INITIAL FRACTURE MECHANISMS IN NICKEL ALLOYED PM STEEL

Initial fatigue crack propagation mechanisms at near threshold conditi ons were studied for four nickel-alloyed, powder-metallurgy (PM) steel s. Fatigue fracture surfaces were obtained by testing smooth rectangul ar specimens at 30 Hz and under constant amplitude and zero mean stres s conditions. Materials based on Distaloy AE were used in two densitie s, namely 7.15 and 7.45 g/cm(3). All the fracture surfaces were compos ed of three morphological regions (i) a macrocrack initiation region R 1 where cracks propagated preferentially through particles (ii) a macr ocrack growth region R2 and (iii) an unstable crack growth region R3 w here cracks propagated preferentially between particles. Initial fatig ue crack growth, in region R1, was controlled by the propagation of sh ort cracks whose dimensions were comparable to the material microstruc ture. The subsequent fatigue crack growth in regions R2 and R3 was con trolled by ductile rupture between microvoids. Transparticle fracture in region R1 was independent of pore distribution, while interparticle fracture in regions R2 and R3 was dependent on pore distribution.