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A procedure and specification for evaluating the degree of spheroidization of graphite in spheroidal graphite iron (SGI), using a computer-based image analyzing system has been developed as an aid to structure-property-quality assessment. Both global and feature-specific numerical indices have been programmed and implemented using a Zeiss Jenaphot 2000 projection microscope and SEM interfaced to a computer-based MACROS III analyzer and a CCD video camera. The modular procedure has been tested and used to evaluate the effect of variation in the degree of spheroidization of graphite on the 0.2 % offset yield strength for an iron series ranging from ASTM type I (fully nodular) to ASTM type II-III-IV (mixtures of nodular and compacted/vermicular graphite) and were found to indicate good correlation.

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This article presents the results of an experimental study of fracture and fatigue in a nanostructured (an average grain size of~23 nm) bulk Ni-18 wt pct Fe alloy that was produced using a pulsed electrodeposition technique. The fracture behavior of the alloy is investigated using fracture resistance experiments, while the fatigue behavior is studied in fatigue crack growth experiments. The alloy exhibits limited toughening as the crack initiates at a fracture toughness of about 25 MPa ffiffiffiffi m p and propagates with a slight increase to a plateau value of about 30 MPa ffiffiffiffi m p. The limited toughening arises from the slight increase in the crack-tip plasticdeformation zone at the early crack growth and ligament bridging due to the microcrack formation ahead of the tip of the main crack. In contrast with a flat fatigue-crack wake, a wavy crack wake was observed under monotonic loading. This trend is attributed to the following: (a) nanovoid coalescence at grain boundaries, (b) microcrack formation by joining nanovoids, and (c) the linking of microcracks with the main crack through the fracture of inclined bridging ligaments. The fractured surface is shown to contain ductile dimple structures with average diameters of~100 nm. Focused-ion-beam (FIB) methods are also used to study fatigue-crack growth. These results show that fatigue crack growth occurs by the coalescence of nanovoids that form ahead of the crack tip. The observed mechanisms of fatigue crack growth are shown to be consistent with the results of prior atomistic simulations.