| Electro less nickel-phosphorous (EN) deposits in as-deposited condition have a non-equilibrium phase structure, which is thermodynamically unstable and would transform into equilibrium state through crystallization reactions. Since the first discovery in 1944 by Brenner and Riddle, various studies have been carried out on these deposits to investigate their microstructure properties and crystallization behavior under the three categories: low (1-5 wt.%), medium (5-8 wt.%) and high (9 wt.% and above) phosphorus deposits. Studies have shown that as-coated low-phosphorus deposits are either crystalline or consist of microcrystalline nickel. As-plated medium-phosphorus deposits are reported to be either fully amorphous phase or mixture of microcrystalline nickel and amorphous phase. Studies on high-phosphorus as-plated deposits have demonstrated that the deposits could be fully amorphous.Amorphous crystallization by laser irradiation is a newly developed technique that received growing interest in material science,especially in fabrication of semiconductor materials. However, there is less knowledge of the influence of laser energy density on process of crystallization of EN amorphous alloy.In this paper, the structure and the crystallization of the pulsed laser irradiated Ni-5wt%P alloy layers are studied with special attention to the effect of the laser energy density on crystallization behavior. SEM has been chosen to characterize the morphologies of Ni-5 wt.% P, both in the as-plated amorphous state as well as after pulsed laser irradiation. X-ray diffraction analysis (XRD) was employed to follow the overall transformational behavior. We present evidence that Laser energy density plays a significant role on growth of dendrite. In order to depict the effect of energy density on the temperature of alloy surface the heating and cooling process is simulated. The cooling rate varied with energy density result in differences in growth morphology of Ni-P alloy. It is important that the change of surface temperature in solidification of alloys after irradiation. So the temperature fields in alloys surface are calculated. The average cooling rates in the solidification process can be approximately calculated.It is a rapid melting and rapid solidification process that laser irradiation. At the area of laser irradiation, the laser energy density had considerable influence on the surface morphology of Ni-P alloy layer. When the energy density is below 8.2×105 W/cm2, no modification could be observed. In contrast, above this threshold needle-like crystals could be detected in SEM. It is indicated that when energy density of pulsed laser is around 8.2×105 W/cm2, the surface of alloy starts to melt and rapid solidification occurs.Structures in conventional casting processes are the result of cooling rates up to approximately 102 K/s. At greater cooling rates than this the structures change, becoming initially more refined. The grain size decrease from several hundred micrometers to several micrometers. If the cooling rate continues to increases and exceeds 108 K/s, Nan crystals can be obtained. In my experiment, pulsed laser can take the surface to above melting point (the melting point of Ni-5 wt.% P alloy is about 1473 K referring to Ni-P equilibrium diagrams) less than 0.1ms. secondary arms of dendrite broadened slightly and dendrites grown remarkably。This can be explained by the effect of energy density on the cooling rate. With the increase of energy density, the cooling rate decreased and more time spent in solidification interval.One exception to this rule is that at very high cooling rates, when primary arm spacing becomes very small, secondary and tertiary may be absent. In our experiment, the energy density varies from 0.82×106 W/cm2 to 1.52×106 W/cm2, corresponding growth morphology from dendrite to cross-like crystals, as a result of absence of secondary arm. When energy density decreases to 8.2 ×106 W/cm2, the dense needle-like crystals appear.As an important parameter laser energ...
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