Development of a high-strength high-conductivity Cu-Ni-P alloy. Part I: Characterization of precipitation products

The precipitation products of a Cu-1.5Ni-0.32P (wt.%) alloy isochronally aged at 400°C and 600°C were investigated by transmission electron microscopy (TEM). There are two different sizes of precipitates formed in the 600°C aged specimen. Large precipitates appear to be lath, the length of which varies from 100 nm to 500 nm, whereas small dotty contrasts around 10 nm in diameter are uniformly distributed in the matrix. Only small dotty contrasts are observed after 400°C aging. In the composition range 0-33.3 at.%P, both the previous studies and the calculated Ni-P binary phase diagram indicated that the possible nickel phosphide phases are Ni₃P, Ni₁₂P ₅, Ni₅P₂, and Ni₂P. However, selected area electron diffraction (SAED) pattern analysis in conjunction with energy dispersive x-ray spectroscopy (EDS) identified the large lath precipitates as Ni₁₂P₅ phase. On the other hand, the small precipitates could not be analyzed by EDS due to thinness; thus, the structure is estimated to be a Ni₅P₄ phase from the relationships between d-spacing and angles between planes measured from a selected area diffraction pattern.