Static and Cyclic Crack Growth Behavior of Ultrafine-Grained Al Produced by Different Severe Plastic Deformation Methods

Halle

et al. 2010

MSF

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: Introductionmentioning

confidence: 99%

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

Halle

et al. 2010

MSF

“…Figure 1 shows the position in the ECAP billet out of which the SEB specimens were extracted. In the choice of the extraction direction, it was considered that the crack is most likely sidetracked in the direction of the last ECAP shear plane [12]. The crack length was measured continuously according to the principle of the indirect electric potential method with crack measurement foils.…”

Section: Methods Of Mechanical Testing and Electron Microscopymentioning

confidence: 99%

“…The matrix material shows the highest thresholds and also a strong load ratio dependency. This can be explained by the well-known effect of the grain size on the crack growth at different load ratios [11][12][13]15,35]. The amount of crack deflection is constant for all load ratios, whereas the effect of roughness induced crack closure is minimized by increasing the load ratio.…”

Section: Fatigue Crack Propagationmentioning

confidence: 97%

“…These investigations show the well-known effect of the grain or particle size on the fatigue crack growth. The grain refinement through ECAP leads to a minimized amount of crack deflection and roughness-induced crack closure, which results in higher crack propagation rates and, therefore, in minor thresholds [11][12][13][14][15][16][17][18]. In reinforced materials, particle sizes larger than 2-5 µm have a contrary effect on crack propagation: by increasing crack deflection and roughness-induced crack closure, crack propagation rates are lowered [19][20][21][22][23][24].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Influence of Particulate Reinforcement and Equal-Channel Angular Pressing on Fatigue Crack Growth of an Aluminum Alloy

Köhler

Hockauf²,

Lampke³

et al. 2015

Metals

Abstract:The fatigue crack growth behavior of unreinforced and particulate reinforced Al 2017 alloy, manufactured by powder metallurgy and additional equal-channel angular pressing (ECAP), is investigated. The reinforcement was done with 5 vol % Al2O3 particles with a size fraction of 0.2-2 µm. Our study presents the characterization of these materials by electron microscopy, tensile testing, and fatigue crack growth measurements. Whereas particulate reinforcement leads to a drastic decrease of the grain size, the influence of ECAP processing on the grain size is minor. Both reinforced conditions, with and without additional ECAP processing, exhibit reduced fatigue crack growth thresholds as compared to the matrix material. These results can be ascribed to the well-known effect of the grain size on the crack growth, since crack deflection and closure are directly affected. Despite their small grain size, the thresholds of both reinforced conditions depend strongly on the load ratio: tests at high load ratios reduce the fatigue threshold significantly. It is suggested that the strength of the particle-matrix-interface becomes the critical factor here and that the particle fracture at the interfaces dominates the failure behavior.

Bulk Nanostructured Materials With Multifunctional Properties

“…The standard crack growth rate da/dN versus stress intensity factor range ΔK plots for the nanostructured materials showed the same stages of crack propagation as in the coarse-grained materials. However, the fatigue 'threshold' ΔK th corresponding to [81] with the permission from the publisher the low-end limit of the curves decreased after SPD processing in pure Al [109], AA5056 [110], and Al-1.5MgScZr [108].…”

Section: Low-cycle Fatigue Behavior Of Nanostructured Metallic Materialsmentioning

confidence: 99%

Multifunctional Properties of Bulk Nanostructured Metallic Materials

Sabirov

Enikeev

Murashkin

et al. 2015

This chapter focuses on multifunctional properties of bulk nanostructured metallic materials and structure-properties relationship therein. The most important structural factors affecting mechanical, physical, and chemical properties of the nanomaterials are discussed, and the strategies to their further improvement are outlined. Special attention is paid to nanostructural design for simultaneous improvement of mutually exclusive properties. Superstrength and Enhanced Mechanical PropertiesAlthough the mechanical and functional properties of all polycrystalline metallic materials are determined by several factors, the grain size of the material generally plays the most significant and often a dominant role. Thus, the strength of different polycrystalline materials is related to the grain size, d, through the Hall-Petch equation which states that the yield stress, σ y , is given bywhere σ o is termed the friction stress and k y the Hall-Petch constant [1,2]. It follows from Eq. 3.1 that the strength increases with a reduction in the grain size and this has led to an ever-increasing interest in fabricating materials with extremely small grain sizes. The fabrication of bulk samples and billets using equal channel angular pressing (ECAP), high pressure torsion (HPT), and other severe plastic deformation (SPD) techniques was a crucial first step in initiating investigations into the properties of bulk nanomaterials because the use of SPD processing permitted, and subsequently fully supported, a series of systematic studies using various nanostructured metallic materials including commercial alloys [3][4][5][6][7][8]