Seizure resistance of leaded aluminium bearing alloys

Abstract

The antifriction and antiseizure characteristics of a number of aluminium bearing alloys varying in lead content from 10 to 50 wt-% have been studied under different lubrication conditions using a bearing test machine designed for the investigation. With continuous oil lubrication, bearings of all the leaded alloys are able to run without seizure up to the regimes of boundary and mixed lubrication, whereas the base alloys seize under conditions in which the leaded alloys are under hydrodynamic lubrication. The load at which bearing seizure takes place is primarily determined by the frictional state of the bearing. In general, the addition of lead to aluminium alloys is found to reduce interface friction and to improve their ability to resist seizure only for concentrations in the range 10–25 wt-%. Lead–containing aluminium alloys also show a capacity to absorb and to store a lubricating oil and to use it to diminish friction in the event of failure of the oil supply.

MST/121     

Some characteristics of icosahedral aluminium alloys

Preliminary results of a study of the transformation behaviour, and those of microhardness measurements on quasi-crystalline Al-14 at.% Mn and Al-22 at.% Mn alloys are reported.     

Creep and creep fracture of commercial aluminium alloys

Using standard power law equations, creep rate and creep life measurements at 373–463 K are analysed for a series of aluminium alloys, namely, 2419, 2124, 8090 and 7010. The seemingly complex behaviour patterns are easily rationalized through a modified power law expression, which incorporates the activation energy for lattice diffusion in the alloy matrices (145 kJ mol⁻¹) and the value of the ultimate tensile stress at the creep temperature. By considering the changes in microstructure and creep curve shape as the test duration and temperature increase, all results are then interpreted straightforwardly in terms of the processes shown to govern strain accumulation and damage evolution. Moreover, the data rationalization procedures are also included in new relationships which superimpose the property sets onto sigmoidal ‘master curves’, allowing accurate prediction of the 100,000 h creep-rupture strengths of 2124 by extrapolation of creep lives determined from tests having a maximum duration of only around 1000 h.     

Age-hardening characteristics of aluminium–chromium alloys

The age-hardening behaviour of Al–Cr alloys containing varying amounts of chromium has been studied, and the effect of silicon on the ageing response of the alloy system was also investigated. Hardness measurement and tensile properties evaluation were used to describe the ageing response of the alloys. With the aid of microstructural observation, the ageing mechanism has been explained. Results of isothermal ageing were used to describe the kinetics of precipitation in both the binary and ternary alloys. It was noticed that the age hardening in aluminium–chromium alloys was mainly due to the precipitation of intermetallics on to the dislocations. The ageing response of the binary alloy containing chromium in excess of its solubility limit was rather poor. However, addition of silicon helped to improve the ageing response of the alloys. Silicon was found to stimulate nucleation of ageing precipitates in the investigated alloys. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the effects of second phase distribution on the fracture behaviour of two superplastic aluminium alloys

The significant scatter observed in the elongation to fracture of specimens taken from a single, industrially processed sheet for two Al alloys has been traced to changes in the value of the strain rate sensitivity index (m) and also to the specimen to specimen variation in the concentration and distribution of second phase particles/inclusions. It is shown that these three variables of m, second phase particle content and distribution affect the width of tear ridges, the size of clusters of grains that pull out as a whole during fracture, their connectivity and the extent to which cavitation can be suffered before final fracture. The consequences of these effects for specimen ductility are discussed.     

In situ monitoring of particle fracture in aluminium alloys

Material damage at the microscale involves both initiation and interaction effects that are typically activated long before the appearance of macroscopically observable failure events. These early appearances of damage have been broadly classified as microstructure‐sensitive damage precursors or indicators. A particular class of such precursors which is of importance to aluminium and other precipitate‐hardened alloys is the focus of this article. Specifically, the hard, intermetallic particles in aluminium alloys fracture before significant failure occur in the surrounding matrix. In this investigation, an effort to directly assess particle fracture activity at the time and scale that it occurs is made by coupling mechanical testing inside a scanning electron microscope with nondestructive evaluation techniques including digital image correlation as well as real‐time acoustic emission monitoring. The use of a surface measurement technique along with a volumetric monitoring method at the microscope scale provides a way for coupling of fracture information at locations which are directly related to the particle activity. In this article, Al2024‐T3 specimens in the as‐received condition were subjected to tension as well as to tension‐tension cyclic loading. The obtained in situ results demonstrate, for the first time to the best knowledge of the authors, that particle fracture occurs early in the damage process which justifies its characterization as a material damage precursor. The overall approach provides datasets capable to detect particle fracture initiation, which could become useful in future structural health monitoring applications.     

Fracture toughness and fracture energy measurements on aluminium alloys

Methods of conducting and analysing instrumented Charpy impact tests have been discussed and applied in measuring the initiation fracture toughness, $\text{K}{}_{\mathrm{Ic}}$, of two precipitation hardened Al-alloys.For full speed impact tests a method for indirectly deriving fracture load from “system stiffness” and “time to fracture” has been found to be the most suitable. In the lower speed impact tests measured fracture load has been used directly to calculate $\text{K}{}_{\mathrm{Ic}}$ In these tests an energy method superficially resembling “specific surface energy” has also been used to calculate $\text{K}{}_{\mathrm{Ic}}$.     

On the fracture toughness of shape memory alloys

International Journal of Fracture - A phenomenological constitutive model for the thermomechanical constitutive behavior of shape memory alloys is implemented within finite element calculations to...     

The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys

A fracture mechanics approach to stress corrosion cracking is highlighted. The mechanisms of stress corrosion cracking is presented. Experiments on 2024 and 7075 aluminium alloys are carried out to determine their mechanical properties, microstructure and plane strain fracture toughness (K_IC). Stress corrosion cracking tests, namely, cantilever beam tests as well as wedge opening loading tests using sea water as a corrosive medium, are conducted to establish the critical stress intensity factor for stress corrosion cracking (K_ISCC) for each aluminium alloy. It is found that the K_ISCC is in the range of (1/5) to (1/6) of the plane strain fracture toughness, K_IC, depending on the alloy. The scanning electron microscopy of fracture surfaces reveals a great dependence of the cracking and/or pit severity on the applied stress intensity factor. A brief discussion on the dislocation's role in stress corrosion cracking is given.     

Synergistic effect of environmental media and stress on the fatigue fracture behaviour of aluminium alloys

The present study aims at explaining the synergistic effect of environmental media and stress/strain on fatigue lives of aluminium alloys. Rotating bending fatigue tests were carried out using four different aluminium alloys LY12‐CZ, 2024‐T4, 7475‐T7351 and 7075‐T651, at air state, 3.5% and 5.0% NaCl aqueous solutions. These results indicated that synergistic actions of the environmental media and cyclic loading accelerated the fatigue crack propagation of aluminium alloys. Furthermore, various influence factors (such as solution concentration, cyclic numbers, high (low) strength aluminium alloys etc.) of the fatigue life at synergistic actions of the environmental media and stress were quantificationally discussed in this paper.     

Fatigue and fracture toughness of aluminium alloys reinforced with SiC and alumina particles

The mechanisms and fracture mechanics of fatigue crack initiation and the growth of small and large fatigue cracks and fracture toughness are reviewed in this paper. It is concluded that: (1) there are many factors which can affect fatigue crack initiation, some of which are understood; (2) small and large fatigue cracks can be correlated with stress intensity factor if closure is excluded; and (3) fracture toughness is mainly related to matrix plasticity but is strongly influenced by particle characteristics.     

Change of the ultrasonic characteristics with stress in some steels and aluminium alloys

Under application of tensile stress to specimens of several steels and aluminium alloys up to a stress close to their yielding stress, a continuous ultrasonic wave of approximately 5 MHz was propagated through the specimens in directions parallel and perpendicular to the tensile axis. The change of ultrasonic resonance frequency with applied stress was measured. The results of the measurement of the ultrasonic resonance frequency showed that the frequency ratio varied with the magnitude of applied stress, the material of the specimen, and ultrasonic wave propagating direction with respect to the direction of the stress. They also showed that the ultrasonic wave resonance frequency measuring method is useful in finding the ultrasonic characteristics of materials under applied stress.List of symbols density - , Lame's constants - l, m Murnaghan's constants - stress - wavelength - n order number of resonance frequency - f _n resonance frequency ofnth order - elastic strain - v Poisson's ratio - C ultrasonic velocity - L length of specimen     

Vapour deposited aluminium alloys

The vapour deposition process and the properties of an Al---Cr---Fe alloy produced by this process are described. Comparisons are made with some of the rapidly solidified alloys that have been reported.     

Powder metallurgy aluminium alloys: characteristics of an Al-Cr-Fe rapidly solidified alloy

An Al-4Cr-1 Fe alloy has been evolved utilizing the advantages of rapid solidification technology. The paper describes the formation of the as-atomized (inert gas) powder microstructure and its decomposition duringin situ heating. It was observed that the most typical powder microstructure had a cellular morphology with a fine intercellular network consisting of ironrich phases. Decomposition of the powder duringin situ heating commenced from the intercellular network, finally resulting in a matrix with a high volume fraction of chromium-rich globular-like precipitates. Consolidation was achieved through cold compaction and hot extrusion; the alloy being easily extrudable. The room-temperature mechanical properties of the alloy were also assessed. The 0.2% proof stress and the tensile strength were below the target limits for dispersion-strengthened alloys, but the elongation and fracture toughness values were very promising. Finally, the extruded microstructure was related to mechanical properties.