Mechanical and thermal properties of nanostructured nickel-phosphorus alloys at grain sizes below 30 nm

Despite the numerous past studies on nanocrystalline materials, the effects of grain size on many properties are still not well understood, mainly because of uncertainties associated with sample imperfections (e.g. pores and cracks), contamination, compositional variations with grain size and the contribution of interfaces.;During calorimetric annealing, two distinct exotherms, heat releases I and II, were observed. During heat release I, strong grain boundary drag by P solute was found to account for the substantial thermal stability up to 400°C. During heat release II, massive grain growth occurred concurrently with the formation of Ni3P particles, demonstrating a weaker grain boundary drag by particles at higher temperatures.;The activation energies for abnormal and normal grain growth were determined to be 1.49 eV and 2.4 to 2.6 eV, respectively. The isobaric heat capacity in the temperature range from 50°C to 120°C was close to the experimental values previously reported for polycrystalline materials, indicating no significant grain size effect.;In this study, a structure model is first presented for the calculation of the interfacial volume fractions in such materials. Fully-dense Ni-P samples were then electrodeposited with a relatively constant P content (∼2.5wt%) and a 4-30 nm grain size range to measure the following properties: Young's modulus, thermal stability and heat capacity. It was shown that for grain sizes larger than 18 nm, the Young's modulus is independent of grain size (∼200GPa). However, for grain sizes less than 18 nm, the Young's modulus decreased continuously towards the value of an amorphous Ni-P structure (144 GPa). It is shown that this is due to the substantial contribution of the interface structures, i.e. the excess free volume in the interface regions, as measured by positron annihilation lifetime spectroscopy. The hardness obeyed the classical Hall-Petch relationship for grain sizes larger than 7 nm with a positive slope of 0.36 MPa-m0.5, but decreased with a negative slope of -0.17 MPa-m0.5 for grain sizes less than 7 nm.