Critical Temperature Range in Standard and Ni-bearing Spheroidal Graphite Cast Irons
Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended.
Mechanical properties and rolling-sliding wear performance of dual phase austempered ductile iron as potential metro wheel material
Abstract Dual phase austempered ductile iron (dual phase ADI) was produced through intercritical austempering with various partially austenitizing temperatures. Partial austenitization within the intercritical interval has a profound influence on the amount and morphology of microstructure in dual phase ADI. The influence of partially austenitizing temperatures on the mechanical properties and rolling-sliding wear performance of dual phase ADI were also investigated. The results show steady increases in the tensile strength and yield strength with increasing partially austenitizing temperature, while the opposite behavior is observed for elongation. The optimum combination including ultimate tensile strength of 746 MPa, impact toughness of 125 J and elongation of 14% is obtained in the dual phase ADI partially austenitized at 810 °C, which is composed of 20% proeutectoid ferrite and 80% ausferrite constituent. Wear behavior could be understood from the manner the hardened layer fragmented and detached. The main wear mechanism under air cooling condition is delamination due to subsurface deformation. Wear rate decreases as matrix hardness increases. According to the results of mechanical and wear tests, dual phase ADI partially austenitized at 810 °C demonstrates superior wear performance as well as relatively reasonable mechanical properties, suggesting the potential of this material for application in metro wheels.
Size-selective neuronal changes in the anterior optic pathways suggest a differential susceptibility to injury in multiple sclerosis.
Axonal damage is found in both acute and chronic lesions of multiple sclerosis. Direct axon counting in post-mortem tissue has suggested that smaller axons might have a greater susceptibility to damage, but methodological limitations have precluded unequivocal interpretation. However, as neuronal and axonal sizes are linked and neuronal changes would be expected with retrograde or transsynaptic degeneration following axon injury, we hypothesized that an alternative strategy for studying this phenomenon would be to define multiple sclerosis-associated changes in neurones. To test this hypothesis, we measured both axonal loss and neuronal size changes in the anterior optic pathway [including the optic nerve (ON), optic tract (OT) and lateral geniculate nucleus] of the brains of eight patients who died with multiple sclerosis and in eight control brains. The ONs and OTs in brains from the multiple sclerosis patients showed a trend to smaller mean cross-sectional areas (ON, multiple sclerosis = 6.84 mm(2), controls = 9.25 mm(2); and OT, multiple sclerosis = 6.45 mm(2), controls = 7.94 mm(2), P = 0.08) and had reduced axonal densities (ON, multiple sclerosis = 1.1 x 10(5)/mm(2), controls = 1.7 x 10(5)/mm(2); and OT, multiple sclerosis = 1.4 x 10(5)/mm(2), controls = 1.8 x 10(5)/mm(2), P = 0.006). Estimated total axonal counts were reduced by 32 (OT)-45% (ON) in the patients relative to controls (ON, multiple sclerosis = 8.1 x 10(5) axons, controls = 14.8 x10(5), P = 0.05; and OT, multiple sclerosis = 9.1 x 10(5), controls = 13.3 x 10(5), P = 0.02). The size distributions of the magnocellular cells in the lateral geniculate nucleus were similar for the two groups, but in multiple sclerosis brains the parvocellular cells were significantly smaller (mean sizes: multiple sclerosis = 226 microm(2), controls = 230 microm(2), P < 0.001) and had a larger variation in size, suggesting a greater proportion of atrophic neurones. Axon loss in the optic nerves of multiple sclerosis patients correlated strongly with measures of increased dispersion of cell sizes in the parvocellular layer (r = 0.8, P < 0.04). These data demonstrate that both atrophy and decreased density contribute to the substantial axonal loss in the anterior visual pathway of these patients. This appears related to a relatively selective atrophy of the smaller neurones of the parvocellular layer in the lateral geniculate nucleus, supporting the hypothesis that smaller axons may be preferentially susceptible to injury in multiple sclerosis.