Transformation-superplasticity of metals and metal matrix composites

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.Includes bibliographical references (p. 156-162).The work covers transformation superplasticity of metals, alloys and metal matrix composites. Fundamental studies of transformation superplasticity in unreinforced metals, which either deform plastically or by creep, form the basis of further investigations in metal matrix composites. Experiments and analytical modeling are complemented by numerical analysis. The transformation superplastic behavior is related to microstructure and chemical composition. Based on an existing linear theory, a non-linear model is developed and applied to the experimental data. Numerical methods are used to model the stress-, strain and temperature evolution during the phase transformation. The results are in good agreement with the experiment and analytical predictions. First, transformation superplasticity of iron and iron-TiC composites is demonstrated with strains of 450% and 230% respectively. The reduction of the transformation superplasticity in the composites is attributed to the dissolution of TiC in iron and effect which is shown for iron-carbon alloys. Effects of transient primary creep, ratchetting and partial transformation through the ferrite-austenite phase field are examined. Second, transformation superplasticity of zirconium is demonstrated for the first time with a strain of 270% without fracture. Partial transformation resulting from high cycle frequencies is analyzed and related to material properties and cycle characteristics. Finally, nickel aluminide with unstabilized zirconia particulates shows significant higher strain rates upon thermal cycling as compared to the unreinforced matrix. Although, the fracture strain of 23% is below the superplastic limit, the composite shows a high strain rate sensitivity of m = 0.71, which is a necessary characteristic of transformation superplasticity.by Peter Zwigl.Ph.D

Vitamin D Status in Bipolar Disorder

Vitamin D status may impact acute affective symptomatology and the severity of symptoms in patients with bipolar disorder (BD). Therefore, this cross-sectional study analyzed 25(OH)D, 24,25(OH)2D, and the vitamin D metabolite ratio (VMR) in BD and correlated the results with clinical affective symptomatology and functionality. The inactive precursor 25(OH)D, and its principal catabolite 24,25(OH)2D, were measured simultaneously with a validated liquid chromatography–tandem mass spectrometry method in 170 BD outpatients and 138 healthy controls. VMR was calculated as follows: VMR = 100×(24,25(OH)2D/25(OH)D). The psychometric assessment comprised: Beck Depression Inventory-II, Hamilton Depression Rating Scale, Young Mania Rating Scale, Global Assessment of Functioning, and number of suicide attempts. We did not find a significant difference between patients and controls in the concentrations of 25(OH)D and 24,25(OH)2D. Additionally, the VMR was comparable in both groups. The calculations for the clinical parameters showed a negative correlation between the Young Mania Rating Scale and 24,25(OH)2D (r = −0.154, p = 0.040), as well as the Young Mania Rating Scale and the VMR (r = −0.238, p = 0.015). Based on the small effect size and the predominantly euthymic sample, further exploration in individuals with manic symptoms would be needed to confirm this association. In addition, long-term clinical markers and an assessment in different phases of the disease may provide additional insights