Elsevier

Acta Materialia

Volume 50, Issue 7, 19 April 2002, Pages 1639-1651
Acta Materialia

Structure and properties of ultra-fine grain Cu–Cr–Zr alloy produced by equal-channel angular pressing

https://doi.org/10.1016/S1359-6454(01)00437-2Get rights and content

Abstract

The structure, thermal stability and properties are investigated of a Cu–Cr–Zr alloy with ultra fine grains (UFG) of 160 nm diameter produced by severe plastic deformation through equal-channel angular pressing (ECAP). Special attention is paid to optimization of multi-functional thermal, electrical and mechanical properties of this alloy by aging after ECAP. Fatigue life and cyclic response under strain-controlled experiments are investigated aiming at clarification of mechanisms of plastic deformation and fracture in the precipitation hardened ECAP materials. It is shown that the precipitation strengthened UFG structure remains stable both under elevated temperatures as high as 500°C and under cyclic loading at room temperature. Substantial improvement of fatigue life is evidenced in comparison with conventional coarse-grain materials. The appearance of cyclic softening is noticed and its nature is discussed in terms of dislocation–particle interaction and possible dissolution of precipitates during fatigue.

Introduction

Interesting physical and mechanical properties of bulk nanostructured (the grain size d is less than 100 nm) and ultra-fine grained (UFG) materials (d is of 100–500 nm) [1] are in focus of many investigations during last decade. The possibility to produce massive UFG specimens via severe plastic deformation makes them attractive for engineering applications and creates new opportunities to explore their specific properties in comparison with ordinary coarse grain materials by using the standard specimens. Equal-channel angular pressing (ECAP) [2], [3] is a technique which allows us to achieve extremely large imposed strains through intensive simple shear in bulk samples. During ECAP, the significant grain refinement occurs together with dislocation hardening, resulting in the spectacular enhancement of strength of a working material [1], [2], [3], [4]. However, many modern engineering applications require a rather sophisticated combination of physical and mechanical properties [5] and these requirements can be quite controversial. For example, in attempts to improve the fatigue properties by strengthening, i.e. by increasing the yield stress and tensile strength, one can obtain an excellent endurance limit in the high-cyclic fatigue (HCF) regime, but the low-cyclic fatigue (LCF) properties and resistance to crack propagation may be reduced considerably due to the loss of ductility [6], as has been observed in severely plastically deformed (SPD) materials [6], [7]. The enhancement of both LCF and HCF properties is possible, in principle, if one improves the tensile strength together with ductility that is not easy to achieve via plastic deformation and a judicious compromise has to be found. It has long been recognized that SPD is capable of producing strong materials with rather good ductility. Nevertheless, the vast majority of experimental data show that the fine-grained SPD materials are less ductile than their coarse grain counterparts. This explains why the low cyclic fatigue performance of ECAP materials appears to be below expectations. The other reasons include their low structural stability, susceptibility to cyclic softening and early strain localization. A high cyclic softening rate in ECAP metals, indicating a high degree of structural instability under load, plays an important role in fatigue degradation [6], [7], [8]. Several causes have been proposed in the literature to account for this phenomenon [6]: (1) dislocation recovery and recovery of highly disturbed grain boundaries, (2) grain coarsening, recrystallization and abnormal grain growth promoted by cyclic stresses, (3) strain localization in the large-scale shear bands appearing during mechanical testing and microcracking. According to [9], grain growth resulting in local softening serves as a precursor of strain localization, which tends to develop in the regions where the resistance to plastic flow is lower. From acoustic emission measurements and surface observations, it has been supposed [10], [11] that the shear bands appear very rapidly as a result of local plastic instabilities initiated along the non-equilibrium grain boundaries in a fashion similar to inhomogeneous plastic deformation of metallic glasses. Being a specific case of plastic instability and intensive damage, shear banding is quite undesirable regardless of its precise microscopic mechanism. Therefore, when fatigue is of major concern, the material should be carefully designed so that to ensure its structural stability and to eliminate or delay strain localization as long as possible. One more problem arises in the development of ECAP materials owing to the fact that SPD results in the dramatic decrease of thermal stability of a working material [1], [12] in view of increasing driving force towards recovery and recrystallization when the imposed strain increases.

Several ways towards structure stabilization of SPD materials can be proposed: (a) annealing below recrystallization temperature [7], [8]; (b) using of other than wavy slip materials [7]; (c) solid solution strengthening [7]; (d) particle strengthening. While the first three approaches have been explored to some extent (although, in our opinion, there is still a great need in more systematic data), the effect of precipitation is unclear to date. The positive effect of precipitation hardening on the thermal stability of ECAP materials has been shown, for example, using the model Cu–ZrO2 composite [13]. Since the cyclic behaviour of ECAP pure copper has been investigated in more detail than that of other materials [6], [7], [8], [9], we chose a commercial precipitation hardened copper based alloy Cu–Cr–Zr for the present study. This alloy is traditionally used in applications where a combination of high mechanical strength, heat resistance and electrical (or thermal) conductivity is demanded (electrodes for point welding, heat exchangers, fusion reactors, etc. [5]). Thus, the present work has a triple purpose. The first is to assess the mechanical properties and, particularly, fatigue performance of this practically important Cu–Cr–Zr alloy subjected to significant grain refinement and strengthened through ECAP. The second is to explore the possibility to enhance the thermal stability and fatigue resistance of ECA-processed metals through subsequent precipitation hardening. The third is to optimize the electrical and mechanical properties by varying temperature–time conditions of the post-ECAP aging. Hence, in addition to attempts to improve the fatigue performance we intend to produce the SPD material which will be heat resistant and will posses a satisfactory electric conductivity. The latter issue is important because obtaining a high enough conductivity is not straightforward in plastically deformed metals since their electrical properties decrease usually with plastic straining and hardening [14], [15]. Emphasis is placed on the cyclic response under strain-controlled conditions since this kind of testing provides valuable information regarding the mechanisms of plastic deformation and because these conditions are most difficult to sustain for pre-deformed metals. A special attention is paid to the effect of processing including the number of ECA-passes and the aging conditions on mechanical properties.

Section snippets

Experimental procedure

Before ECAP the Cu–0.44Cr–0.2Zr billets (the concentration of alloying elements is given in wt.%) of 14×15×175 mm were solution treated at 1040°C for 30 min and quenched in 5% water solution of NaCl. After quenching, the samples had the Vickers microhardness of 50 kgf/mm2. Multiple pressing through intersecting at 90° square channels was performed from 1 to 12 times with 0.4 mm/s velocity at room temperature via the so-called “route Bc” when the sample was rotated through 90° clockwise about

Microhardness, electric conductivity, thermal stabi1lity and aging conditions

The significance of aging for deformation-processed Cu-based alloys has been well understood for optimization of both strength and electrical conductivity [14], [15]. The DSC curve of the ECAP specimen, Fig. 1, reveals the complexity of multi-stage structural transformations in Cu–Cr–Zr under linear heating. The enthalpy release peak, which can be associated with recrystallization, starts at a rather high temperature of 525°C and its maximum is reached around 650°C. The onset of the first

Discussion

In the preceding section we have underlined the difference in the cyclic response of as-fabricated ECAP and aged after ECAP specimens: the former demonstrates the steady stress amplitude during cycling whereas the latter exhibits notable softening which is particularly pronounced at the beginning of fatigue loading. It has been recognized that cyclic hardening and softening in precipitation strengthened alloys is closely related to the type, morphology and density of second-phase particles. The

Summary and conclusions

The ultra-fine grain Cu–Cr–Zr dilute alloy has been fabricated by ECAP. It is undoubtedly shown that post-ECAP aging makes the precipitation hardened ultra-fine grain structure rather stable under both thermal and mechanical influence. Optimal aging conditions are found to ensure the best high-cyclic fatigue performance in combination with satisfactory good electric properties and thermal stability. At the same time, the low cyclic fatigue properties remain high despite some loss of ductility

Acknowledgements

The authors are indebted to M. Kawazoe (YKK Corporation, Japan) and T. Yamasaki (Doshisha University, Japan) for their skilful help in TEM observations. One of the authors (V.K.) wishes to thank the Japanese Society for the Promotion of Science for the fellowship awarded that allowed completing the present work during his staying in Japan in 2000–2001.

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