Lamellar instabilities during scanning laser melting of Al–Cu eutectic and hypoeutectic thin films

Abstract

Scanning laser melting was employed to investigate directional solidification in Al–Cu thin films, 200 and 500 nm thick. The Al–Cu alloy films, with both eutectic and hypoeutectic compositions, were co-deposited on fused silica substrates by magnetron sputtering. Melting of the films was carried out in air using a CW laser, as a function of the scan velocity, v, up to 5 cm/s. For eutectic films, regular lamellar microstructure was observed over the entire range of velocities studied. Interlamellar spacings, λ, below 50 nm were readily produced. Results were found to be consistent with studies on bulk Al–Cu alloys, obeying the classic steady state lamellar growth relationship, λ2v = K. However, the value of the constant, K is significantly larger in thin films when compared to bulk. This is attributed to constraints on mechanisms for spacing adjustment in two-dimensional eutectic systems. In the hypoeutectic thin films, a complex lamellar structure was observed as the scan velocity was reduced. Regions of straight lamellae, oscillatory lamellae, solitary tilt waves, and recurring extreme lamellar branching events coexisted across the melt track. Fourier analysis was used to quantify the chaotic area fraction of the films. The transition to chaos occurred as the G/v ratio increased, attributed to approaching a re-entrant eutectic-to-dendritic transition from above. Approaches to further refine laser melting in thin eutectic films in order to better control solidification processes are discussed.