Laser-Clad NiAICrHf alloys with improved alumina scale retention

Abstract

A new mechanism for the improved retention of alumina scales formed on laser-clad NiAICrHf alloys has been observed. Laser cladding is the process where fine metal powders are rapidly melted and fused to a solid substrate using a CO2 laser. The effects of laser cladding upon scale retention on NiAICrHf alloys after cyclic and isothermal exposure to air were investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The calculated compressive stress in the scale due to constrained cooling exceeded the probable compressive strength of alumina. Additions of up to ≈ 2.5 wt pct Hf increasingly promote retention of scales grown at 1200 °C, with laser-clad samples of ≈ 2.5 wt pct Hf alloy retaining almost completely intact scales. The improvement in scale retention is due to improved toughness in scales containing hafnia polycrystallites, possibly_via microcracking initiated by anisotropic thermal contraction of the hafnia. Laser cladding the 2.5 wt pct Hf alloy provides a large concentration of ~ 1 µm Hf-rich particles that are precursors of the hafnia in the scale as well as a finer dendrite spacing that reduces the mean free distance between particles.