Mechanical properties and microstructural changes of ultrafine-grained AA6063T6 during high-cycle fatigue

Halle

et al. 2010

MSF

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Abstract. In the present work, the near-threshold fatigue crack propagation (FCP) at different load ratios is studied for an aluminium alloy processed by equal-channel angular pressing (ECAP). The conditions under investigation represent different stages of microstructural refinement as well as a ductility-optimized condition with superior crack growth properties, obtained by a combination of ECAP and aging. The results show a strong dependency of the threshold and its load ratio sensitivity on the grain size and grain size distribution. These observations can be rationalized on the basis of crack path tortuosity and the contribution of (roughness-induced) crack closure. Moreover, the experimental data is evaluated using the two-parametric concept of Vasudevan and Sadananda, which employs two necessary minimum conditions for crack growth, namely a critical cyclic ∆K * th , and a critical maximum stress intensity K * max . The application of this concept shows a strong interaction of both parameters for all ECAP-processed conditions, where the ductility-optimized condition reveals superior FCP properties compared to the "as-processed" conditions.

Section: Resultsmentioning

confidence: 99%

Near-Threshold Fatigue Crack Propagation in an ECAP-Processed Ultrafine-Grained Aluminium Alloy

Halle

et al. 2010

MSF

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“…The high strength of the CM materials is hypothesized to be the reason behind the delayed crack initiation and would most likely lead to longer HCF lifetimes consistent with other UFG materials. [31,37] 2. The CM materials exhibited cyclic softening on the first cycle before hardening to saturation.…”

Section: Discussionmentioning

confidence: 99%

“…The microstructure of these materials before testing was briefly described earlier and has been presented in detail elsewhere. [31,32] Before cycling, solid solution AA5083-H131 alloy had a grain size of 200 lm and contained limited subgrain structure. Strain at a grain boundary can be seen ( Fig.…”

Section: F Microstructural Observationsmentioning

confidence: 99%

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

Walley

Lavernia

Gibeling

2009

Metall Mater Trans A

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 lm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation.

“…In this state, the microstructure can be considered as homogeneously refined, consisting fully of ultrafine grains in the range of 1µm and smaller. As described in [10], LABs which are introduced during the first passes, are successively converted into HABs during further ECA-extrusions. For the optimized condition, ECAE-processing from the solid-solutionized state, followed by a subsequent short aging, leads to a large amount of LABs.…”

Section: Nanomaterials By Severe Plastic Deformation IVmentioning

confidence: 99%

“…For the material under investigation, detailed studies on the microstructural development with successive ECAE-processing have been conducted previously [10]. Furthermore, in a study regarding combination of ECAE and a subsequent heat treatment, the microstructural features of the …”

Section: Microstructurementioning

confidence: 99%

Microstructure and Mechanical Properties Affecting Crack Growth Behaviour in AA6060 Produced by Equal-Channel Angular Extrusion

Meyer

Sommer

Halle

et al. 2008

MSF

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Crack growth in AA6060 after two and eight equal-channel angular extrusions (ECAE), showing a bimodal microstructure and a homogenous ultrafine-grained microstructure, respectively, are compared to the coarse grained counterpart. Furthermore, an optimized condition, obtained by combining one ECA-extrusion and a subsequent short aging treatment is included. Fatigue crack growth behaviour in the near-threshold regime and the region of stable crack growth is investigated and related to microstructural features such as grain size, grain size distribution, grain boundary characteristics and ductility. Micrographs of crack propagation surfaces reveal information on crack propagation features such as crack path deflection and give an insight to the underlying microstructure. Instrumented Charpy impact tests are performed to investigate crack initiation and propagation under impact conditions. Due to the recovery of ductility during the post-ECAE heat treatment, the optimized condition shows improved fatigue crack properties and higher energy consumption in Charpy impact tests, when compared to the as-processed conditions without heat treatment.