Pulsed Ultraviolet Laser Quenched Surfaces
Al- and Fe- Based Alloys
J.G. Hoekstra, P.M. Mackey,
J.M. Fitz-Gerald, G.J. Shiflet, and J.R. Scully
University of Virginia, Dept. of Materials Science & Engineering

Al- and Fe- based metallic glasses demand attention due to their unique mechanical hardness, corrosion, and magnetic properties. The synthesis of amorphous Al- based alloys has achieved limited success and are of interest for high strength to weight applications. The ability to process Fe- based glassy alloys is of particular interest in ferroelectric applications. Mechanical quenching methods have created glassy materials for nearly a quarter of a century with cooling rates on the order of 10⁶ - 10⁷ K/s.¹ Irradiation of a material with a short laser pulse establishes rapid melting and solidification rates at the surface. Nanosecond laser pulses (3 – 100 ns) create solidification velocities in the range 10⁹ – 10¹¹ K/s and 10^-1 – 10¹ m/s, respectively.² Due to the fact that the UV absorbance is on the order of hundreds of nm, the absorbed energy densities and resultant melting and quenching remains localized to the surface with respect to the bulk. This study aims to investigate the atomic scale relationships of amorphous layer formation as a function of chemistry and laser processing conditions.

In this preliminary study, a KrF excimer laser (l=248 nm, 25 ns FWHM) operating between 0-50 Hz at fluences ranging from 0-10 J/cm² irradiated a target surface with corresponding velocity between 0-50 mm/s in a controlled atmosphere ranging from 50-500 mTorr. The material systems under investigation include the following: Al 2024, Al₈₄Co₈Ce₆ powder, melt spun Al₉₀Co₃Ce₇ metallic glass ribbon, and Fe₅₁Mo₁₀Cr₄Mo₁₄C₁₅B₆ bulk metallic glass ingots. The goal of this research is to laser surface modify a continuous, homogenous, amorphous surface on a crystalline substrate suitable for intensive characterization. From the nonequilibrium thermal nature of the process, complicated microstructures exhibiting cellular resolidification patterns in the quarter mm regime have been observed in Al- based samples, as seen in figure 1. Metastable microstructural features such as these have been reported in rapidly-solidified laser-melted steel.³ Laser surface treatment suppressed the polyphase crystalline microstructure of Fe- based bulk ingots, as seen in figure 2. Current and future research/ characterization is focusing on processing conditions optimization, cross sectional high resolution SEM and TEM (both with quantitative EDS), and electrochemical analysis of laser surface modified samples.

1. “Topics in Applied Physics: Glassy Metals II”, H. Beck and H.J. Guntherodt ed., Springer-Verlag, New York, Vol. 53, 1981.

2. P. Baeri, “Pulsed Laser Quenching of Metastable Phases”, Materials Science and Engineering, A178, 1994, 179-183.

3. “Metastable Microstructures”, D. Banerjee and L.A. Jacobson ed., Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1993.

4. S. Cao, A.J. Pedraza, L.F. Allard, and D.H. Lowndes, “The Effects of the Atmosphere on the Surface Modification of Alumina by Pulsed-Laser-Irradiation”, Journal of Materials Research, Vol. 12, No. 7, July 1997.

Acknowledgments: This work is supported by the Multi University Research Initiative Grant No. F49602-01-1-0352, the Advanced Laser Processing Laboratory, and the Center for Electrochemical Science and Engineering.