Mechanical properties of cast lead alloy/silicon carbide particulate composites

This paper describes a study of the mechanical properties of cast lead-antimony alloy composites containing silicon carbide (SiC) particles of size 90–150 μm and of contents ranging from 0% to 5% by weight. The ‘vortex method’ of production was employed in which the SiC particles were poured into the vortex created by stirring the molten metal at 400°C by means of a mechanical agitator. The results of this study revealed that as SiC composition was increased, there were significant increases in the ultimate tensile strength (UTS), hardness, torsional strength and impact strength of the composite, accompanied by a reduction in its ductility. An attempt is made in the paper to provide explanations for these phenomena.     

Microscopic observation of cold-deformed Al–4Cu–Mg alloy samples after semi-solid heat treatments

Wrought aluminum alloys can be effectively fabricated by a strain-induced, melt-activated (SIMA) process. The SIMA method involves plastic deformation of an alloy to some critical reduction point and a semi-solid heat treatment in the solid–liquid temperature range. The semi-solid heat treatment is a key process to control the semisolid microstructures. In this paper, the microscopic morphology of a cold-deformed SIMA treated Al–4Cu–Mg alloy has been investigated, and the effects of microstructural evolution, precipitation behavior and dislocation morphology on the mechanical properties are discussed. The experimental results show that the number of CuAl₂ (θ phase) precipitates and the dislocation density of Al–4Cu–Mg alloy decreased gradually by the semi-solid heat treatment. Moreover, unique dislocation morphologies including helical dislocations and dislocation loops appeared and evolved to reduce the stored energy. With an increase of the holding time in the semi-solid heat treatment, the ultimate strength and yield strength decreased. The reduction of these mechanical properties of the SIMA treated Al–4Cu–Mg alloy is mainly due to the decrease of refinement strengthening, solution strengthening, and dislocation strengthening in the semi-solid heat treatment.     

Aging behaviour of Al–Cu–Mg alloy–SiC composites

The age-hardening kinetics of powder metallurgy processed Al–Cu–Mg alloy and composites with 5, 15 or 25 vol.% SiC reinforcements, subjected to solution treatment at 495 °C for 0.5 h or at 504 °C for 4 h followed by aging at 191 °C, have been studied. The Al–SiC interfaces in composites show undissolved, coarse intermetallic precipitates rich in Cu, Fe, and Mg, with its extent varying with processing conditions. Examination of aging kinetics indicates that the peak-age hardness values are higher, and the time taken for peak aging is an hour longer on solutionizing at 504 °C for 4 h, due to greater solute dissolution. Contrary to the accepted view, the composites have taken longer time to peak-age than the alloy, probably due to lower vacancy concentration, large-scale interfacial segregation of alloying elements, and inadequate density of dislocations in matrix. The composite with 5 vol.% SiC with the lowest inter-particle spacing has shown the highest hardness.     

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M_s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an + β titanium alloy. The high temperature β transforms to either ′ or + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of have been studied and discussed in this paper.     

Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy

Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of magnesium matrix and Mg₁₂Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO₁₉ structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.     

Sliding wear behaviour of SiC particle reinforced 2024 aluminium alloy composites

Abstract

Sliding wear tests on SiC particle reinforced 2024 aluminium alloy composites fabricated by a powder metallurgy technique were carried out, and the effects of SiC particle content, size, and the wear load on the wear properties of the composites were systematically investigated. It was found that the wear resistance of the composites was about two orders of magnitude superior to that of the unreinforced matrix alloy, and increased with increasing SiC particle content and size. Under the conditions of sliding wear used, the effect of SiC particle size on the wear resistance was more significant than that of particle content.

MST/3161     

Studies on the development of aluminium alloy–mild steel reinforced composite

With a view to developing a new metal–metal cast composite material as a possible substitute for ferrous materials in wear resistant applications, Al alloy (LM11) is reinforced with mild steel (ms) wires and it is heat treated to get ‘reaction interface’ (RI). Microhardness, tensile properties and wear characteristics of the matrix, as-cast and heat treated composites have been determined. While microhardness of the composite showed variation from 150 to 45 VHN across the interface in the as-cast composite, annealed (500–525°C) composite showed a microhardness of 350–420 VHN at the interface indicating the effectiveness of the heat treatment. It is seen that the % improvement in wear resistance increased with increase in number of wires when embedded in the aluminium alloy matrix. Further imrpovement of about 30% was observed when heat treated at 500°C for 15 h. These results have been discussed in terms of wetting between ms wires and the matrix, particularly the increase of hardness and tensile strength to the formation of ‘reaction interface’ due to annealing. The width of the interface increased with annealing time and temperature and the kinetics of reaction followed logarithmic and parabolic growth rate. The activation energy for the formation of intermetallics constituting the reaction interface is found to be 20.7 KJ mol⁻¹. From the measured hardness and ultimate tensile strength of the constituents and composites an empirical relation was deduced.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 μm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macro- and microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^′ (Al₃Li) and S^′ (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

Metallurgical assessment of an emerging Al–Zn–Mg–Cu P/M alloy

The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of an emerging Al–Zn–Mg–Cu powder metallurgy (P/M) alloy known as Alumix 431D. A variety of techniques were considered including optical microscopy, X-ray diffraction, electron-probe micro-analysis, thermal dilatometry, and differential scanning calorimetry as well as apparent hardness, tensile testing, and bending fatigue. Alumix 431D exhibited many of the same attributes found in wrought counter parts such as 7075. A sintered density of approximately 99% of theoretical was achieved, indicating that the alloy was highly responsive to sintering. Once heat treated, a T6 hardness of 86 HRB and a room temperature ultimate tensile strength of 448 MPa were noted. Thermal analyses implied that the precipitation behaviour of Alumix 431D closely mimicked comparable 7XXX series wrought alloys and was largely premised on the precipitation of η-phase variants. Tensile properties of the alloy in a T1 temper were found to be relatively stable at temperatures up to 150 °C and 1000 h of exposure time. Those of T6 specimens degraded under the same exposure conditions to the point where equivalency with the T1 product was noted.     

In situ synthesis mechanism and characterization of ZrB2–ZrC–SiC ultra high-temperature ceramics

The synthesis route, microstructure and properties of ZrB₂–ZrC–SiC composites prepared from a mixture of Zr–B₄C–Si powders by in situ reactive synthesis were investigated. The reactive path and synthesized mechanism of ZrB₂–ZrC–SiC composite were studied through series of pressureless heat treatments ranging from 800 °C to 1700 °C in argon. The in situ ZrB₂–ZrC–SiC composites were fabricated under different synthesis processing. The one with 88.4% relative density performed poorly in mechanical properties due to the occurring of self-propagating high-temperature synthesis (SHS). The fully dense ZrB₂–ZrC–SiC composite was obtained under the optimized synthesis processing without SHS reactions. Its Vickers hardness, flexural strength and fracture toughness were 20.22 ± 0.56 GPa, 526 ± 9 MPa and 6.70 ± 0.20 MPa m^1/2, respectively.     

Wear behaviour of an Al–12% Si alloy reinforced with a low volume fraction of SiC particles

Aluminium–silicon alloys reinforced with low volume fractions of SiC particles were prepared by the compocasting process. The wear behaviour of the unreinforced Al–12Si alloy and metal-matrix composites (MMCs) was investigated by using a block-on-ring test at room temperature under dry conditions. The results showed that the addition of a low volume fraction of SiC particles (2–8 vol%) is a very effective way of increasing the wear resistance of the matrix alloy. Metallographic examinations revealed that the wear zone of the Al–12Si alloy consists of both hardened and deformation layers. The depth of the hardened layer depended on the applied load and was in the vicinity of 10–50 μm. The formation of the hardened layer was related to the alignment and redistribution of fragmented eutectic phase to the surface region during sliding wear. Furthermore, the delamination of debris from the hardened layer was responsible for a higher wear loss observed in the Al–12Si alloy. The thickness of the hardened layer formed on the MMC specimens was reduced considerably by the incorporation of fragmented SiC particles. This layer exhibited higher hardness and wear resistance than that developed in the unreinforced alloy.     

Stress corrosion cracking model in 7075 aluminium alloy

The stress corrosion cracking is a typical fracture process in metals and alloys. Among aluminium alloys, the 7075 alloy, presents a high performance in the mechanical properties but it is susceptible to stress corrosion cracking. This paper presents a semiempiric model of crack growth by stress corrosion cracking for the above alloy. This model only uses macroscopic parameters from fracture mechanic theory and experimental tests which are easy to obtain. The model quantifies the fissure rate related to environmental condition, microstructure and loading level, permitting the evaluation of the crack growth process at different environmental conditions and heat treatments. The model results are compared with the experimental data obtained. The theoretical model reproduces adequately the stress corrosion cracking process for the 7075 alloy.     

Microstructure and grain refining performance of Al–5Ti–1B master alloy prepared under high-intensity ultrasound

The microstructure and grain refining performance of an Al–5Ti–1B master alloy prepared under high-intensity ultrasound were investigated. With applying continuous high-intensity ultrasound vibrations in the reaction, the Al–5Ti–1B master alloy is successfully manufactured in 4 min. Compared with conventional Al–5Ti–1B master alloys, the mean size and the size spread of TiB₂ particles in the prepared master alloy are evidently decreased. The narrower particle size spread significantly improves the grain refining performance of the master alloy, which proves the calculation predictions by Greer. Consequently, the limiting grain size of commercial purity aluminium refined by the new master alloy can reach 45 μm.     

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni₅Al₃ phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.     

Vacuum brazing of aluminium metal matrix composite (55 vol.% SiCp/A356) using aluminium-based filler alloy

Aluminium matrix composites with high volume fractions of SiC particles, as the reinforcements, are potentially suitable materials for electronic packaging. These composites, due to their poor weldability, however, have very limited applications. The microstructure and shear strengths of the bonds made in 55 vol.% SiC_p/A356 composite, using an aluminium based filler alloy containing Cu, Si, Mg and Ni, were investigated in this paper. The brazing temperature had a clear effect on the bond integrity, and the samples brazed at 560 °C demonstrated good bonding between the filler alloy and the SiC particles. The maximum shear strength achieved in this work was 102 MPa.     

On the sequence of clustering and ordering in a meltspun Cu–Ti alloy

Although it is well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu–Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573–723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N₃M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu₄Ti (D1_a).     

Effect of prior cold work on age hardening of Cu–3Ti–1Cr alloy

The influence of 50%, 75% and 90% cold work on the age hardening behavior of Cu–3Ti–1Cr alloy has been investigated by hardness and tensile tests, and light optical and transmission electron microscopy. Hardness increased from 118 Hv in the solution-treated condition to 373 Hv after 90% cold work and peak aging. Cold deformation reduced the peak aging time and temperature of the alloy. The yield strength and ultimate tensile strength reached a maximum of 1090 and 1110 MPa, respectively, following 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation twins. The maximum strength on peak aging was obtained due to precipitation of the ordered, metastable and coherent β′-Cu₄Ti phase, in addition to high dislocation density and deformation twins. Over-aging resulted in decreases in hardness and strength due to the formation of incoherent and equilibrium β-Cu₃Ti phase in the form of a cellular structure. However, the morphology of the discontinuous precipitation changed to a globular form on high deformation. The mechanical properties of Cu–3Ti–1Cr alloy are superior to those of Cu–2.7Ti, Cu–3Ti–1Cd and the commercial Cu–0.5Be–2.5Co alloys in the cold-worked and peak-aged condition.     

Microstructure – A Dominating Parameter for Chip Forming During High Speed Milling

High speed milling experiments with cutting speeds of up to 7000 m/min have been performed on an aluminium alloy (AlZnMgCu1.5, AA 7075) and a steel (40CrMnMo7). Both materials were heat treated to produce a variety of microstructures in order to investigate the influence of microstructure on segmentation during chip formation. In case of the aluminium alloy it has been shown that chip formation is governed by the precipitation state, but not by the hardness of the material. In contrast, hardness can be used for a qualitative prediction of chip shape of the steel. In all cases, the chip formation mechanisms remain essentially unaffected by cutting speed, i.e. no transition from continuous to segmented chips occurred.     

Investigation of the rotary swaging and heat treatment on the behavior of W- and γ-phases in PM 92.5W–5Ni–2.5Fe–0.26Co heavy alloy

The effects of rotary swaging and different heat treatment procedures on the W- and γ-phases behavior of PM 92.5W–5Ni–2.5Fe (wt.%) heavy alloy microalloyed with cobalt have been studied. The investigation was performed on sintered and cold rotary swaged samples deformed with area reduction from 5 to 30%. One batch of swaged samples was annealed in vacuum at 1473 K for 7.2 ks and then furnace-cooled to the room temperature, whereas another batch of swaged samples was previously deformed 30% and strain aged in argon and nitrogen in the temperature range between 473 and 1123 K for 3.6 ks. Strengthening of W- and γ-phases was investigated by applying microhardness measurements. Effects of the degree of deformation, parameters of heat treatment and strain aging on microstructural changes have been studied. Mechanical properties, hardness and microhardness of phases as a function of the degree of deformation and heat treatment were analyzed by applying statistical modeling. A correlation between deformation behavior of phases, effect of heat treatment and alloy properties was also discussed.     

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10^–4s^–1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.