Key Engineering Materials Vols. 592-593

Abstract: The paper presents the results of fatigue tests where temperature changes on specimen surfaces were registered. Some different materials were tested. A relation between the crack growth and temperature changes in the propagation place was found. The highest temperature gradients were measured on the crack growth path, and it was caused by molecular friction.

Abstract: The paper presents joint zone microstructure and the test results obtained for fatigue cracks in zirconium-steel bimetal under bending. The materials were joined with explosive welding technology, and special attention was paid to characteristics of the joint zone.

Abstract: The paper presents the results of research on the changes in microstructure of GX12CrMoVNbN9-1 cast steel subject to aging at the temperature of 600°C and holding time of 8000 hours, followed by low-cycle fatigue. The characteristics of the microstructure of the examined cast steel after ageing and low-cycle fatigue was described using transmission electron microscopy (study of the dislocation microstructure) and morphology of precipitations. It has been shown that low cycle fatigue leads to the matrix softening as a result of the processes of recovery, polygonization and repolygonization. Moreover, the processes of precipitation of Laves phase and coagulation of M₂₃C₆ carbides were observed in the microstructure. Intensity of these processes depended not only on the temperature of fatigue tests, but also on the level of total strain amplitude ε_ac.

Abstract: Fracture surfaces of two sets of rotor blades failed after strong testing procedures of aircraft engines in the test-stand are investigated by means of SEM. While the process of fatigue fracture in the first set of blades did not reveal any defect features, both the initiation and propagation of fatigue cracks was strongly influenced by casting defects.

Abstract: Room temperature fatigue behaviour of two variants of diffusion aluminide coatings and conventional air plasma sprayed CoNiCrAlY + ZrO₂-Y₂O₃ thermal barrier system, which were deposited on a cast nickel-based supperalloy substrate, was studied under symmetrical bending loading regime. The study elucidates the differences in a fatigue crack initiation micromechanism with respect to applied stress level and its impact on the fatigue performance of studied coating systems.

Abstract: In this work we studied the microstructure evolution due to equal channel angular pressing of Cu-2wt.%Co alloy after various heat treatments. Several subsequent passes were performed at room temperature. The microstructure was characterized using electron backscatter diffraction technique in a scanning electron microscope. Local mechanical properties were studied by means of nanoindentation experiments using a Hysitron PI85 picoindenter operated inside an electron microscope.

Abstract: Variations of a true stress vs. true strain illustrate behaviour of materials during plastic deformation. Stress-strain relationship is generally evaluated by a torsion, compression and tensile tests. Results of these tests provide crucial information pertaining to the stress values which are necessary to run deformation process at specified temperature and cooling rate. Uniaxial compression tests at temperatures below the γ solvus were conducted on nickel based superalloy CMSX-4, to study the effect of temperature and strain rate on its flow stress. On the basis of received flow stress values activation energy of a high-temperature deformation process was estimated. Mathematical dependences (σ_pl -T i σ_pl - ἐ) and compression data were used to determine material constants. These constants allow to derive a formula that describes the relationship between strain rate, deformation temperature and true stress.

Abstract: Ti-(Al₃Ti+Al) and Ti-Al₃Ti laminated composites have been fabricated in vacuum using 50, 100, 150, 200, 400 and 600 μm thick titanium and 50 μm thick aluminum foils. The composites were synthesized with controlled temperature and treating time. Microstructural examinations showed that Al₃Ti was the only phase formed during the reaction between Ti and Al. The initial foil thicknesses only affected the volume fraction of the resultant Ti, Al and Al₃Ti layers. Treating time at 650 °C was a main factor determining microstructures and properties of the composites. After 20 minutes not all aluminum was consumed and therefore the composites consisted of alternating layers of Ti, Al and Al₃Ti. After 60 minutes aluminum layers were completely consumed resulting in microstructures with Ti residual layers alternating with the Al₃Ti layers. Mechanical tests were performed on the materials with different microstructures to establish their properties and fracture behavior. The results of investigations indicated that mechanical properties of the composites strongly depended on the thickness of individual Ti layers and the presence of residual Al layers at the intermetallic centerlines.

Abstract: Specimens of Al-and Ti-alloys and turbine blades of Ni-based alloy were considered in the case of crack origination at the surface, subsurface, and at the inner surface of turbine blades. Aircraft structures such as hydrocylinders of Al-based alloy and landing gear of high-strength-steel were also considered. There was used acoustic emission (AE) monitoring to fix moment of crack occurring and, then, analyzing its growth. Fracture surface features appearance was compared with AE-signals regularities based on fractographic analyses. To indicate the crack origination moment it was introduced α-criterion. This criterion characterized drastically increase in summarized AE-signals appearing in dependence on cyclic loads number. During crack propagation, there was used Elber-portion of opened crack to register AE-signals in cyclic loads. It was demonstrated crack increment in unloading portion of cyclic loads with fatigue striations formation. Two aircraft structures of hydrocylinders and shock strut of landing gear were tested on the special test facilities and AE-monitoring was conducted. Effectiveness of α-criterion using for crack registration in the high-cycle fatigue regime was demonstrated applicably to crack appearance on the internal surface and subsurface. The discovered dependences of summarized AE-signals on number of cycles were compared with fracture surface patterns and briefly discussed.

Abstract: This investigation aims to examine the influence of asymmetric fatigue cycling on the nature of strain accumulation and its influence on post fatigue tensile properties of three materials namely 6063 aluminum alloy, interstitial free steel and α-brass. The results highlight that strain accumulation provides improvement in yield and tensile strength of the investigated materials, with associated reduction in their %uniform elongation. However, %total elongation is found to be strong function of post-necking elongation which is governed by substructural features of the ratcheted material. Increase in strength values has been correlated with increased cyclic damage of the materials.