Influence of grain size and grain boundary misorientation on the fatigue crack initiation mechanisms of textured AZ31 Mg alloy

ABSTRACTZinalco alloy (Zn-21mass%Al-2mass%Cu) specimens were deformed superplastically with a strain rate (ε) of 1×10-3 s-1 at homologous temperature (TH) of 0.68 (5 ). It was observed neck formation that indicate nonhomegeneus deformation. Grain size and grain boundaries misorientation changes, due superplastic deformation, were characterized by Orientation Imagining Microscopy (OIM) technique. It was studied three regions in deformed specimens and the results were compared with the results for a specimen without deformation. Average grain size of 1 mm was observed in non-deformed specimen and a fraction of 82% for grain boundary misorientation angles with a grain boundaries angles between 15° and 55° was found. For deformed specimen, the fraction of angles between 15° and 55° was decreced to average value of 75% and fractions of low angle (<5°) and high angle (>55°) misorientations were 10% and 15% respectively. The grain size and high fraction of grain boundary misorientation angles between 15° and 55° observed in the alloy without deformation, are favorable for grain rotation and grain boundary sliding (GBS) procces. The changes observed in the fraction of favorable grain boundary angles during superplastic deformation, shown that the superplastic capacity of Zinalco was reduced with the deformation.

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Effect of Heat Treatment on Microstructure and Thermal Fatigue Properties of Al-Si-Cu-Mg Alloys

High Temperature Materials and Processes ◽

10.1515/htmp-2016-0199 ◽

2018 ◽

Vol 37 (4) ◽

pp. 289-298

Author(s):

Wei Chao ◽

Liu Guang-lei ◽

Wan Hao ◽

Li Yu-shan ◽

Si Nai-chao

Keyword(s):

Heat Treatment ◽

Fatigue Crack ◽

Crack Initiation ◽

Grain Boundary ◽

Crack Propagation ◽

Thermal Fatigue ◽

Fatigue Crack Initiation ◽

Fatigue Properties ◽

Eutectic Si ◽

Al Matrix

AbstractThe effect of heat treatment on the microstructure and thermal fatigue properties were studied by means of optical microscope (OM) and scanning electron microscope (SEM). Energy dispersive X-ray detector (EDX) was used to analyze the role of phase composition in fatigue crack propagation. The results show that after heat treatment, the ultimate tensile strength increased from 285 MPa to 368 MPa and the elongation increased from 5.8 % to 6.5 %. During the initiation of fatigue crack, the crack was mainly propagated through eutectic Si area. With the long needles of eutectic Si particles spherodized after heat treatment, the split action from brittle Si particles to α-Al matrix was reduced and prolonged the fatigue crack initiation period. After aging for 6 h, the dispersed precipitation of secondary phases (Al2Cu, Mg2Si) elevated the driving force of crack propagation, blocked the spread of crack in the grain boundary, decreased the rate of fatigue crack growth and improved the fatigue resistance of alloy at the same time. In the process of crack initiation, the surplus-phase around the grain boundary fell off from α-Al matrix under thermal cycling stresses. The combination of interfaces was weaken by cycling stress and the fatigue crack was finally grown up in the weakness area between matrix and secondary phase.

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Interactions between geometrical defects and microstructure during high cycle fatigue of polycrystalline aluminium with different grain sizes

MATEC Web of Conferences ◽

10.1051/matecconf/201816514004 ◽

2018 ◽

Vol 165 ◽

pp. 14004

Author(s):

Benoît Bracquart ◽

Charles Mareau ◽

Nicolas Saintier ◽

Franck Morel

Keyword(s):

Grain Size ◽

Fatigue Crack ◽

Crack Initiation ◽

High Cycle Fatigue ◽

Fatigue Behavior ◽

Fatigue Crack Initiation ◽

Cycle Fatigue ◽

Grain Sizes ◽

Average Grain Size ◽

Geometrical Defects

In this work, the influence of the geometrical defect size on the high cycle fatigue behavior of polycrystalline aluminium with different grain sizes is investigated, to better understand the role of internal length scales. Two sizes of grains and defect are used: 100 μm and 1000 μm, the grain size being controlled with thermomechanical treatments. Fully reversed stress-controlled fatigue tests are then carried out. According to fatigue test results, surface crack initiation is delayed when the grain size is reduced, while an approximation of the fatigue limit shows that it is not much influenced by the average grain size. The relative defect diameter (compared to the grain size) seems to be the leading parameter influencing fatigue crack initiation from a defect. Finally, Electron BackScattered Diffraction (EBSD) maps are collected for specimens with large grains and small defects. Fatigue crack initiation from a defect is found to be strongly impacted by the crystallographic orientation of the surrounding grain, crack initiation preferably occurring in crystals being favorably oriented for plastic slip.

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