Hip replacement implants fabricated using the ASTM F-75 alloy sometimes fail in a sudden catastrophic way. In general, fractures start at microstructural defects subjected to stress-corrosion under chemical attack by body fluids. In this paper the results of a study on the effect of casting parameters on the microstructure of ASTM F-75 are presented. The preheating mold temperature and the liquid temperature were varied between 900 and 1000 degrees C, and 1410 and 1470 degrees C, respectively. Optimum static strength and ductility were obtained when shrinkage microporosity and the volume fraction of M23C6 "eutectic" carbides precipitated at grain boundaries were minimized by increasing the preheating mold temperature to 1000 degrees C and by using intermediate pouring temperatures of 1455 degrees C. Under these casting conditions, however, the solidification rates are low, leading to large grain sizes, which, in turn, reduce the strength of the material under dynamic loading conditions. The volume fraction of the M23C6 "blocky" carbides appears to be independent of the casting conditions; however, their size and spatial distributions determine the strength of the as-cast alloys.

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ABSTRACT:The influence of citric acid on the hydration and strength development of a calcium sulphoaluminate cement was investigated. Cement pastes were prepared by mixing calcium sulphoaluminate (C 4 A 3 Ŝ) with 15, 20 and 25wt% of hemihydrate (CŜH 0.5 ). Citric acid was added as a retarder at 0 and 0.5wt%. The samples were cured at 20 °C for periods of time from 1 to 28 days to evaluate their compressive strength and to characterize the hydration products by scanning electron microscopy and X-ray diffraction. Calorimetric curves showed that the retarding agent considerably decreases the heat release rate and the quantity of total heat released. The main product after the curing was ettringite (C 6 AŜ 3 H 32 ). The morphology of this phase consisted of long and thin needles growing radially on the cement grains. Samples with 15wt% of hemihydrate and 0.5wt% of citric acid developed the highest compressive strength (70 MPa) at 28 days of curing. RESUMEN: Efecto de la adición de ácido cítrico y la cantidad de yeso sobre las propiedades del cemento de sulfoaluminato de calcio.Se investigó el efecto del ácido cítrico sobre la hidratación y propiedades mecánicas de un cemento de sulfoaluminato de calcio. El C 4 A 3 Ŝ se mezcló con 15, 20 y 25% e.p. de hemihidrato (CŜH 0.5 ). Se agregó ácido cítrico como retardante en 0 y 0.5% e.p. Las muestras fueron curadas a 20 °C por periodos de 1 a 28 días para realizar mediciones de resistencia a la compresión y caracterizar los productos de hidratación mediante microscopía electrónica de barrido y difracción de rayos X. Las curvas de calorimetría mostraron ue el ácido cítrico disminuye la velocidad de liberación de calor y la cantidad de calor liberado durante la hidratación. La resistencia a la compresión alcanzó un máximo de 70 MPa en muestras con 15% e.p. de hemihidrato y 0,5% e.p de ácido cítrico. Los resultados muestran a la etringita (C 6 AŜ 3 H 32 ) como principal producto de hidratación. Se observa a esta fase con morfología acicular creciendo sobre las partículas de cemento.

We conducted a retrospective study with 750 peritoneal dialysis (PD) patients in a Spanish multicenter registry between 1993 and 1999 to analyze comorbidity and mortality in type 1 diabetes (T1D), type 2 diabetes (T2D) and nondiabetic (ND) patients. 163 patients (21.7%) were diabetic – 96 T1D (58.8%) and 67 T2D (42.2%) – while 587 were not (78.3%). Different comorbidity factors such as the presence of cardiovascular disease, age over 70 and dyslipidemia at the start of PD were analyzed as well as the incidence of peritonitis, the peritonitis-free interval, need for hospitalization, mortality rate, early mortality rate, survival curves (log rank) and the impact factor (Cox) on mortality for the different variables. The comorbidity index (number of comorbidity factors when starting the treatment) and the peritonitis incidence were higher for T2D. Hospitalization rates were similar, but mortality rates were higher for T2D and early mortality rates (death during the 1st year of treatment) were higher for T1D. The actuarial survival curves showed a higher mortality for T2D with no differences between ND and T1D after adjustment for age. The mortality odds ratio was 1.78 for T2D and 1.13 for T1D, differences which were not significant after adding age over 70 and cardiovascular disease to the variables analyzed. Our results show that associated comorbidity is the most important difference between ND, T1D and T2D. While cardiovascular comorbidity is responsible for the higher percentage of early mortality found in T1D when compared to ND, both age and cardiovascular disease are responsible for the higher comorbidity and mortality found in T2D.