PSI - Issue 7

M. Goto et al. / Procedia Structural Integrity 7 (2017) 248–253

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M. Goto et Al./ Structural Integrity Procedia 00 (2017) 000–000

However, there are difficulties in processing and application owing to the toxicity of beryllium present. Cu-Ni-Si alloy developed by Corson (1927) has a prospect for substituting for toxic Cu-Be alloy because of their good combined properties of electrical conductivity and strength. The Cu-Ni-Si alloy is strengthened by nano-size intermetallic compounds ( δ -Ni 2 Si) precipitated during aging (e.g., a few to a few tens of hours at 450-500 °C). The aging condition depends on alloy compositions and thermomechanical treatment (Locker and Nobel 1994, Fujiwara et al. 1998). To reduce the aging time, Han et al. (2016) used Cu-Ni-Si alloy with high solute concentrations (Cu-6Ni-1.5Si), showing that a 0.5 h-aging after the solution heat treatment yielded a near maximum hardness. However, some parts of microstructure were transformed into discontinuous precipitate (DP) phases sporadically distributed in the matrix. It has been known that DP phases bring a detrimental effect on mechanical properties. For the actual structural application of Cu-Ni-Si system alloys, the fatigue characteristics should be clarified. Unlike tensile property, relatively little work has been conducted on the fatigue behaviour of alloys, and only a few reports on the fatigue mechanism can be found, especially for alloys with low solute concentrations such as Cu–2Ni–1Si (Locker and Nobel 1999), Cu–2.16Ni–0.72Si (Sun et al. 2011), Cu–2.2Ni–0.5Si (Fujii et al. 1287). The present work was performed in an attempt to obtain a better understanding on fatigue damage of precipitate strengthened Cu-6Ni-1.5Si alloy with sporadically distributed DP phases. In this regard, clear evidence of the crack initiation from GBs and the effect of sporadic DP phases on fatigue strength were shown. In addition, the physical background of the role of microstructure on the fatigue strength of the alloy was discussed. 2. Experimental procedures Cu and Si 99.99% pure, and 99.9% pure Ni were used as alloying elements for the fabrication of the Cu–6wt%Ni– 1.5wt%Si alloy ingots by induction melting. The ingots were cold-rolled with 80% reduction in thickness, and were subsequently solution heat-treated at 980 °C for 1 h and aged at 500 °C for 0.5 h. The microstructure was observed using an optical microscope (OM) and a scanning electron microscope (SEM). The characterization of precipitates was carried out using a 200 kV field-emission transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectroscopy (EDS) detector along with a scanning TEM. The 3-mm-diameter, 100- µ m-thick disk was prepared for TEM observation by mechanical polishing with a digitally enhanced precision specimen grinder and dimpling by a dimple grinder. The microhardness was measured using a Vickers hardness tester with an applied load of 1 N and tensile tests (4 mm diameter specimen) were performed on a tensile testing machine with a loading speed of 10 mm/s at room temperature. The Cu–6Ni–1.5Si alloy in this study showed a Vickers hardness of 259, a tensile strength of 820 MPa, a yield strength of 634 MPa and a tensile elongation of 14.3%. Round bar fatigue specimens with 5 mm diameter were machined from the aged samples. To remove any surface layer affected by the preparation, a layer of approximately ≈25 µ m was electrolytically polished from the surface of the specimen prior to fatigue testing. All fatigue tests were carried out at room temperature using a rotating bending fatigue machine (constant bending-moment type) operating at 50 Hz. The fatigue damage on the specimen surface and on the fracture surface was observed by using OM and SEM. The crack length, l , was measured along the circumferential direction of the surface using a plastic replication technique. The stress value referred to is that of the nominal stress amplitude, σ a , at the minimum cross-section (5 mm diameter). 3. Experimental results and discussion Fig. 1 shows (a) the microstructure of pre-aged and (b) aged alloys. Aged alloy had bright grains and sporadically distributed dark phases. Fig. 1c shows high-resolution TEM (HR-TEM) images of the matrix (bright grains). Disc shaped precipitates from aging were observed, which were identified as δ -Ni 2 Si intermetallic compounds by an optical diffractogram. SEM observation of the dark phases at high magnification showed the cellular structure consisted of fibre-like Ni 2 Si precipitates (Fig. 1d). The Ni 2 Si intermetallic compounds were found to precipitate as spherical particles, and the origin of DPs formed simultaneously in a small fractioned area in the matrix, in particular near the GBs. The DPs are generally formed by the growth of newly generated moving GBs that act as a solute diffusion path (Findik 1998). The precipitates grow perpendicular to their moving direction along the lower interface energy between the precipitates and the alloy matrix, forming a cellular structure with precipitates with an extremely high aspect ratio.