Effect of aluminum on microstructure and property of Cu–Ni–Si alloys

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Abstract

The effect of aluminum on the microstructure and properties of Cu–Ni–Si alloys has been investigated using hardness test, electrical conductivity measurement, optical microscopy, X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. Compared with Cu–Ni–Si alloy, Cu–Ni–Si–Al alloy had finer grains. After homogenization treatment at 940 °C for 4 h, hot rolling by 80% at 850 °C, solution treatment at 970 °C for 4 h, cold rolling by 50% and ageing treatment at 450 °C for 60 min, properties better than Cu–Ni–Si alloy have been obtained in Cu–Ni–Si–Al alloy: hardness was 343 HV, electrical conductivity was 28.1% IACS, tensile strength was 1080 MPa, yield strength was 985 MPa, elongation percentage was 3.1% and stress relaxation rate was 9.83% (as tested at 150 °C and loading for 100 h). β-Ni3Si and δ-Ni2Si formed during the ageing process and the crystal orientation relationship between matrix and precipitates was : (02̄2̄)Cu(01̄1̄)β(010)δ, [100]Cu[100]β[001]δ; (111̄)Cu(111̄)β(02̄1)δ, [112]Cu[112]β[012]δ. Addition of Al promoted the precipitation, and effectively enhanced the anti-stress relaxation property. Quasi-cleavage fracture with shallow dimples appeared in designed Cu–Ni–Si–(Al) alloy.

Introduction

High strength copper alloys play a significant role in the automation industry, and automobile industry, as well as electrical and electronics industry [1], [2]. Cu–Be alloys are the most widely used elastic copper, owing to their high strength of ∼1200 MPa and electrical conductivity of ∼22% IACS [3]. However, the high toxicity of beryllium in Cu–Be alloys limits their processing and application. Moreover, stress relaxation property of Cu–Be alloys is poor as it is performed in elevated temperature environment (∼25%, as tested at 150 °C and loading for 100 h). Many researchers have put great effort to develop new elastic copper alloys without beryllium, such as Cu–Ni–Sn, Cu–Ti and Cu–Ni–Al system alloys to replace the harmful Cu–Be alloys [4], [5], [6], [7], [8]. These alloys have high strength (>1000 MPa) and good anti-stress relaxation property; however, their electrical conductivities (<12% IACS ) are much lower than that of Cu–Be alloys (∼22% IACS). Encouragingly, Cu–Ni–Si system alloys with high strength and good electrical conductivity have been developed recently [9]. Some Cu–Ni–Si alloys with low concentrations of Ni and Si have been developed for lead-frames [10], [11], such as KLF-1, C70250, MAX251, etc. [12], [13], [14], [15], [16], owing to their high strength (∼800 MPa) and good electrical conductivity (∼45% IACS). Meanwhile, Cu–Ni–Si alloys with high concentrations of Ni and Si have high strength (>1000 MPa), which would have a bright application prospect in elastic materials to replace the present Cu–Be alloys [17], [18, [19], [20].

Effects of addition of Sn [17], Mg [21], Cr [22], Zn [23] and P [24] on the microstructure and properties of Cu–Ni–Si alloys have been reported, showing that the addition of alloying elements could effectively improve some properties of alloy. Additions of previous elements play a limited role in further developing Cu–Ni–Si alloys. Therefore, addition of new suitable alloying elements may be an effective way to develop super-high strength Cu–Ni–Si alloys. However, effect of addition of Al element on the Cu–Ni–Si alloys has not been investigated yet. In this paper, Cu–Ni–Si alloys with and without Al have been designed and investigated, with an aim to reveal the effect of aluminum on the microstructure and properties of Cu–Ni–Si alloys.

Section snippets

Experimental details

The nominal compositions of the designed Cu–Ni–Si alloys are listed in Table 1. Pure copper, pure nickel, pure silicon, pure aluminum, pure chromium and Cu–10 wt% Mg master alloy were prepared and melted in the furnace. The melting and casting operations were carried out in a N2 atmosphere to prevent the alloys from oxidation. After surface defects were removed, the casting ingots were homogenization treated at 940 °C for 4 h and subsequently hot rolled at 850 °C by 80%, reducing the thickness of

Thermo-mechanical treatment

Fig. 1 shows the microstructure of designed Cu–Ni–Si–(Al) alloys in as-cast state or homogenizing state. Interdendritic first-arm space of as-cast CuNiSi alloy was 60 μm (Fig. 1a), while that of CuNiSiAl alloy was only 40 μm (Fig. 1c). After the same homogenizing treatment of heat at 940 °C for 4 h, grain size of CuNiSiAl alloy (Fig. 1d) was still less than that of CuNiSi alloy (Fig. 1b). Addition of Al refines the structure of as-cast alloy effectively. Typical SEM micrographs and elemental

Conclusions

  • 1.

    The addition of aluminum refines the microstructure of Cu–Ni–Si–(Al) alloy with as-casting state and solution treatment state, promotes the precipitation, and effectively enhances the anti-stress relaxation property.

  • 2.

    After homogenization treatment at 940 °C for 4 h, hot rolling by 80%, solution treatment at 970 °C for 4 h, cold rolling by 50%, and ageing treatment at 450 °C for 60 min, CuNiSiAl alloy achieved excellent combination properties : hardness was 343 HV, electrical conductivity was 28.1%

Acknowledgment

The authors are pleased to acknowledge the financial supply supported by the National Natural Science Foundation of China (51271203), the Hunan Provincial Natural Science Foundation of China (11JJ2025), the Undergraduate Innovative Research Training Program of Central South University (CL11060, AL11491), the Hunan Provincial Innovation Foundation for Postgraduation (CX2011B107), the Excellent Doctor Degree Thesis Support Foundation of Central South University (2012ybjz013), the Scholarship

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