Effect of Low Temperature on Fatigue Life and Cyclic Stress-Strain Response of Ultrafine-Grained Copper
- PDF / 452,007 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 5 Downloads / 190 Views
INTRODUCTION
BULK nanostructured materials prepared by severe plastic deformation (SPD) have shown promise in application areas. The potential of the SPD materials is given basically by their high tensile strength and hardness with fairly large ductility. This especially concerns materials prepared by the method of equalchannel angular pressing (ECAP) producing materials with an ultrafine grain size in the range of 100 nm to less than 1 lm. Their fatigue properties have been studied on a number of pure metals and alloys both in the lowcycle fatigue (LCF) and high-cycle fatigue (HCF) regimes. Besides several tens of primary articles, perhaps even a few hundred of them, a few overview articles have also been written (e.g., References 1 through 3).[1,2,3] The main results can be summarized as follows. (1) Fatigue life of ECAP materials determined under conditions of stress-controlled cycling (this concerns mainly the HCF region) is superior to that using their conventional counterparts. (2) On the other hand, the strain-controlled cycling (this concerns exclusively the LCF region) of UFG copper, which is the most deeply studied material, leads to shorter life (expressed on the basis of the Coffin–Manson plot) in comparison to the conventional-grain (CG) counterpart. The critical issue of the fatigue resistance of these materials is the thermal and mechanical stability of the highly deformed structure. Fatigue life shortening under strain-controlled tests in the LCF region is closely related to grain coarsening, cyclic softening, and strain P. LUKA´Sˇ, Director, L. KUNZ and M. SVOBODA, Heads of Department, are with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zˇizˇkova 22, 616 62, Brno, Czech Republic. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Ultrafine-Grained Materials: from Basics to Application,’’ which occurred September 25–27, 2006 in Kloster Irsee, Germany. Article published online May 1, 2007. 1910—VOLUME 38A, SEPTEMBER 2007
localization, in other words, to instabilities of the structure produced by ECAP. The degree of the stability of the structure can also explain the discrepancies of the results obtained from stress-controlled tests in the HCF region in different laboratories on the same or similar materials. As an example, the results of stress-controlled cycling of UFG copper in the HCF region can be mentioned. The fatigue strength of UFG copper of purity 99.9 pct was shown[4,5] to be higher by a factor of about 2 in the entire HCF region (105 to 107 cycles to failure), while fatigue strength of UFG copper of purity 99.99 pct is higher only by a factor of about 1.3 for 105 cycles and by a factor of about 1.1 for 107 cycles.[6] This can be explained by the fact that the lower purity copper exhibits a more stable grain structure (very limited or no grain coarsening) than the higher purity copper. It is also assumed in the literature that not only the purity but also the ECAP route applied may affect the fatigue life di
Data Loading...
