Low cycle fatigue behaviour of a precipitation hardened Cu-Ni-Si alloy

https://doi.org/10.1016/j.ijfatigue.2016.07.019Get rights and content

Highlights

  • Investigated Cu-Ni-Si contains δ-Ni2Si nano precipitates.

  • Cyclic hardening followed by softening is observed.

  • Dissolution of δ-Ni2Si results from repetitive cutting.

  • Deformation is localised in precipitate-free bands.

Abstract

Low cycle fatigue tests were performed at room temperature to investigate the role of the microstructure of a Cu-Ni-Si alloy on the stress response to strain cycling and on the fatigue resistance. The cyclic accommodation consisted of a hardening followed by a softening. TEM analysis showed that in some grains dislocations remained isolated and confined between precipitates while in other grains dislocations piled up at δ-Ni2Si precipitates and then cut them. Repetitive cutting allows their dissolution and formation of precipitate-free bands where the plastic deformation is localised. The Manson-Coffin diagram exhibited two regimes according to the proportion of grains involved in the plastic deformation accommodation.

Introduction

A large amount of engineering applications, like railway equipment, marine hardware or lead frames, requires materials that exhibit high mechanical strength as well as good electrical and thermal conductivities. In that way, copper alloys are widely used although the enhancement of mechanical properties usually comes along with a decrease of the electrical performances [1], [2]. To face this problem, oxide-dispersion-strengthened (ODS) copper alloys, where aluminium oxide nanoparticles are dispersed in the matrix, is one of the existing solutions [3], [4], [5]. However, their mechanical properties might not be enough for the cited applications. To overcome this aspect, precipitation hardened copper alloys have been developed, such as copper-beryllium alloys. Those latter exhibit a good balance between mechanical, thermal and electrical properties [1], [6]. But due to the price and the toxicity of beryllium and its compounds, alternative copper alloys have been formulated. Cu-Ni-Si alloys, with a nickel content from 1.5 to 8.0 wt.% and a quantity of silicon included between 0.3 and 1.8 wt.%, are known to be one of the best replacement options [4], [5], [6]. Their good property balance is mostly attributed to the formation of nanosized disc-shaped coherent δ-Ni2Si precipitates identified by transmission electron microscopy (TEM) [7], [8], [9], [10], [11]. Currently, the scientific literature on these alloys mainly deals with the microstructure evolution combined with the optimisation of the mechanical (hardness and tensile properties) and electrical behaviour thanks to their elaboration process [12], [13], [14], [15], [16], [17], [18], their heat treatments [7], [12], [19], [20], [21] or their chemical composition [14], [22], [23], [24], [25], [26], [27], [28]. The scope of applications of Cu-Ni-Si alloys contains the transport sector, in electric engines for example. It implies that they are submitted to cyclic loading and are therefore subjected to fatigue failure. The fatigue behaviour of Cu-Ni-Si alloys has however only been investigated in few articles, especially on high-cycle fatigue (HCF) [29], [30]. Low cycle fatigue (LCF) was just briefly studied on polycrystalline material by Lockyer and Noble [29], and on Cu-Ni-Si single crystals by Fujii et al. [31]. The LCF behaviour seems to exhibit cyclic softening but has not been fully investigated at this point. Very recently, Goto et al. [32] investigated the role of microstructure on short crack propagation in a Cu-Ni-Si alloy by pointing out the importance of precipitation process. Indeed, particles and precipitate-free zones formed in their material as a result of the high level of Ni and their heat treatment. Both induced localised high stress/strain distribution which led to the crack initiation, followed by the growth along slip planes in grains sharing grain boundaries.

The aim of the present work is therefore to improve the knowledge on a Cu-Ni-Si alloy by providing a complete description of the LCF behaviour. The study of cyclic accommodation and of fatigue resistance will be linked to the microstructure investigated by scanning and transmission electron microscopes in order to propose a fatigue mechanism of the Cu-Ni-Si alloy.

Section snippets

Material and experimental procedures

The Cu-Ni-Si alloy used in the present work is a CuNi2Si (CW111C) provided by the company Le Bronze Industriel (France). The casted material was first solution treated around 950 °C for 2 h, then hot formed, rapidly quenched and finally aged at a temperature between 450 °C and 500 °C for a duration between 2 h and 4 h. At last, the plate fatigue specimens were machined with a thickness of 3 mm, a width of 6 mm and a gauge length of 12 mm. This design of specimen ensures that no bending occurred during

Metallographic characterisation and microstructure

The microstructure of the material is shown in Fig. 1. EBSD data reveal that the studied alloy presents equiaxed grains with a size ranging from 20 to 50 μm. A large number of annealing twins is also observed.

Concerning the precipitates formed during the aging treatment, they have been identified by using selected area diffraction mode in TEM. Disc-like δ-Ni2Si intended for the hardening of the alloy have been indeed detected. The precipitate density is high and their width is close to 5 nm. For

Discussion

The present investigation which aimed at identifying the low cycle fatigue properties of a Cu-Ni-Si alloy shows that the material exhibited high stress values. This is the result of the efficient hardening heat treatment that precipitated δ-Ni2Si nanosized particles. Compared to other ODS copper such as Cu-Al2O3 material, the Young modulus as well the yield stress of the considered Cu-Ni-Si alloy are respectively 30% and 65% higher [3]. Both materials exhibit nearly the same size range of

Conclusion

The investigation of the microstructure of the tested Cu-Ni-Si alloy has exhibited a high density of δ-Ni2Si precipitates resulting in the fatigue properties summarised as follows.

  • 1.

    The cyclic accommodation of the alloy is composed of a hardening step at the beginning of the test, followed by a continuous softening until the specimen fracture. Moreover, two plastic regimes have to be considered (0.8%  Δεt  1.0% and 1.2%  Δεt  1.5%) in order to characterise its fatigue resistance. Planar slip is

Acknowledgments

This work has been financially supported by Bpifrance and the Conseil Regional du Nord-Pas de Calais.

The authors thank A. Addad, D. Creton and J. Golek for their technical assistance.

The SEM and TEM national facility in Lille (France) is supported by the Conseil Regional du Nord-Pas de Calais, the European Regional Development Fund (ERDF).

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