Y.Q. Wu
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X. Y. Yang and Y.B. Xu (State Key Laboratory for Fatigue and Fracture of Materials
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Institute of Metal Research
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The Chinese Academy of Sciences
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Shenyang 110015
,
China)(Laboratory of Atomic Imaging of Solids
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Institute of Metal Research
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The Chinese Academy of Sciences
,
Shenyang 110015
,
China)
金属学报(英文版)
A cross-sectional sample of indentation in [001] silicon was made and a lateral microcrack beneath the indentation was observed by high resolution electron microscope (HREM). The HREM images around the microcrack tip show an alternate distribution of the amorphous and lattice structure. The crack does not propagate by the successive debonding between certain atomic planes, meaning that before a crack completely opens, there may be a stage of amorphization, resulting from severe distortion of lattice.
关键词:
amorphization
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Journal of Alloys and Compounds
In the gigacycle fatigue regime (106 to 109 Cycles), the micro-mechanism of fatigue crack propagation for a forged Mg-Zn-Y-Zr alloy subjected to temper treatment (T5) has been studied. High-magnified images of the different crack propagation regions indicate that when the effective stress intensity factor K(eff) is below 3.2 MPa m(1/2), in Region 1, the fracture surface is smooth covered with many flat facets. However, when the stress intensity factor is higher than 3.2 MPa m(1/2), in Region 2, many larnellar semi-cleavage planes caused by rod-like beta(1)', precipitates can be observed, resulting in a rather rough fracture surface. However, in Region 3, no lamellar semi-cleavage planes but lots of zonal distributed Mg(3)Zn(3)Y(2) phases can be observed. It suggests that with the increase of the crack-tip driving force, the preferential path for fatigue crack propagation changes from the rod-like beta(1)' precipitates to the zonal distributed Mg(3)Zn(3)Y(2) phases. In addition, two kinds of fatigue striations, i.e. striations formed on the flat fracture surface and striations formed on the cleavage steps, have been observed. Based on double-slip and plastic obtuseness at the crack tip, the forming mechanism of the fatigue striations on the cleavage steps has been discussed. (C) 2006 Elsevier B.V. All rights reserved.
关键词:
M-Zn-Y-Zr magnesium alloy;gigacycle fatigue;micro-mechanism;magnesium alloy;re alloys;mechanical-properties;cycle fatigue;life;regime;behavior;steels
Ieee Transactions on Magnetics
The spin orientation and spontaneous magnetostriction of multicomponent TbxDy1-x-yPry(Fe0.9B0.1)(1.93) (0.10 less than or equal to x less than or equal to 0.25, 0.30 less than or equal to y less than or equal to 0.60) Laves phases were studied by step-scanning their (440) X-ray diffraction lines. The easy magnetization direction (EMD) of the Laves phases changed from the (100) to (111) axis when x was increased from 0.10 to 0.25. With increasing y, the EMD of Tb0.15Dy0.85-yPry(Fe0.9B0.1)(1.93) and Tb0.2Dy0.8-yPry(Fe0.9B0.1)(1.93) Laves phases reoriented from the (100) to (111) axis. The magnetostriction coefficient lambda(111) of TbxDy1-x-yPry(Fe0.9B0.1)(1.93) Laves phases increased with increasing x (or y) when y (or x) was kept constant.
关键词:
Laves phases;magnetostriction;spin orientation;X-ray diffraction;anisotropy compensation;alloys
Acta Materialia
The fatigue behavior of as-cast Mg-12%Zn-1.2%Y-0.4%Zr alloy has been investigated. The S-N curve showed that the fatigue strength at 107 cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 X 10(7) cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 x 10(5) cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 x 10(5) cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m(1/2), respectively. When the maximum stress intensity factor K(max) was lower than 4.2 MPa m(1/2), in the steady crack propagation region, due to the retarding effect of I-phase/alpha-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/alpha-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m(1/2), in the sudden failure region, the rigid bonding of I-phase/alpha-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/alpha-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
关键词:
mg alloy;I-phase;fatigue crack initiation;casting porosity;fatigue;behavior;magnesium alloy;re alloys;deformation;mechanism;growth
ZHANG Hongxue XU Zhichao CHEN Guoliang University of Science and Technology Beijing
,
Beijing
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China Professor
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Faculty of Superalloy
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University of Science and Technology Beijing
,
Beijing
,
China
金属学报(英文版)
The relative ratio of fatigue resistance to creep resistance of materials varies with test temper- ature.As the temperature decreases,the creep resistance,since it is a thermal activation pro- cess,becomes relatively larger than fatigue resistance.Therefore the fatigue damage becomes predominant,and results in expansion of fatigue fracture region(region F),and shrinkage even complete elimination of creep fracture region(region C).A materials parameter Ω can be defined to estimate the temperature at which the creep fracture region is completely de- pressed.This phenomenon could be understood on the basis of the integrated model of compet- itive and cumulative models of fatigue creep interaction.
关键词:
creep fracture
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WANG Xu
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ZHANG Shouhua
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CUI Peiyong Beijing University of Science and Technology
,
Beijing
,
China. Central Iron and Steel Research Institute
,
Beijing
,
China.
材料科学技术（英文）
Rolling Contact Fatigue(RCF) is a cumulative damage phenomenon when metals are subjected to repeated contact stresses. The fomation of pitting on the contact surface is the result of the rolling contact fatigue. The morphologies of rolling contact fatigue fracture of the har- dened steels (86CrHoV7, 42CrMo) show that strong resemblance in fractuye mechanisms exists between rolling contact fatigue and uni-axial fatigue. Since fatigue striations are hardly observed in hardened steels under uni-axial fatigue, it is interesting to note that the state of stress in rolling contact fatigue is more favor- able to ductile fractures than in uni-axial fatigue.
关键词:
rolling contact
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