Microstructure and mechanical behavior of spray-deposited Al-Cu-Mg(-Ag-Mn) alloys

The effect of alloy composition on the microstructure and mechanical behavior of four spray-deposited Al-Cu-Mg(-Ag-Mn) alloys was investigated. Precipitation kinetics for the alloys was determined using differential scanning calorimetry (DSC) and artificial aging studies coupled with transmission electron microscopy (TEM) analysis. DSC/TEM analysis revealed that the spray-deposited alloys displayed similar precipitation behavior to that found in previously published studies on ingot alloys, with the Ag containing alloys exhibiting the presence of two peaks corresponding to precipitation of both Ω-Al₂Cu and θ′-Al₂Cu and the Ag-free alloy exhibiting only one peak for precipitation of θ′. The TEM analysis of each of the Ag-containing alloys revealed increasing amounts of Al₂₀Mn₃Cu₂ with increasing Mn. In the peak and over-aged conditions, Ag-containing alloys revealed the presence of Ω, with some precipitation of θ′ for alloys 248 and 251. Tensile tests on each of the alloys in the peak-aged and overaged (1000 hours at 160 °C) conditions were performed at both room and elevated temperatures. These tests revealed that the peak-aged alloys exhibited relatively high stability up to 160 °C, with greater reductions in strength being observed at 200 °C (especially for the high Mn, low Cu/Mg ratio (6.7) alloy 251). The greatest stability of tensile strength following extended exposure at 160 °C was exhibited by the high Cu/Mg ratio (14) alloy 248, which revealed reductions in yield strength of about 2.5 pct, with respect to the peak-aged condition, for the alloys tested at both room temperature and 160 °C.     

Cast aluminum alloys containing dispersions of zircon particles

A process for preparing Al-alloy castings containing dispersions of zircon particles is described. Composites were prepared by stirring zircon particles (40 to 200 μm size) in commercially pure Al (99.5 pct)* and Al-11.8 pct Si melts and subsequently casting these melts in permanent molds. It was found to be necessary to alloy the above two melts with 3 pct Mg to disperse substantial amounts of zircon particles (25 to 30 pct). Further, it was possible to disperse up to 60 wt pct zircon by adding up to 5 pct Mg; however, the melts containing above 30 wt pct zircon showed insufficient fluidity for gravity diecasting and had to be pressure diecast. Microstructural studies of cast composites indicated the presence of a reaction zone at the periphery of zircon particles, and electron probe microanalysis showed concentrations of Mg and Si at the particle-matrix interface. Hardness, abrasive wear resistance, elastic modulus, 0.2 pct proof stress, and tensile strength of cast Al-3 pct Mg alloy were found to improve with the dispersions of zircon particles. Scanning electron micrographs of abraded and fractured surfaces did not show any evidence of particle pull-outs or voids at the particle matrix interface, indicating strong continuous bonding.     

Compositional analysis of the icosahedral phase in rapidly quenched Al-Mn and Al-V alloys

The formation ranges and alloy compositions of icosahedral phases in rapidly quenched Al-Mn and Al-V alloys containing 12.5 to 25 at. pct Mn and V, respectively, were examined by X-ray diffractometry and analytical transmission electron microscopy. The icosahedral phase was found to appear in a wide range of compositions below about 23 at. pct Mn and below about 18 at. pct V, but the formation of the icosahedral single phase was limited only in the vicinity of about 22.5 at. pct Mn. The analytical solute concentration in the icosahedral phase is not always constant and increases continuously from about 17 to 23 at. pct Mn and about 18 to 21 at. pct V with increasing nominal solute concentration. Thus, the icosahedral phase in rapidly quenched Al-Mn and Al-V alloys can be approximately formulated to be Al₄Mn and A1₄V with a maximum deviation of about 3 at. pct Mn or 2 at. pct V from the stoichiometric ratio.     

Coprecipitation of Ω and σ phases in Al-Cu-Mg-Mn alloys containing Ag and Si

The individual influence of small additions of Ag and Si on the nucleation of the Ω (i.e., a chemically modified coherent form of ϑ-Al₂Cu) and σ (Al₅Cu₆Mg₂) phases, respectively, in Al-Cu-Mg alloys has been known. These phases nucleate directly and exhibit reduced rates of coarsening at the commercial aging temperatures. Alloys containing a uniform and fine distribution of these phases may, therefore, be of interest for further investigation for applications at temperatures below 200 °C. In a recent study, using an Al-Cu-Mg-Mn-Ag-Si alloy aged at 180 °C, it was shown that the Ω phase formed as a major precipitate nucleated predominantly upon the Mn-bearing dispersoids, while the σ phase was present as a minor one. This article describes the conditions under which widespread nucleation of σ phase may occur at unidentified sites in the matrix as well as upon the Mn-bearing dispersoids in the alloy. Widespread nucleation of σ phase begins in the alloy following the onset of dissolution of Guinier-Preston-Bagaryatski (G-P-B) zones that form early in the aging cycle as the major precipitate. It is established that nucleation of σ as a major phase precipitate requires a critical minimum supersaturation of Si in the solid solution. This article further points out that several constituent phases (implying those which form as solidification products and survive the homogenization treatment) together with the Mn-bearing dispersoids dissolve Si, thereby considerably reducing the Si supersaturation in the solid solution. The implications of these observations are discussed in view of the available data on the nucleation of σ phase in such alloys.     

Study of Cu-rich phase precipitation in austenitic antibacterial stainless steel

Cu precipitation behaviors in two Cu-bearing austenitic antibacterial stainless steels,type 304 and type 317L,were systematically studied by using relatively simple methods for materials analysis,including micro-hardness,electrical resistivity,electrochemical impedance spectroscopy,X-ray diffraction and differential scanning calorimetry.The results indicated that after aging at elevated temperature,the micro-hardness, electrical resistivity,electrochemical impedance and lattice constant of the steel were all varied at different degrees due to the precipitation and growth of Cu-rich phases.The results also showed that the heat evolution during the process of Cu precipitation could be sensitively detected by means of differential scanning calorimetry,obtainning the starting temperature,peak temperature,peak area of the Cu-rich precipitation,and even the activation energy by calculation.The results confirmed that the Cu-rich phased precipitation in the Cu-bearing austenitic antibacterial stainless steel should be a thermal activation process controlled by Cu diffusion.All the materials analysis methods used in this study can be more simple and effective for application in R & D of the Cu-bearing antibacterial stainless steels.     

Effects of cold work on precipitation in Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys

A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry, and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM), selected area electron dif-fraction (SAED), and quantitative stereology. Particular attention has been given to the phases Ω andT ₁ that form on the {111 }_α planes, although information on the precipitates θ′,S′ (orS), and δ′ is also reported. Although Ω andT ₁, have similar morphologies and habit planes, their response to cold work prior to aging is different. Deformation promotesT ₁ formation at the expense of the δ′ phase in Al-Cu-Li alloys and at the expense of δ′, θ′, andS′ in Al-Cu-Li-Mg-Ag alloys. On the other hand, in Al-Cu-Mg-Ag alloys, deformation assists precipitation of θ′ at the expense of Ω phase, and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions. Formerly Research Fellow, Department of Materials Engineering, Monash University     

Solid-liquid phase characterization of several rheocast high performance alloys

High performance alloys, such as nickel-base superalloys and specialty steels, are amenable to the Rheocast process. Through heat treatment and quenching studies on these alloys, it is possible to determine their volume fraction of solid versus liquid as a function of temperature, as well as the partitioning of chemical elements between the two phases. The coarsening of primary solid particles in the liquid occurs by both diffusion-controlled growth and particle agglomeration. The rate of coarsening is sufficiently low that it should not interfere with thixotropic processing.     

Analysis of softening alloys of the Al-Ni system containing particles of variable dispersity

Regularities of the deformation strengthening and softening of aluminum alloys containing second-phase Al₃Ni particles 0.3 to 2.2 μm in size with a volume fraction from 0.03 to 0.1 are investigated during cold deformation and subsequent annealing at 0.6t _m. It is shown that the largest hardness increment is observed for alloys with a maximal fraction of fine particles (d = 0.3 μm) after rolling deformation larger than 0.4. Fine particles prevent the development of crystallization upon true deformation up to 2.3, thereby effectively inhibiting softening. An increase in the particle size to 1.2–2.2 μm stimulates nucleation during recrystallization, substantially accelerating this process. For example, in order to ensure recrystallization uniformly over the entire sheet volume at d = 2.2 μm, cold deformation with ? = 0.4 is sufficient.     

The assessment of creep damage in certain aged alloys based on Al-Cu-Mg system

The mechanism of creep failure was investigated for three different test alloys namely Al-4.0%Cu-0.3%Mg, Al-4.0%Cu-0.3%Mg-0.4%Cd and Al-4.0%Cu-0.3%Mg-0.4%Ag at 125 and 150°C in the fully hardened condition. The room temperature tensile properties of these alloys increased in the order of ternary alloy, Cd-containing alloy and Ag-containing alloy. The creep performance of these alloys also improved in the similar order. The present studies revealed the dominance of intercrystalline creep failures in all the alloys at both the test temperatures. The grain boundary microstructures contained precipitates with narrow Precipitate Free Zones (PFZ’s) with large difference in particle spacings. Ag-containing alloy recorded minimum grain boundary particle spacing as compared to that of ternary and Cd-containing alloys. The creep damage assessment in terms of damage distribution in the gauge portion showed maximum damage in the Ag-containing alloy as compared to other two alloys. In all these alloys, failures occurred by the coalescence of several cracks and the negotiations of few boundary junctions rather than the propagation of single major crack.     

Wear behavior,microstructure, and dimensional stability of as-cast zinc-aluminum/SIC (metal matrix composites) alloys

The wear behaviors of five different zinc-aluminum (ZA)-based alloys containing silicon, copper, and 8 and 16 pct on volume of reinforcing silicon carbide (SiC) particles were analyzed. Hardness, dimensional stability, and wear tests were performed on these five alloy samples. Microstructural investigation and semiquantitative chemical analysis of the different alloying characteristics of the cast samples, the wear surface, and the wear debris were obtained by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDXA), and X-ray diffraction (XRD). The addition of Si, Cu, and SiC has a significant effect on the solidification process and final morphology of the alloys. The five cast alloys tested showed dimensional stability for a period of 1000 hours at 165°C±2.5°C. The wear tests were performed using a pin-on-disc apparatus under dry and lubricated conditions. Loads of 29.43 N (3 kg), 49.05 N (5 kg), and 78.48 N (8 kg) and a velocity of 250 rpm (2 m/s) were used. The results indicate that the wear rate of ZA alloys is strongly dependent on test load in a non-linear relationship and that the addition of SiC particles improved the wear properties of the matrix alloys. Under dry conditions, there was considerable loss of material, particularly in the nonreinforced alloys. In addition, the nonreinforced alloys presented substantial local plastic deformation and transfer of elements between the disc, the sample, and the debris. The amount of element transfer can be correlated with the elements presented. The proposed wear mechanisms are discussed.     

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.     

Magnetic studies of phase equilibria in Ti-Ai (30 to 57 At. Pct) alloys

Room temperature magnetic susceptibility studies have been made on a series of Ti-Al alloys in the composition range 30 to 57 at. pct Al, quenched from anneals at 900, 1065, 1165, 1215, 1265, 1315, and 1365°C. As a result of this work an equilibrium partial phase diagram has been constructed, compared with literature data, and discussed with reference to the results of a comprehensive metallographic and microhardness study.     

Effect of alloying elements on martensitic transformation in the binary NiAl(β) phase alloys

The characteristics of the B2(β) to L1₀(β′) martensitic transformation in NiAl base alloys containing a small amount of third elements have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It is found that in addition to the normal Ll₀ (3R) martensite, the 7R martensite is also present in the ternary alloys containing Ti, Mo, Ag, Ta, or Zr. While the addition of third elements X (X: Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Ta, W, and Si) to the binary Ni₆₄Al₃₆ alloy stabilizes the parentβ phase, thereby lowering the M_s temperature, addition of third elements such as Co, Cu, or Ag destabilizes theβ phase, increasing the M_s temperature. The occurrence of the 7R martensite structure is attributed to solid solution hardening arising from the difference in atomic size between Ni and Al and the third elements added. The variation in M_s temperature with third element additions is primarily ascribed to the difference in lattice stabilities of the bcc and fcc phases of the alloying elements.