On the variation of mechanical properties with solute content in Cu–Ti alloys

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Abstract

The variation of mechanical properties and electrical conductivity of Cu–Ti alloys of four compositions, viz. Cu–1.5 wt%Ti, Cu–2.7 wt%Ti, Cu–4.5 wt%Ti, and Cu–5.4 wt%Ti, have been studied in solution treated (ST), solution treated+peak aged (ST+PA), and solution treated+cold worked+peak aged (ST+CW+PA) conditions. In the ST condition, Ti is found to be a potential solid solution strengthener of copper showing greater effect than other elements like Zn, Ni, Al, Si, Be, and Sn. Solid solution strengthening in Cu–Ti alloys is attributed to the interaction of titanium atoms with screw dislocations and the effective interaction is more due to modulus mismatch than size misfit. Further, a marked change in the linear variation of tensile strength and elongation with Ti content is observed at about 4.0 wt%Ti beyond which, tensile strength increases sharply while elongation decreases further, which is attributed to fine scale precipitation formed during quenching of Cu–4.5 Ti and Cu–5.4 Ti alloys. On the other hand, hardness and tensile properties increase linearly up to 5.4 wt%Ti in the peak aged condition with or without prior cold work, due to uniform precipitation of Cu4Ti, βl phase in all the four alloys. The increase in yield and tensile strengths due to solid solution strengthening, cold work, and precipitation have been determined quantitatively in ST+CW+PA alloys. While electrical conductivity is less, the mechanical properties of Cu–Ti alloys are comparable with those of commercial Cu–Be alloys.

Introduction

Binary Cu–Ti alloys have good potential as a substitute for expensive and toxic Cu–Be alloys. The mechanisms of spinodal decomposition and precipitation strengthening in Cu–Ti alloys have been studied extensively 1, 2, 3, 4, 5, 6, 7, 8, 9. It was reported earlier by us that hardness and yield strength in solution treated (ST) Cu–Ti alloys increase linearly up to about 4.0 wt%Ti beyond which, a sharp increase is observed with further additions of Ti, due to fine scale precipitation in the form of modulations in Cu–4.5 Ti and Cu4Ti, βl precipitate in Cu–5.4 Ti alloy formed during quenching [10]. Further, a similar behaviour has been observed in the case of fatigue strength [11]and electrical resistivity [12]as well. On the other hand, variation of fatigue strength and electrical resistivity in peak aged (PA) condition has been reported to be linear up to 5.4 wt%Ti and this has been attributed to uniform precipitation of ordered, metastable, and coherent Cu4Ti, βl phase in all the four Cu–Ti alloys 11, 12. However, little work has been reported on the variation of tensile strength and elongation in ST condition and mechanical properties (hardness, yield and tensile strengths, and elongation) in aged condition with and without prior cold work (CW). The aim of the present study has been to investigate the variation of mechanical properties with Ti content and correlation of yield and tensile strengths with volume fraction of the Cu4Ti, βl precipitate in Cu–Ti alloys. The results obtained on the effect of Ti content on hardness and tensile properties in ST, ST+PA, and ST+CW+PA conditions are presented in this paper. Further, tensile strength and electrical conductivity of Cu–Ti alloys are compared with those of Cu–Be alloys.

Section snippets

Experimental

A 30 kg melt of each of the four Cu–Ti alloys with the nominal composition (in wt%) of 1.5, 3.0, 4.3, and 5.5 Ti have been made in a Stokes vacuum induction melting (VIM) furnace with oxygen free electronic (OFE) copper and Cu–26 wt%Ti master alloy as charge materials. The ingots were homogenised at 850°C for 24 h and analysed for Ti content. The analysed composition of the ingots (in wt%Ti) is 1.5, 2.7, 4.5, and 5.4 and oxygen (in ppm) is 5.7, 6.0, 6.6, and 6.4, balance being copper. The

Solid solution strengthening

The variation of hardness and yield strength with Ti content in ST Cu–Ti alloys as reported earlier [10], is reproduced in Fig. 1. Both hardness and yield strength increased linearly up to about 4.0 wt%Ti beyond which, a sharp increase was observed with further additions of titanium. The sharp rise in hardness and yield strength was attributed to fine scale precipitation in the form of modulations in Cu–4.5 Ti and Cu4Ti, βl precipitate in Cu–5.4 Ti alloy formed during quenching itself. The

Conclusions

  • 1.

    Titanium is a potential solid solution strengthening element in copper resulting in higher strength than that of Zn, Ni, Al, Si, Be, and Sn. Solid solution strengthening in Cu–Ti alloys is attributed to the interaction of titanium atoms with screw dislocations and the effective interaction is more due to modulus mismatch than size misfit. The slope of the straight lines showing solid solution strengthening in copper by different solutes, increases in the order: Zn, Ni, Al, Si, Be, Sn, and Ti

Acknowledgements

The financial support of the Defence Research and Development Organisation is gratefully acknowledged. One of the authors (S. Nagarjuna) is grateful to Dr K.K. Sharma, Director-II, DMRL for encouragement and support.

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