High-temperature hardness and scratch behaviour of differently strengthened iron aluminide laser claddings

Abstract

Fe₃Al-based iron aluminides provide notable high-temperature properties at a comparatively low overall ecological impact. To improve their wear resistance, different strengthening strategies are studied for which Fe₃Al-based laser claddings (30 at.% Al) are alloyed either with Si, C, or Ti and B. Detailed microstructural investigations after laser metal deposition (LMD) including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) were performed. Results show that Si-alloyed claddings are single-phase with equiaxed grains of average sizes from ∼350 μm (1 at.% Si) decreasing with increasing Si content to ∼50 μm at 3 and 5 at.% Si. Contrary, the C-alloyed claddings microstructures are dendritic and ledeburite-like with perovskite-type carbides Fe₃AlC₀.₆. The Ti- and B-alloyed cladding exhibits finely dispersed TiB₂-type precipitations; at low contents in the sub-micron range and mainly present at the grain boundaries, at higher additions are quite large (3–5 μm) and present within individual grains. The individual hardphases, as quantified via nanoindentation, of Fe₃AlC₀.₆-type carbides or TiB₂-type borides exhibit average hardness values of ∼7.5 GPa and ∼ 45.5 GPa, respectively. Therefore, the hardness of C- and Ti & B-alloyed claddings increases from ∼260 HV10 (Fe₃Al) to 405 HV10 (Fe₃Al plus 5 at.% C) and to 340 HV10 (Fe₃Al plus 3 at.% and 6 at.% B). The higher hardness of the C-alloyed cladding stems from the s higher hardphase content; the matrix hardness ranges between 4 and 5 GPa for all precipitation-strengthened claddings. Contrary, the Si-alloyed cladding exhibits a pronounced increase to 5.7 ± 0.8 GPa upon adding up to 5 at.% Si. Thus, an overall hardness of ∼350 HV10 is quantified on the expense of ductility (relaxation cracking after LMD). High-temperature hardness and scratch tests as well as wear investigations prove that the C- as well as Ti and B-alloyed claddings are superior to the plain Fe₃Al and Si-alloyed ones, making them promising alternatives to other wear protection materials based on Co, Ni or Cr.