Cooling precipitation and strengthening study in powder metallurgy superalloy U720LI

The excellent mechanical properties of powder metallurgy (P/M) superalloys strongly depend on the microstructure, such as grain size, and morphology and size distribution of the γ’ precipitates. The microstructure is, in turn, determined by the heat treatment, viz., solution annealing, quenching, and subsequent aging. To study the effect of the quenching process, two types of quenching methods were used to produce different quenched microstructures in a UDIMET 720LI (U720LI) alloy. One was a continuous quenching method, where samples were colled along linearly controlled cooling profiles, each at a fixed cooling rate. This test studied the effect of cooling rate on the size of cooling γ’ precipitates (formed during quenching) and the consequent strengthening effect. The other test was the interrupted quenching test, which allowed tracking the growth of cooling γ’ precipitates with decreasing temperature during quenching at a given cooling rate. The strengthening response at each interrupt temperature was also studied. Results from the continuous cooling tests showed that the relationship between the size of the cooling γ’ precipitate and the cooling rate obeys a power law, with an exponential being about 0.35. The tensile strength was found to increase linearly with the cooling rate. Strengthening due to the subsequent aging treatment occurred regardless of cooling rates. The interrupted cooling tests showed that γ’ growth is a linear function of decreasing temperature for a given cooling rate. A nonmontonic degradation of tensile strength against interrupt temperature was discovered.     

The yeast telomere length counting machinery is sensitive to sequences at the telomere-nontelomere junction

Saccharomyces cerevisiae telomeres consist of a continuous 325 +/- 75-bp tract of the heterogeneous repeat TG1-3 which contains irregularly spaced, high-affinity sites for the protein Rap1p. Yeast cells monitor or count the number of telomeric Rap1p molecules in a negative feedback mechanism which modulates telomere length. To investigate the mechanism by which Rap1p molecules are counted, the continuous telomeric TG1-3 sequences were divided into internal TG1-3 sequences and a terminal tract separated by nontelomeric spacers of different lengths. While all of the internal sequences were counted as part of the terminal tract across a 38-bp spacer, a 138-bp disruption completely prevented the internal TG1-3 sequences from being considered part of the telomere and defined the terminal tract as a discrete entity separate from the subtelomeric sequences. We also used regularly spaced arrays of six Rap1p sites internal to the terminal TG1-3 repeats to show that each Rap1p molecule was counted as about 19 bp of TG1-3 in vivo and that cells could count Rap1p molecules with different spacings between tandem sites. As previous in vitro experiments had shown that telomeric Rap1p sites occur about once every 18 bp, all Rap1p molecules at the junction of telomeric and nontelomeric chromatin (the telomere-nontelomere junction) must participate in telomere length measurement. The conserved arrangement of these six Rap1p molecules at the telomere-nontelomere junction in independent transformants also caused the elongated TG1-3 tracts to be maintained at nearly identical lengths, showing that sequences at the telomere-nontelomere junction had an effect on length regulation. These results can be explained by a model in which telomeres beyond a threshold length form a folded structure that links the chromosome terminus to the telomere-nontelomere junction and prevents telomere elongation.     

Contribution of flow deformation to the shrinkage of injection molded PVC

The viscoelastic nature of PVC cannot be ignored during the injection molding process. Strains imposed on the compound during the injection stage display recovery dynamics that are indicative of the temperature at which the strains were imposed. A mathematical model, employing “approximate” time-temperature modulus relationships, provided the basis for interpreting the shrinkage data. A distribution map of the various relative temperatures attained within the part during injection was determined.     

Analysis of the effects of temporally distant events on school behavior.

The purpose of this study was to assess the relationship between temporally distant antecedent events (TDEs) and the frequency and function of aggressive and out-of-seat behaviors occurring in the school setting. A checklist of potential TDEs was developed for each of three participants (aged 4–5 yrs) with behavior problems, based on interviews with caregivers and school personnel. Each day, parents completed the checklist and a functional analysis of problem behavior was conducted. For each participant, initial functional analyses yielded undifferentiated results. When data from the functional analysis were graphed according to the occurrence or nonoccurrence of a specific TDE, results demonstrated a momentary change in both the frequency and function of the target behavior. These results are consistent with previous literature that describes certain antecedent events as establishing operations. Discussion focuses on (a) determining which antecedent events serve as potential establishing operations; (b) how these events interact to alter the functional properties of temporally distant problem behavior; and (c) the implications for school-based interventions. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Characterization of commercial double-walled carbon nanotube material: composition, structure, and heat capacity

A purified commercial double-walled carbon nanotube (DWCNT) sample was investigated by transmission electron microscopy (TEM), thermogravimetry (TG), and Raman spectroscopy. Moreover, the heat capacity of the DWCNT sample was determined by temperature-modulated differential scanning calorimetry in the range of temperature between −50 and 290 °C. The main thermo-oxidation characterized by TG occurred at 474 °C with the loss of 90 wt% of the sample. Thermo-oxidation of the sample was also investigated by high-resolution TG, which indicated that a fraction rich in carbon nanotube represents more than 80 wt% of the material. Other carbonaceous fractions rich in amorphous coating and graphitic particles were identified by the deconvolution procedure applied to the derivative of TG curve. Complementary structural data were provided by TEM and Raman studies. The information obtained allows the optimization of composites based on this nanomaterial with reliable characteristics.