Influence of casting heterogeneities on microstructure and mechanical properties of austempered ductile iron (ADI)

An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructureand mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutecticductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a corelessinduction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austemperingheat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with imageanalysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanicalproperties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicatethat as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanicalproperties of ADI. During austempering, non uniform distribution of graphite nodules and segregation ofalloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellularregions displayed inhomogeneous microstructures containing eutectic carbides and martensite.Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfacesshowed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidificationcarbides

Thymoma followed by aplastic anemia: two different responses to immunosuppressive therapy

Aplastic anemia is an uncommon complication of thymoma and is extremely infrequent after the surgical removal of a thymic tumor. Aplastic anemia is a result of marrow failure and is characterized by peripheral pancytopenia and severely depressed marrow cellularity; it may be an autoimmune manifestation of thymoma. As thymoma-associated hematological dyscrasias, which include pure red cell aplasia, aplastic anemia and myasthenia gravis, are supposed to be of immunologic origin, two cases of very severe aplastic anemia following the resection of lymphocytic thymomas treated with immunosuppression are herein presented