together with a dispersed ultra-fine-grained gamma + alpha-2 mixture microconstituent. High-temperature isothermal aging treatments changed the microstructure to a stable mixture of gamma-phase grains (matrix) and coarse alpha-2-phase particles, having size distributions and volume fractions that were specific to the aging temperature. A concurrent strength loss reflects this trend and results in a stable strength level of 550 MPa upon aging at 1000 °C for 144 h. Using composition estimates from the phase-boundary shifts that occur from the Nb addition to a Ti–45Al base alloy and, the rule of mixtures, an analysis was made to show that the gamma-phase matrix has an intrinsic strength of 178 MPa. This is a significant intrinsic strength level, well over that of 70 MPa for the Ti–45Al binary alloy. This is rationalized as the solid-solution strengthening effect from shifts of the Ti and Nb levels in the gamma phase and, by an added effect due to increased oxygen solubility in the gamma phase. The overall strength of Ti–45Al–5Nb, however, is roughly the same as that of Ti–45Al, and this is explained by a drastic reduction in the volume fraction of alpha-2-phase in Ti–45Al–5Nb alloy, which is a result of the Nb-induced phase-boundary shifts.     

The role of Zr-rich cores in strength differential effect in an extruded Mg–Zn–Zr alloy

This work examines the effects of extrusion parameters namely ratio and temperature on recrystallization behavior of a Mg–Zn–Zr alloy, and their consequent effects on anisotropy in the mechanical properties. Upon extrusion, the characteristic Zr-rich cores that do not recrystallize form the so-called “soft stringers” which are deformed bands elongated in the extrusion direction and curled around the extrusion axis. At higher extrusion ratio, there is more twinning contribution and the DRX response is improved, making the recryatallized grains finer and increasing the proportion of recrystallized Zr-rich cores. A basal texture after extrusion and the directional activation of tensile twinning cause anisotropy in the mechanical properties. In addition, the microstructural features such as large unrecrystallized regions and coarse crystallized grains also contribute in the strength differential effect. Further slip in the strain-hardened unrecrystallized grains is inhibited while twin activation under favorable orientation becomes easier in the coarse recrystallized grains. A higher proportion of large unrecrystallized and coarse crystallized gains in the case of lower extrusion ratio result in a much higher strength differential effect (100 MPa) in comparison to the one caused by the crystallographic texture only (25 MPa).     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.     

Physical properties of tin oxide thin films improved by vapour chopping

The vapour chopping technique has been successfully used to lower the ambient air ageing effect on the tin oxide thin films. The films were prepared by thermal oxidation (in air) of vacuum evaporated vapour chopped and nonchopped tin thin films. The films showed SnO and SnO₂ phases with tetragonal and orthorhombic structure. All the films showed increase in optical transmittance with increase in oxidation temperature and duration. The vapour chopped films showed higher refractive index and band gap than those of nonchopped films. The refractive index was found to increase with the thickness. Due to air ageing, the refractive index of both the films was found to increase. The ageing effect was found lower on the vapour chopped (0.008) than those on nonchopped (0.02) tin oxide thin films. These films can have potential use in optical waveguides.     

Creep feed grinding of gamma titanium aluminide and burn resistant titanium alloys using SiC abrasive

Following a brief introduction to titanium alloys and their machinability, the cutting performance of a gamma titanium aluminide intermetallic (γ-TiAl) alloy: Ti–45Al–8Nb–0.2C wt% and a burn resistant titanium (BuRTi) alloy: Ti–25V–15Cr–2Al–0.2C wt%, is compared with creep feed grinding using SiC abrasive. The work utilised 2 separate L9 Taguchi fractional factorial arrays. Typically G-ratios were a factor of 10× greater for γ-TiAl than BuRTi, with on average 10% lower maximum power and 25% lower maximum specific energy for the γ-TiAl alloy. A combination of a moderately high wheel speed: 35 m/s, low depth of cut: 1.25 mm and low feed rate: 150 mm/min, produced the lowest average workpiece surface roughness (Ra1.4 μm). Workpiece surface integrity evaluation indicated that with lower operating parameter levels, in particular a wheel speed of 15 m/s, surfaces free of burn and cracks could be produced, while at higher wheel speeds: 35 m/s, extensive workpiece surface burn was evident, with the γ-TiAl alloy suffering extensive cracking. Microhardness measurements showed in some instances slightly increased workpiece surface hardness of around 50–60HK_0.025 for the BuRTi alloy and 200HK_0.025 for the γ-TiAl material over respective bulk hardness values of 375HK_0.025 and 400HK_0.025.     

Drill geometry and operating effects when cutting small diameter holes in CFRP

The paper details experimental results when drilling small holes (1.5 mm diameter cemented carbide drills with varying end point and helix geometry) in thin quasi-isotropic, unbacked carbon fibre reinforced plastic (CFRP) laminate (typical cutting time 0.4 s/hole). The study utilised an L12 Taguchi fractional factorial orthogonal array with analysis of variance (ANOVA) employed to evaluate the effect of drill geometry and drilling conditions on tool life and hole quality. Main effects plots and percentage contribution ratios (PCR) are detailed in respect of response variables and process control factors. More conventionally, tool wear and cutting force data are plotted/tabulated, together with micrographs of hole entry/exit condition and internal hole damage. Drill geometry and feed rate in general had the most effect on measured outputs. Thrust force was typically below 100 N at test cessation; however, drill wear progression effectively doubled the magnitude of force from test outset. Entry and exit delamination factors (F_d) of 1.3 were achieved while the maximum number of drilled holes for a tool life criterion VB_Bmax of ≤100 μm was 2900 holes using a stepped, uncoated drill with a feed rate of 0.2 mm/rev.     

Structural refinement and deformation mechanisms in nanostructured metals

Deformation mechanisms in metals deformed to ultrahigh strains are analyzed based on a general pattern of grain subdivision down to structural scales 10 nm. The materials analyzed are medium- to high-stacking fault energy face-centered cubic and body-centered cubic metals with different loading conditions. The analysis points to dislocation glide as the dominant deformation mechanism at different length scales supplemented by a limited amount of twinning at the finest scales. With decreasing deformation temperature and increasing strain rate, the contribution of twinning increases.     

The ultimate sharpness of single-crystal diamond cutting tools—Part II: A novel efficient lapping process

As the theoretical predictions presented in Part I, the ultimate sharpness of (1 1 0)–(1 0 0) oriented diamond cutting tool can reach up to 16 nm, and the (1 0 0)–(1 0 0) oriented cutting tool up to 25 nm [W.J. Zong, K. Cheng, D. Li, T. Sun, Y.C. Liang, The ultimate sharpness of single-crystal diamond cutting tools—Part I. Theoretical analysis and predictions, International Journal of Machine Tools and Manufacture 2005, submitted]. In order to validate the theoretical predictions, a novel efficient lapping process, namely thermo-mechanical lapping, is developed in the present work, which is regarded as an effective extension of the traditional mechanical lapping process. By this developed lapping method, a large number of lapping experiments are carried out on the (1 1 0)–(1 0 0) oriented cutting tools. AFM measurement results illustrate that lapping time has little effects, but both lapping compression force and lapping velocity have enormous influences on the finished cutting-edge radius. Furthermore, based on the optimized lapping compression force and lapping velocity, an elegant diamond cutting tool oriented along (1 1 0)–(1 0 0) is sharpened. Atomic force microscope (AFM) and scanning electron microscope (SEM) measurement results indicate the cutting-edge radius has been sharpened down to 29 nm, which agrees well with the theoretical predicted value, i.e. 16 nm.     

Quantifying the strain-induced dissolution of precipitates in Al alloy microstructures using nuclear magnetic resonance

Nuclear magnetic resonance (NMR) has been used for the first time to directly monitor the dynamic partitioning of Cu atoms from shearable precipitates into the solid solution as a function of straining at room temperature in two Al–Cu-based alloys. Al–3Cu–0.05Sn (wt.%) and Al–2.5Mg–1.5Cu (wt.%) alloys were heat-treated to provide a fine distribution of 5 nm Guinier–Preston (GP) zones and <1 nm Guinier–Preston–Bagaryatsky (GPB) zones, respectively, and were then subjected to rolling strains up to 100%. It is shown that in the Al–Cu–0.05Sn alloy, strains up to 40% can pump solute from the 5 nm GP zones back into solid solution for the temperature and strain-rate of deformation employed here. In the case of the Al–Cu–Mg alloy, no dissolution of the GPB zones is observed. A simple model for the strain-induced dissolution of the shearable precipitates is given and compared with the experimental results. The dependence of the Cu repartitioning process on the precipitate size is emphasized. These observations and modeling give guidelines for the design of Al–Cu-based alloys to exploit the dynamic interplay of strain-induced Cu partitioning between metastable states, e.g. solid solution and GP (or GPB) zones, for tailoring ultimate mechanical properties. It is proposed that this strain-induced phase transformation is a form of dynamically responding microstructure that can be employed to obtain aluminum alloys with well-designed microstructures.     

Deformable alloys based on the Al - Mg - Se system

Aluminum alloys with magnesium that are deformable and not strengthened by heat treatment are widely used as a structural meterial due to their good weldability, high corrosion resistance, and high ductility. However, even the strongest alloys of this system, containing 5–6% Mg, have low strength characteristics. For example, annealed sheets of alloy AMg6 have _b = 340 N/mm² and _0.2 = 180 N/mm². The present work concerns the possibility of strengthening AI - Mg alloys by an additional alloying with scandium.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 33 – 36, June, 1996.     

Impact of donor, acceptor, and blocking layer thickness on power conversion efficiency for small-molecular organic solar cells

We investigated the dependency of the power conversion efficiency on the thickness of donor (copper phthalocyanine; CuPc), acceptor (fullerene; C₆₀), and hole/exciton blocking (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; BCP) layers in the OPV devices fabricated with double small-molecular layers. The power conversion efficiency peaked at a specific layer thickness, 12.7 nm for the donor layer, 17.5 nm for the acceptor layer, and 8.0 nm for the hole/exciton blocking layer. This trend of power conversion efficiency was determined by short-circuit-current rather than open-circuit-voltage after light absorption. In addition, the donor layer thickness was more sensitive than the thickness of the acceptor or hole/exciton blocking layers in improving power conversion efficiency; i.e., 330% for the donor layer, 118% for the acceptor layer, and 112% for the hole/exciton blocking layers.     

The origin of non-uniform microstructure and its effects on the mechanical properties of a friction stir processed Al–Mg alloy

The microstructure of the stirred zone (SZ) resulting from friction stir processing or welding (FSP/FSW) has usually been assumed to be uniform when discussing the mechanical properties. However, numerous works have indicated that the fine-grained microstructures in the SZ were non-uniform, with precipitate, texture and grain size gradients caused by the severe plastic deformation and heat distribution. In this work commercial aluminum alloy 5083-H112 was subjected to FSP and fine-grained microstructures with an average grain sizes of 2.7–13.4 μm were obtained by controlling the FSP conditions. The stress–strain curves exhibited stepped yield point elongation, which was suggested to be associated with these characteristic non-uniform microstructures. Tensile tests indicated that the Hall–Petch relationship held in this FSP alloy when taking account of the average grain size. Toughness analysis indicated that the optimum toughness was anticipated to be obtained around a grain size of 1 μm for this FSP alloy.     

A theoretical interpretation of the pressure–composition isotherms of RNi5 (, Pr, Nd and Sm) systems based on statistical mechanics

A simple model to understand the phase behavior of RNi₅–H (R=La, Pr, Nd and Sm) systems is proposed based on the statistical mechanics. The essential parameters in the present model, hydrogen site energies and lattice relaxation term, were optimized so as to reproduce the experimentally determined pressure–composition isotherms (PCIs) of these four alloys. The qualitative difference in the PCI of LaNi₅ from other alloys – at room temperature the former tends to show a single plateau while the latter have two plateaux – is explicable in terms of hydrogen site energies as follows. In case of LaNi₅, four hydrogen sites, which are considered to belong to 3f, 12n, 6m and 12o sites, are stable in α phase, leading to a wide single plateau upto N6 where N is in RNi₅H_N below room temperature. At higher temperature, small differences in site energies for these sites brings the appearance of the second plateau. In case of other three alloys, on the other hand, the 6m site (or 12o) is less stable than LaNi₅ in the α phase. After obtaining enough lattice relaxation energy by around N=4 through hydrogenation, the 6m site becomes favorable for hydrogen, bringing the sencond plateau spanning from N4 to N6. Predicted T–C phase diagrams are also shown.     

Micro-forming of Zr65Al10Ni10Cu15 metallic glasses under superplastic condition

Micro-formability behavior of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ alloy sheet fabricated by using squeeze casting method, which exhibited Newtonian behavior in the supercooled liquid region, was examined by finite element methods and experiments. In micro-forming simulation on a single V-groove, micro-formability index increased with time and nearly completed die-filling was achieved in 100 s when deformed under a constant pressure of 52.6 MPa at 696 K. In micro-forming on the die with multiple V-grooves in array along its width, the degree of die-filling was predicted to depend on the V-groove position. The V-grooves near the center of the die exhibited the high degree filling ratio of 1 but it was only 0.1 near the free end. This trend was confirmed to agree with the experimental results. According to the simulation result, higher friction coefficient and longer loading time could improve the die-filling ability near the free end.     

Effects of C particle size on the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system

C particle size plays an important role in the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system. When coarse C particles (38 and 75 μm) are used, the SHS reactions consist of two different combustion stages with different brightness intensity of the combustion wave; XRD results indicate that the first and second combustion stages mainly correspond to the formation of Ni–Ti compounds and TiC ceramics, respectively. However, the final reaction is incomplete with a few Ni–Ti compounds and unreacted C. In contrast, when the fine C particle (1 μm) is used, the SHS reaction consists of only one combustion stage with high brightness intensity of the combustion wave; XRD result indicates that final products consist of TiC and Ni, without any intermediate phase. With the decrease of C particle size, the wave velocities increase, and the ignition time becomes shorter. In addition, the morphology of TiC particulate changes to near-spherical, as C particle size decreases.     

Electrochemical behaviour and corrosion performance of Mg–Li–Al–Zn anodes with high Al composition

This study investigated the electrochemical and corrosion performance of Mg–Li–Al–Zn anodes with Al compositions of 3 wt.% and 9 wt.%. Mg–Li–Al–Zn alloy with 9 wt.% Al had a relatively negative open-circuit potential and a high discharge voltage in MgCl₂ electrolyte, owing to the distribution of numerous AlLi particles in the matrix of the alloy. AlLi particles were believed to transform to Al particles during the corrosion of the Mg–Li–Al–Zn anode. The high-Al anode material exhibited good corrosion performance since a dense and continuous Mg(OH)₂/Al composite layer covered the surface of the high-Al anode. Experimentally, increasing the Li⁺ concentration in the electrolyte improved the corrosion performance of the Mg anode.