HVEM study of recrystallisation in aluminium alloys containing second phase particles

An investigation of the recrystallisation behavior of two Al alloys containing fine (Al-Fe) and coarse (Al-Si) particles is presented. In situ anneals in a 1 MeV electron microscope revealed different recrystallisation mechanisms in each case and allowed direct measurements of grain boundary velocities and subgrain sizes. Multiple nucleation around coarse particles is observed at 250°C for the Al-Si alloys. Conversely, small precipitates strongly pin a subgrain structure up to 400°C for the Al-Fe alloy. Grain boundary migration rates are compared to values deduced from bulk-annealed specimens for the Al-Si alloy. The stability of the subgrain structure in the Al-Fe alloy is discussed with respect to recent models.     

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.     

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     

先进高强韧Al-Zn-Mg-Cu合金凝固和均匀化组织及相构成

采用热力学计算与实验相结合的方法,研究了两种高强韧Al-Zn-Mg-Cu合金铸态及均匀化态的显微组织和相构成.铸态A合金主要由Mg（Zn,Al,Cu）₂相和少量Al₂Cu相组成,而铸态B合金仅含Mg（Zn,Al,Cu）₂相.热力学计算显示,A和B两种合金的实际凝固过程介于Lever Rule和Scheil Model两种模拟结果之间,由于合金成分不同而导致的铸态A和B合金中各相含量差异与Scheil Model模拟所得到的各相摩尔分数变化规律基本一致.经常规工业均匀化处理（460℃保温24 h）,铸态A和B合金中存在的Mg（Zn,Al,Cu）₂或Al₂Cu相均能充分回溶,并得到单相α（Al）基体,这与热力学计算所得到的AlZn-Mg-Cu四元系统在7.5%Zn条件下460℃等温相图相符合.     

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.     

Modeling matrix grain growth in the presence of growing second phase particles in two phase alloys

In a number of two phase systems such as in α-β titanium alloys, second phase particles as well as the matrix grains grow simultaneously. A systematic study has been undertaken to understand matrix grain growth behavior of such materials in the presence of growing second-phase particles. Using the classical expressions of the driving force for grain growth and a modified form of the Zener retardation to account for preferential location of particles at triple junctions, grain growth of α and β matrices in the presence of growing second phase particles have been modeled by first order nonlinear differential equations. The contributions of grain boundary curvature, nonspherical nature of particles and nonorthogonal nature of particle grain boundary contact angles have also been incorporated into these models. Matrix grain sizes predicted by these methods were found to correlate well with the experimental results.     

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.     

Fracture in equiaxed two phase alloys: part ii. fracture in alloys with isolated plastic particles

A model, describing fracture of two phase equiaxed alloys containing isolated plastic particles within an elastically deforming matrix, has been developed. The model relates fracture toughness parameters to the microstructure and mechanical characteristics of the individual phases. The model utilizes the concepts of a process zone and crack closing forces in the process zone along with recent developments in the fracture mechanics of toughened ceramics. One adjustable parameter, either the extent of the process zone or the effective “gauge length” of plastic particles within the process zone, is used in the analysis. The values of these parameters, as deduced from experimental fracture mechanics studies in Co-CoAl alloys, are reasonable in their magnitude and depend on alloy microstructure in the manner predicted from the analysis. M. A. PRZYSTUPA, formerly Graduate Research Assistant, Department of Metallurgical Engineering, Michigan Technological University     

Rheology of Al-Cu-Mg pre-alloyed powder in supersolidus liquid phase sintering process

Sintering process was performed on cylindrical consolidates in dry nitrogen atmosphere at various temperatures ranging from the solidus-to-liquidus temperatures for 30?min at a heating rate of 10°C?min^?1. Beam bending technique has been used to measure the macroscopic apparent viscosity of Al-Cu-Mg pre-alloyed powder (2024 Al alloy). Slumping test has been used to measure the yield strength at temperatures above solidus temperature. Wetting angle and capillary stress were calculated. Results showed that the rheological behaviour of this system follows as Bingham model. The yield stress varies from 0.37?kPa at 580°C to 0.3?kPa at 610°C on air atomised Al-Cu-Mg having 0.1% Sn. The correlation between variations of viscosity as function of temperature has been derived. The activation energy of viscous flow was estimated to 15.164?kJ?mol^?1. Capillary and yield stress decrease by increasing temperature. Maximum densification with minimum distortion is obtained at 600–610°C.     

A micromechanical incremental modelling of damage in materials containing second phase particles

A micromechanical model is derived to simulate the nucleation and evolution of damage in a material containing second phase particles, which can exhibit fracture or decohesion. The damaged particle is replaced by an equivalent one having anisotropic properties, whose interaction with the matrix is solved by means of the Eshelby method applied in incremental elastoplasticity. Two bounds are determined with this method. The results show typically a strong dependence upon the loading path. Quantitative measurements are made to characterize the damage existing in the material prior and during testing. Comparisons are made between theoretical and experimental results, and show that the latter fall in between the bounds determined in the model. It is also shown that using a single fitting parameter in the model allows to reproduce all experimental results.     

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.