Effect of hydrogen absorption on the low-cycle fatigue strength of 3V and 7M titanium alloys

The paper presents results of an investigation of the low-cyclic fatigue strength of titanium alloys 3V (3.5 – 5% Al. 1.2–2.5%V) and 7M (1.8–2.6% Al, 2–3% Zr, 0.1% C).Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 33 – 34, April, 1996.     

High-temperature low-cycle fatigue behavior of superalloy MA 760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen orientations. The cyclic hardening curves at different strain am-plitudes were determined at 650, 900, and 950 °C, and the stress-strain response of the material was sub-stantiated by detailed fractographical scanning electron microscopy (SEM) and microstructuralJoptical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or soften-ing, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The “brittle” nature of deformation behavior of MA 760, especially at lower test tempera-tures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical studies of the fracture surfaces. At 650 °C, evidence of dislocation cutting of γ was ob-served in transmission micrographs, but at higher temperatures, dislocation climb and by-passing of γ particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particles, was frequently observed.     

A mechanism governing oxidation-assisted low-cycle fatigue of superalloys

A model capable of characterizing oxidation-assisted low-cycle fatigue is described. It involves the following steps. After a few strain cycles, because of creep, a tensile stress develops during the de-straining phase of the cycle. This stress opens cracks present in the material and exposes the surfaces to the atmosphere, causing thermally grown oxide (TGO) growth. Dilatation takes place upon converting the alloy to oxide, with an associated strain rate that induces a compressive growth stress. Thereafter, during the re-straining phase of the cycle, transverse extension of the substrate induces in-plane tension in the TGO, which “pushes” the TGO into the substrate along the crack front. Finite element simulations of this process have been presented that predict crack growth per cycle, da/dN, comparable with experimental measurements. Trends in da/dN with the TGO dilatation rate and the creep strength of the superalloy have been elucidated.     

Uniaxial ratcheting and low-cycle fatigue failure behaviors of AZ91D magnesium alloy under cyclic tension deformation

Uniaxial ratcheting and low-cycle fatigue failure behaviors of hot-rolled AZ91D magnesium alloy were studied by uniaxial cyclic stress-controlling tension deformation experiments. The effects of stress amplitude, mean stress and stress rate on the uniaxial ratcheting response and fatigue life of the hot-rolled AZ91D magnesium alloy were analyzed. Results show that (1) the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy both increase with the increase of stress amplitude or mean stress; (2) increasing stress rate will decrease the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy; (3) the increase of stress amplitude and mean stress can both reduce the fatigue life of the hot-rolled AZ91D magnesium alloy, while the fatigue life will be prolonged with the increase of stress rate.     

高载荷条件下TC17钛合金低周疲劳和保载疲劳损伤行为研究

研究了高应力水平下不同加载波形对TC17钛合金疲劳损伤的影响,结果表明,对于在峰值应力处保载120S的加载波形,TC17钛合金的保载疲劳寿命低于无保载加载波形试验的寿命,在相同的保载疲劳条件下,片层组织的保载疲劳敏感性较等轴组织的疲劳敏感性低。断口分析表明,保载疲劳试样裂纹源出现在样品表面和亚表面处,常规疲劳试样裂纹源出现在试样表面处,保载疲劳断口较常规疲劳断口附近小平面平坦,保载条件并未从根本上改变断裂模式。     

A microstructural and low-cycle fatigue investigation of weld-repaired heat-resistant cast steels

The multi-pass weld-repair of heat-resistant cast steels is carried out using an automated shielded metal arc welding (SMAW) process, with various filler materials and pre-heating at 400 °C. Specimens weld-repaired with a filler material more resistant than the heat-resistant cast steel (over-matching) generally crack within the base metal following the tenth filling pass, whereas specimens buttered with a soft alloy prior to welding remain free of cracks.The high temperature strain-controlled fatigue lifetime of material weld-repaired without buttering is lower than that of bulk initial material. This is due to an increase of the stress amplitude as a result of the so-called over-matching. In the case of material welded following a prior buttering, the fatigue lifetime is reduced because of the stress tri-axiality generated in the thin soft layer which prevents its plastic flow. As a consequence, it is concluded that even though buttering prevents cracking efficiently during welding, it is not acceptable as far as fatigue performance, especially lifetime, is concerned.     

Investigation of the low-cycle fatigue mechanism for micron-scale monocrystalline silicon films

This study investigated the cyclic and static fatigue properties of 10 μm thick, deep reactive ion etched, monocrystalline silicon films. Stress–life fatigue curves and fatigue degradation rates vs. stress curves were generated at both 4 and 40 kHz, at 30 °C, 50% relative humidity (RH). A significant frequency effect was observed, with shorter fatigue lives and faster damage accumulation rates at 4 kHz. Static fatigue was also observed with shorter static lifetimes at 80 °C, 90% RH than at 30 °C, 50% RH. Fracture surface evaluation did not reveal any major difference between cyclically and statically fatigued devices. These experimental results confirm that the fatigue of micron-scale silicon is not purely mechanical. The study also proposes a fatigue scenario based on time-dependent subcritical crack growth to account for the low-cycle fatigue regime.     

Fretting fatigue studies of titanium nitride-coated biomedical titanium alloys

Fretting fatigue is an adhesive wear mechanism caused by repetitive tangential micro-oscillation between two contacting materials pressed together under cyclic load. Bioimplants, such as hip joints and bone plates, are prone to undergo fretting fatigue failures during their service within the body. This article presents the fretting fatigue damage characterization of physical vapor deposition (PVD) TiN-coated biomedical titanium alloys (Ti-6Al-4V and Ti-6Al-7Nb) subjected to cyclic loads. The PVD TiN layer delayed the damage because of superior tribological properties compared with uncoated alloys. Delamination and abrasive wear damage of TiN at contact caused failure of the alloy. Friction coefficient curves of the PVD TiN-coated pair showed an irregular pattern caused by the influence of wear particulates and Ringer fluid at the contact.     

Effect of low-cycle fatigue on the hot ductility of plain carbon steel

In a continuous casting steelmaking operation, the surface of a slab is under a condition that can be characterized as high-temperature, low-cycle fatigue in which the tensile and compressive stress is repeatedly developed. For this reason, for the evaluation of the hot ductility of a slab, considering the fatigue deformation is more feasible before a tensile or compressive test. In this study, the effects of low-cycle fatigue on the hot ductility of steels with a carbon content of 0.06–0.8 wt.% are investigated at various temperatures. For a carbon content of 0.06%, there were no significant differences between the RA values from a simple tensile test and those from a tensile test after fatigue deformation. The tendency of ductility deterioration with fatigue deformation is evident in 0.1 %C steel, and is due to the deformation-induced ferrite film that forms around the prior austenite grain. Conversely, high carbon steel containing 0.8 %C did not show a recovery of hot ductility in a low temperature region, and the specimen on which the tensile was measured after fatigue showed a higher hot ductility in the low temperature region, which is thought to result from the pearlite refinement effects. As the results obtained in this work showed noticeable differences in the hot ductility of carbon steel through the test conditions, it is suggested that for more accurate data, fatigue deformation be adopted in which the temperature range in an unbending operation is determined in the steelmaking factory.     

Effect of heat treatment on the low-cycle fatigue strength of aluminum alloy 01420

Conclusions Isothermal hardening improves the ductility and increases the static fatigue strength of alloy 01420 by 9–12%.Tashkent Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 59–61, January, 1978.     

The effect of Zr on the low-cycle fatigue behavior of NiAl at 1000 K

The effect of a 0.1 at. % alloying addition of Zr on the low-cycle fatigue behavior of polycrystalline NiAl was determined at 1000 K and compared to that of binary NiAl. Samples of binary NiAl and the Zr-doped alloy were processed by either HIP consolidation or extrusion of prealloyed intermetallic powders. The cyclic stress response, cyclic stress-strain behavior, and strain-life relationships were all significantly influenced by the microalloying addition of Zr, regardless of the processing technique. A detailed examination of the post-tested low-cycle fatigue (LCF) samples was conducted by optical and electron microscopy to determine variations in fracture and deformation modes and to characterize any microstructural changes that occurred during LCF testing. Differences in LCF behavior due to the Zr addition are attributed to the strong effect that Zr has on modifying the deformation behavior of the intermetallic.     

盘型悬式绝缘子钢脚材料热处理工艺优化及其低周疲劳性能

输变电线路盘型悬式绝缘子钢脚的疲劳断裂会导致绝缘子掉串事故的发生,不合理的热处理工艺是钢脚失效的主要原因.针对几种常用盘形悬式绝缘子钢脚金属材料,对其开展淬火+回火及正火热处理试验,采用光学显微镜、扫描电镜、电子万能材料试验机和低周疲劳试验等研究了其显微组织、力学性能和低周疲劳性能,以便获得其优化的热处理工艺.结果 表...