A study was conducted on the influence of grain boundary elements (GBE) on several P/M nickel base superalloys. Evaluations included microstructural response to chemistry and processing, tensile, creep, rupture, and fatigue crack growth testing (FCG). Boron was found to be the most influential element in the study, with increased boron leading to increased grain boundary triple point void formation and tendency for Thermally Induced Porosity (TIP). Boron reduced creep life but increased both rupture life and rupture ductility and slightly reduced hold time crack growth rates. Carbon also influenced microstructure by slightly refining grain size, and slightly reduced creep and hold time crack growth rates. Hafnium had slight impacts on yield strength and hold time crack growth rates. Magnesium overall had negligible effects. Tantalum had several significant effects, improving creep life and reducing crack growth rates. Overall, the results confirm that careful control and optimization of grain boundary chemistries and grain boundary microstructure are necessary to achieve maximum performance of P/M superalloys.
Although the Third International Mathematics and Science Study found that most 8th‐grade students like science and feel that they are doing well in it [Geary, 1997], fewer than one‐quarter of U.S. adults can define the term DNA and only one in 11 knows what a molecule is [Augustine, 1998]. Hence the motivated, bright young people described by the study somehow become scientific illiterates despite the best efforts of elementary, secondary, and college‐level instructors.
This phenomenon has prompted various investigations into reasons why students have difficulty learning science. One possibility is illustrated by the famed video [Shapiro etal, 1988] showing that most of the graduating Harvard seniors surveyed confidently attributed the cause of the seasons to changes in the distance between the Earth and Sun rather than to the Earth's tilt. They had a clear conception of the answer, but it was wrong.
A statistically based approach was completed to assess the effect of microstructural (and associated heat treatment processing parameters) on the 649 o C capability of two advanced powder metallurgy (P/M) superalloys. The results showed that microstructure and processing play a key role in achieving a balance of mechanical properties. For a desired combination of good high temperature capability the results indicate that a supersolvus heat treatment followed by a fast cooling rate is desired. In addition, selection of a stabilization heat treat cycle would depend upon the balance of creep and crack growth capability desired. The process described above would nominally produce an ASTM 6 grain size with reasonably fine cooling ' and aging ' as defined by the stabilization cycle.
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