Effect of tungsten alloying on short-to-medium-range-order evolution and crystallization behavior of near-eutectic amorphous Ni–P

Abstract The crystallization of near-eutectic amorphous Ni–P can be significantly retarded by alloying a small fraction of tungsten. To understand this phenomenon, amorphous Ni0.80 P0.20 and Ni0.76 W0.04 P0.20 with the same phorphous fraction were fabricated by electroless plating as 10 μm-thick continuous layers. Subsequently, the alloys were tempered to three different stages: (i) an “incubation stage” (still completely amorphous), (ii) a “partially crystalline” stage (amorphous and crystalline phases), and (iii) a “completely crystalline” stage (only crystalline phases). To understand the strong effect of tungsten alloying on the crystallization behavior, we investigate the atomistic structure of as-plated and tempered Ni0.76 W0.04 P0.20 and compare it with that of correspondingly treated amorphous Ni0.80 P0.20. The study focuses on SRO (short-range order) and MRO (medium-range order). SRO, characterized by typical nearest neighbors, was studied by XPS (X-ray photoelectron spectrometry). MRO, which includes neighbor atoms up to about 2 nm distance, was studied by FEM (fluctuation electron microscopy). In addition, the crystalline phases and microstructure of both alloys were studied by XRD (X-ray diffractometry) and TEM (transmission electron microscopy), respectively. Combining results of these complimentary techniques leads to models for the atomistic structure of amorphous Ni–P and Ni–W–P and models for the micromechanism of crystallization. Specifically, these models explain why tungsten reduces the driving force for crystallization and slows the crystallization kinetics. The principles we have recognized may be useful for identifying alloying elements that are potentially even more effective in retarding crystallization.