Stability of ultrafine-grained structure of copper under fatigue loading (original) (raw)
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Fatigue strength, microstructural stability and strain localization in ultrafine-grained copper
Materials Science and Engineering: A, 2006
Fatigue lifetime under stress control of ultrafine-grained Cu of 99.9% purity prepared by equal channel angular pressing is shown to exceed that of conventionally grained cold worked counterparts by a factor of 1.7 in the low-, high-and very-high-cycle region. The electron back scattering diffraction technique did not reveal changes of bulk microstructure due to cyclic loading. Minor changes of dislocation microstructure were detected by transmission electron microscopy. Qualitative change from moderate cyclic hardening to cyclic softening was observed with increasing stress amplitude.
Stability of Microstructure of Ultrafine-Grained Copper Under Fatigue and Thermal Exposition
Strain, 2010
Effect of cyclic loading and thermal exposition on microstructure of bulk ultrafinegrained Cu prepared by equal channel angular pressing was experimentally investigated by means of electron backscattering diffraction and by transmission electron microscopy. Stability of the microstructure under stress-controlled cyclic loading with a tensile mean stress of 200 MPa was shown to be high. Neither dynamic grain coarsening nor development of bimodal microstructure was observed. However, annealing at 250°C for 30 min resulted in formation of bimodal microstructure. Consequently, fatigue strength of annealed specimens was low.
Change in surface states of ultrafine grained copper due to cyclic stresses
Surface Effects and Contact Mechanics X, 2011
High-cycle fatigue tests were carried out on smooth specimens of ultrafine grained copper produced by equal channel angular pressing for 8 passes. The change in surface states during stressing was monitored, showing that shear bands (SBs) were formed at an early stage of stressing. These SBs were oriented along one set of maximum shear planes which correspond to the shear plane of the final pressing. After the formation of SBs, the surface damage was slowly extended with further stressing, where a significant extension of the damaged areas occurred in the latter half of its fatigue life. Correspondingly, the surface hardness exhibited an initially moderate drop, which was closely followed by a more severe reduction in the latter half of the fatigue life. This initial drop in hardness was found to strongly depend on the decreased dislocation density caused by the dynamic recovery due to stressing, while the more severe reduction in hardness may be attributed to the grain coarsening: coarsened grains in the post-fatigued samples were observed by the transmission electron microscopy. In addition, the growth behavior of a major crack, which led to the final fracture of the specimens, was monitored by the plastic replication technique. Fatigue cracks were initiated from SBs. The growth behavior of the major cracks was estimated quantitatively from a small-crack growth law.
Fatigue damage formation and microstructure of ultrafine grained copper under two-step loading
Computational Methods and Experimental Measurements XVI, 2013
In order to study the effect of stress change on the surface damage formation of ultrafine grained copper, two step fatigue stress tests were conducted. The microstructural evolution and crack growth direction of ultrafine-grained copper fatigued under constant stressing depends on the magnitude of the applied stress amplitude. The growth direction occurred perpendicular to the loading axis at low stresses, and changed to 45° to the loading axis at high stresses. In the case of high-to-low block stressing, the 45° inclined growth direction under high stress changed to the perpendicular direction under subsequent low stress. In low-to-high block stressing, although the growth direction perpendicular to the loading axis at low stresses was still retained at subsequent high stress, the degree of zigzag behaviour in the crack growth drastically increased after the stress change. The sizes and topographies of evolved microstructures, which have a great effect on the crack path formation, depended on stress histories (order, cycle number and magnitude of the applied stress). The formation behaviour of the fatigue damage under high-and low-stress amplitudes and the effect of pre-stressing on crack growth direction are discussed from the viewpoints of the microstructural evolution under different stress histories.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2009
Dynamic deformation and fracture behavior of ultra-fine-grained pure copper fabricated by equal channel angular pressing (ECAP) was investigated. The 1-pass ECAP'ed specimen consisted of fine dislocation cell structures, which were changed to very fine, equiaxed subgrains of 300-400 nm in size with increasing number of ECAP pass. The dynamic torsional test results indicated that maximum shear stress increased with increasing number of pass, but that the rate of the strength increase was higher in the 4-pass or 8pass specimen than in the 1-pass specimen. This was because boundaries of subgrains formed during the 4-pass or 8-pass ECAP were more stabilized and high angled. Most of the ultra-fine-grained pure copper specimens were not fractured because adiabatic shear bands were hardly formed as the pure copper specimens were ductile and had high thermal conductivity, although a weak adiabatic shear band was observed in the 8-pass specimen. These findings suggested that the grain refinement by the ECAP was effective in strengthening of pure copper, and that the ECAP'ed pure copper could be used without much loss in fracture resistance under dynamic loading.
Microstructural Features of Ultrafine Grained Copper under Severe Deformation
In this work the microstructural features of pure copper were studied using two methods of severe plastic deformation: equal-channel angular pressing (ECAP) and hard cyclic viscoplastic (HCV) deformation. During the first step the metal was severely deformed up to 10 B c routes of ECAP. The ultrafine grained microstructure was received. The elongated laminar substructure has low angle and diffuse grain boundaries, but high dislocation density. Metal shows high hardness and strength but low ductility at tension straining. During the second step – HCV deformation – the strain amplitude of tension-compression cycles was stepwise increased from 0.2 % up to 2.5 % for 30 cycles and for five series. The results show, that under HCV deformation the ultrafine grained microstructure with high-angle grain boundaries was formed. The mechanism of microstructure evolution contains the elongated (ECAP processed) subgrains fracture under shear stresses by atomic layers of crystals and new microstru...
Development of fatigue damage in ultrafine-grained copper
Ecf19, 2013
Equal channel angular pressing is one of severe plastic deformation methods often used for preparation of ultrafine-grained (UFG) materials in bulk. Mechanical and, particularly, fatigue properties of UFG structures has been a subject of recent intensive research. The fatigue lifetime of UFG materials is closely related to the fatigue crack initiation. For conventionally grained (CG) materials the crack initiation mechanism is well described and understood. However, this is not the case of UFG structures. Furthermore, the knowledge obtained on CG materials cannot be straightforwardly transferred to UFG structures just from the reason of the grain size, which is smaller than the characteristic dimension of dislocation structures, which develops due to fatigue in CG materials. Copper is one of the most thoroughly studied model materials as regards the investigation of fatigue crack initiation mechanisms. The cyclic slip bands, which develop on the surface of cycled Cu and which are sites of crack initiation exhibit very similar features in CG and UFG material. However, the mechanism of the cyclic slip localization known from the CG Cu cannot work in UFG structure. The characteristic dislocation structures formed by fatigue and known from CG Cu cannot simply develop in UFG material unless the grain coarsening takes place. This contribution brings results of an experimental investigation of fatigue crack initiation in UFG copper. Development of fatigue slip bands was studied by means of focused ion beam (FIB) technique in scanning electron microscope. The observation of cyclic slip bands and material microstructure just beneath them shows that the grain coarsening is not a necessary prerequisite for crack initiation and development of surface relief. Observation of dislocation structure on thin foils prepared by FIB reveals well-developed cell structure below slip bands. The technique of ionchanneling contrast indicates that slip bands develop in regions of grains, which exhibit only very small mutual disorientation (low angle boundary regions). Based on these observations the mechanism of fatigue crack initiation in UFG Cu is discussed and compared with that known from CG copper.
Mechanical properties of copper processed by Equal Channel Angular Pressing - a.PDF
The Equal Channel Angular Pressing is a hardening treatment with which ductile metals can be processed to refine their grain and sub-grain structure. This process enhances the mechanical strength of metals in terms of tensile strength, stress-controlled fatigue strength, and fatigue crack growth resistance. In this paper the authors draw a review of the major results of a wide research activity they carried out on a copper microstructure processed by Equal Channel Angular Pressing. The essential results are that tensile and fatigue strengths of the so obtained refined structure are improved by a factor of two with respect to the original coarse-grained metal. The fatigue crack initiation mechanism and the stability of the refined microstructure under cyclic loading are topics also discussed, evidencing the essential role of the process and of the material parameter, as the content of impurities in the microstructure. In this review, the authors also underline some critical aspects that have to be more investigated.
Mechanical properties of copper processed by Equal Channel Angular Pressing - a review
Frattura ed Integrita Strutturale, 2012
The Equal Channel Angular Pressing is a hardening treatment with which ductile metals can be processed to refine their grain and sub-grain structure. This process enhances the mechanical strength of metals in terms of tensile strength, stress-controlled fatigue strength, and fatigue crack growth resistance. In this paper the authors draw a review of the major results of a wide research activity they carried out on a copper microstructure processed by Equal Channel Angular Pressing. The essential results are that tensile and fatigue strengths of the so obtained refined structure are improved by a factor of two with respect to the original coarse-grained metal. The fatigue crack initiation mechanism and the stability of the refined microstructure under cyclic loading are topics also discussed, evidencing the essential role of the process and of the material parameter, as the content of impurities in the microstructure. In this review, the authors also underline some critical aspects that have to be more investigated.