Advanced Materials Research Vols. 891-892

Abstract: Cyclic deformation characteristics of a rare-earth (RE) element containing extruded Mg-10Gd-3Y-0.5Zr (GW103K) magnesium alloy were evaluated via strain-controlled low-cycle fatigue tests under varying strain amplitudes. Microstructural observations revealed that this alloy consisted of fine equiaxed grains and a large number of RE-containing precipitates. Unlike the RE-free extruded magnesium alloys, this alloy exhibited essentially cyclic stabilization and symmetrical hysteresis loop due to relatively weak crystallographic textures and reduced twinning-detwinning activities. The fatigue life of the present alloy was observed to be longer than that of the RE-free extruded magnesium alloys, which could also be described by the Coffin-Manson law and Basquins equation. Fatigue crack was observed to initiate from the specimen surface and crack propagation was basically characterized by fatigue striations.

Abstract: Fatigue strength, crack initiation and microstructure were experimentally investigated in an as-cast AZ91 alloy and in ultrafine-grained (UFG) AZ91 alloy processed by equal channel angular pressing (ECAP). The microstructure after ECAP is bimodal, consisting of fine-grained regions and clusters of larger grains with lower density of intermetallic particles. It has been found that the ECAP substantially increases the tensile strength (factor of two), improves ductility (factor of five) and improves the fatigue strength in low-cycle fatigue region. The improvement of the endurance limit based on 10⁷ cycles is weak. The cyclic slip bands, as sites of the fatigue crack initiation on material surface, were investigated. Focussed ion beam technique (FIB) was applied to reveal the surface relief and the microstructure in the vicinity of early fatigue cracks. No grain coarsening was observed in the close vicinity of the initiated cracks. Fatigue cracks in ultrafine-grained structure develop both in the regions of larger grains and also in the fine grained areas. Two types of crack initiation were observed.

Abstract: In order to increase the strength and maintain the ductility of austenitic steels high Nitrogen austenitic steels (AHNS) emerged of which Ni was substituted by Mn so that up to 1 w% N could be alloyed and kept in solid solution. Cold working was added to gain strength values up to 3000 MPa. Still the endurance limit did not follow this trend. The low stacking fault energy was thought being the main reason for the solely planar slip but it became clear that other near-field effects might govern this behaviour as well. Thus the density of free electrons could be identified as being one for CrMn-steels being mainly influenced by the sum and the ratio of C and N. In order to investigate this strain-controlled fatigue tests are carried out. This contribution presents the results of strain-controlled fatigue tests and discusses them on the basis of SEM-EBSD and TEM investigations in relation to the microstructural characteristics.

Abstract: Orientation changes during fatigue crack initiation in ferrite and ferritepearlite steel were evaluated by electron backscatter diffraction (EBSD). Ferrite steel with different grain sizes and ferritepearlite steel with different carbon contents were prepared. EBSD measurements and fatigue tests were alternately performed using a small specimen. The tests on both ferrite and ferritepearlite steel suggest that the initial cracks were observed in the ferrite matrix. Thus, crystal rotation induced by fatigue in ferrite matrix is quantitatively evaluated by two misorientation parameters: grain reference orientation deviation, which is the misorientation between measuring points and the average orientation in each grain, and crystal misorientation at the same point before and after fatigue testing.

Abstract: In this study, low-cycle fatigue (LCF) tests at 500 °C in the <001>, <011> and <111> directions have been performed for the Ni-based single-crystal superalloy MD2. All tests were carried out in strain control with R_ε=-1. The <001> direction has the lowest stiffness of the three directions and also shows the best fatigue properties in this study followed by the <011> and <111> directions, respectively. It is well recognised that Ni-based single-crystal superalloys show a tension/compression asymmetry in yield strength and this study shows that a tension/compression asymmetry also is prevalent during LCF conditions. At mid-life, the <001> direction generally has a higher stress in tension than in compression, while the opposite is true for the <011> direction. For the <111> direction the asymmetry is found to be strain range dependent. The <011> and <111> directions show a cyclic hardening behaviour when comparing cyclic stress-strain curves with monotonic stress-strain curves. In addition, the <011> and <111> directions show a serrated yielding behaviour for a number of cycles while the yielding of the <001> direction is more stable.

Abstract: During low-cycle fatigue test with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in aximum load. This criterion cannot be directly related to the actual measure of interest - the crack length. By relating data from controlled crack growth tests under low-cycle fatigue conditions of a high strength Titanium alloy at 350°C and numerical simulation of these tests, it is shown that it is possible to determine the relationship between load drop and crack length, provided that care is taken to consider all relevant aspects of the materials stress-strain response.

Abstract: The increased attention that ultrafine grained (UFG) materials have received over the last decade has been provoked, not least, by their high strength in combination with remarkable ductility. The main focus of our investigation was the evaluation of the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steel. In our case HPT increased the hardness by a factor of 1.75 - 3.2 compared to the initial states. The achieved hardness maximum was 726 HV. The amount of increase depended on the initial carbide morphology. By stress controlled cyclic four point bending tests with a load ratio of 0.1 endurance limits were determined for the initial and HPT states. The endurance limit increased linearly with hardness until 500 HV and independently of the carbide morphology. All fracture surfaces were investigated by SEM after the fatigue tests. They revealed pretty flat fatigue fracture surfaces with crack initiation at the surface or rather at non-metallic inclusions for the UFG states. Morphology and crack initiation mechanisms were changed by severe plastic deformation compared with the coarse grained initial state. Residual fracture surfaces with a spheroidal initial microstructure showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with initial tempered microstructure showed rather brittle and rough residual fracture surfaces.

Abstract: In order to examine the period of fine granular area (FGA) formation of bearing steel in very high cycle fatigue regime, rotating bending fatigue tests were carried out at the stress amplitude 1100 MPa below the fatigue limit. The tests were interrupted at the cumulative damage values ranging from 0.1 to 0.5 with an increment of 0.1 to charge hydrogen to the specimens. After the charge, the rotating bending tests were continuously carried out. The crack origin areas on all fracture surfaces were checked by a scanning electron microscope (SEM), and it was discovered that FGA was not formed in some of them. From a view point of fracture mechanics, the stress intensity factor ranges of FGA areas, ΔK_FGA, were calculated by using Murakamis area model. The ΔK_FGA values increase with the increase of the cumulative damage values. Furthermore, ΔK_FGA values in this study were smaller than 5 MPam which was obtained from usual fatigue testing. Therefore, we conclude that the stable crack growth stage starts when the threshold stress intensity factor range decreases due to hydrogen embrittlement in the middle of formation of FGA.

Abstract: The effect of inclusions on the VHCF properties of a metastable austenitic stainless steel in undeformed and predeformed condition was studied. The material contains an inhomogeneous distribution of elongated oxide inclusions. TEM investigations of foils extracted by means of FIB technique show that the stress concentration at the inclusions is compensated by plastic deformation in the austenite phase preventing internal crack initiation in the VHCF regime for the non-predeformed, i.e., almost martensite-free condition. The effect of the spatial distribution and geometry of the inclusions on the VHCF strength was systematically investigated for the predeformed condition. Samples were monotonically predeformed at -80°C resulting in a martensite content of about 60% and then fatigued in high frequency testing machines. Since mechanical components are in practice subjected to complex cyclic loading situations, samples were tested both parallel and transversal to the rolling direction, in order to cover a broad field of applications. The higher notch sensitivity of the martensite phase leads to internal crack initiation from inclusions supported by the formation of a fine granular area (FGA). The change in testing direction perpendicular to the rolling direction reduces the number of cycles to failure due to the increased stress intensity factor at inclusions which leads to internal crack initiation without the formation of a fine granular area. These findings are discussed on the basis of a detailed microstructural characterization of the material focusing on the effect of martensite content, the inclusion morphology with respect to the rolling direction and the load axis applied

Abstract: The main focus of this investigation is to clarify the influence of variable amplitude loadings on subsurface crack initiation and crack growth. Therefore, differently reconstructed load sequences on the basis of a standardized load time history called FELIX are investigated with an R-ratio of -1. The major amount of cycles is situated beneath the fatigue strength. A new damage calculation approach considering inclusion sizes is presented. Thus, the stress amplitude in the S-N curve was normalized with a calculated fatigue limit σ_w(area), which is defined by Murakami. Afterward, the fatigue life depending on the inclusion size is calculated using a Palmgren/Miner rule. The largest inclusion in the measurement volume was determined using extreme value statistics. Fatigue lives for each investigated load sequence were calculated taking the scatter of inclusion sizes into account.