Materials Science Forum Vols. 561-565

Abstract: A holistic model of the atmospheric corrosion of metals is being developed. The model is based on integrating modules that define such processes as marine aerosol production by oceans and breaking surf, transport of marine aerosols across landscapes, deposition of aerosols onto structures, cleaning of surfaces by wind and rain, and the wetting and drying of surfaces throughout surface temperature and relative humidity cycles. The integration of these modules into a software framework enables the user to extract accurate estimates of surface conditions for structures located at any geographical location in Australia. Current research is aimed at developing a more fundamental approach to estimating corrosion based on the response of a metal to its environment.

Abstract: Fully reversed tension-compression fatigue tests were performed on solid-solutioned Al-0.7mass%Mg single crystals with a single slip orientation under constant plastic-strain amplitudes. Dislocation microstructures were quantitatively examined by transmission electron microscopy. The cyclic stress–strain curve (CSSC) exhibited three distinct regions with a short plateau region in the intermediate plastic-strain amplitude range, and the plateau stress was 26MPa. Characteristic microstructures were developed corresponding to the three regions in the CSSC. Vein structure was observed at the low strain-amplitude region. In the plateau regime, the persistent slip bands (PSBs) were observed. Labyrinth structure was also observed at the higher strain-amplitude region. The plateau stress, the cyclic flow stress of PSBs, can be explained by considering not only the Orowan bowing stress and the dipole passing stress of screw dislocations but also solid-solution hardening by Mg atoms.

Abstract: The apparent activation energy for rupture life sometimes changes from a high value of short term creep to a low value of long term creep. This change results in overestimation of rupture life recognized recently in advanced high Cr ferritic steels. The present study examined how to detect the decrease of activation energy in 9-12 %Cr steels with tempered martensitic lath microstructure. During aging without stress hardness of the tempered martensite microstructures remains almost constant in short term, whereas it decreases with increasing time after long term exposure. The onset of hardness drop can be a good measure of the decrease of activation energy. Causes of the hardness drop and the decrease of activation energy are discussed.

Abstract: Thermolastic Stress Analysis (TSA) has been recently developed as a direct investigating method for the study of the stress field around the crack tip of a cyclically loaded structure. The advantage of using measurement techniques based on the thermoelastic effect lays in the fact that stress intensity factors may be determined based on the effective stress distribution around the crack tip rather than calculated from the crack length and amplitude of cyclic loads. This paper reports results related to fatigue tests on Friction Stir Welded alluminium alloys sheets. Fatigue crack propagation experiments were performed by employing single-edge notched specimens, in tensiontension condition with R=0.1, up to failure. The application of TSA allowed the monitoring of crack formation and growth in real time, providing the actual stress distribution around the crack tip for the different technological parameters used in the welding process. Stress intensity factors were determined based on the TSA data and compared to those calculated using an ABAQUS FE model.

Abstract: The notch is not only one of the primary reasons of component fracture, but also the key factor in life assessment of shaft. The stress and displacement field near the tip of annular notch should be obtained at first for safe design and life assessment. A quadric coordinate system suited for annular notch is proposed and the stress and displacement component of cylindrical shaft with annular notch are derived by using the modified Galerkin displacement function. This method can be used to solve various problems of cylindrical shaft with annular notch by selecting an appropriate displacement function and provides the basis for safe design and life assessment of this kind of components. A solution of torsional problems of cylindrical shaft with annular notch is calculated as a demonstration of the method presented.

Abstract: The deformation degrees in near-surface zones and related microstructural changes in CuZn30 were determined after shot peening and ball-burnishing. The induced depths of plastic deformation increase with Almen intensity and rolling pressure in shot peening and ball-burnishing, respectively. As the ball size in burnishing increases, the depth of plastic deformation also increases. The local degrees of cold work within the affected depths were estimated by comparing micro-hardness values, penetration depths of induced slip bands as well as X-ray diffraction peak widths with those measured in blanks cold rolled to fixed degrees of plastic strain. The results clearly indicate that very high local deformation degrees above 80% are typically induced by mechanical surface treatments in areas very close to the surface with this deformation degree gradually decreasing with increasing distance to the surface. Depending on the intensity of ball-burnishing, deformation degrees still of the order of 10% were observed in regions having distances to the surface of as large as 1000 (m.

Abstract: Creep deformation mechanism of the steels with a different matrix, α (ferrite), α’ (martensite) and γ (austenite), and precipitates such as MX and M23C6 has been analyzed using positron annihilation lifetime measurement. The positron annihilation lifetime has been found to be a very useful tool for evaluating the characteristic creep damage of the steels with different microstructure and the corresponding microstructural evolution during creep deformation. The creep deformation process of the α steel is heterogeneous, while the α’+M23C6 steel exhibits gradual changes in the creep rate in both transient and acceleration creep regions with the largest off-set strain, implying the homogeneous creep deformation. The α’+M23C6+ MX steel is in between the α and α’+M23C6 steels. The homogeneous creep deformation takes place in the γ steel.

Abstract: Polymer wear debris particles undergo a thermal and mechanical (shear) cycle since their generation and therefore contain information on the friction and wear processes, while it is often difficult to draw quantitative data from them relating to transitions in tribological behaviour. Results from thermal DTA/TGA analysis and morphological pattern spectra of debris are presented and related to a transition at 180°C sliding temperatures own to hydrolysis and imidisation.

Abstract: Several premature failures were occurred in the high-pressure section of an industrial gas turbine compressor due to the fracture of Titanium blade roots. In this work, the failure process of the compressor blades was investigated based on the experimental characterisation. Macro/microfractographic studies were carried out on the fracture surfaces. Optical and scanning electron microscopy of the blade airfoil and root were performed. Mechanical properties of the blade alloy were also evaluated and compared with the standard specifications. The experimental results showed no metallurgical and mechanical defects for the blade materials. Microstructures of the blade root and airfoil as well as the hardness and tensile properties were all comparable with those reported in the standard specification AMS 4928Q. Fractography experiments showed clearly multiple crack initiation sites and fatigue beach marks. Debris particles were observed on the fracture surface of samples and in the mouth of initiated cracks. The blade surface in contact to the disc in the dovetail region showed a higher surface roughness than the other surfaces. Based on the results obtained, the fretting fatigue mechanism was proposed for the premature failures. It was concluded that the stress concentration has been caused by either unsuitable curvature ratio of the disk dovetail, incorrect design of the blade or insufficient distance between the blade root and the disk in dovetail region.

Abstract: The present work studied the corrosion property around the fusion boundaries of a dissimilar steel welded joint in as-welded condition and after post-weld heat treatment (PWHT) in H₂S containing solution with different times. An in-situ-tracking observation was proceeded for evaluating corrosive progress at the fusion boundary by using a scanning electron microscope (SEM). The results revealed that the fusion boundary was the worst region for corrosion resistance when comparing with other zones, and a broad boundary had a stronger resistance for hydrogen induced disbonding than a narrow one.