Abstract
Fly-ash/NiCr-alloy functionally graded materials (FGMs) plates were prepared by spark plasma sintering (SPS); the plates were 100 mm in diameter and layer thicknesses of 0.6 mm and 1 mm. The plates consisted of stacked layers; the layers had volume ratio compositions of fly-ash/NiCr = (100/0 (A), 60/40 (B), 40/60 (C), 60/40 (B), 100/0 (A)). Two sorts of FGMs specimens with three layers prepared by machined from the FGMs plates with A/B/C/B/A five layers were investigated using a three-point flexural test. One sort of specimens had the normal stacking structure A/B/C, and the other had the reverse structure C/B/A, for the loading nose. After investigating the strengths of these two stacking structures, the fracture origins were observed at the surface of the extending C layer of the A/B/C structures, and of the A layer surface of the C/B/A structures. The deformation mechanism, average strength, and Weibull modulus were ductile, 12.0 MPa, and 5.5, respectively, for the normal A/B/C structure, and brittle, 46.7 MPa, and 13.1, respectively, for the C/B/A reverse structure. The different strengths of the two structures were explained on the basis of the strength of the extending surface layer, A or C, and the residual stress caused by the difference in thermal expansion coefficients of the stacking layers in the two structures. It was concluded that the fracture strengths of stacked FGMs materials were dominated by the stress field between the constituent layers and by the loading direction.
