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The tensile and compressive properties of six NiAl-base single-crystal alloys have been investigated at temperatures between 77 and 1200 K. The normalized critical resolved shear stresses (CRSS/E) and work-hardening rates (Theta/E) for these alloys generally decreased with increasing temperature. However, anomalous peaks or plateaus for these properties were observed in conventional purity (CPNiAl), Si-doped (NiAl-Si), C-doped low Si (UF-NiAl1), and Mo-doped (NiAl-Mo) alloys at intermediate temperatures (600 to 1000 K). This anomalous behavior was not observed in high-purity, low interstitial material (HP-NiAl). Low or negative strain-rate sensitivities (SRS) also were observed in all six alloys in this intermediate temperature range. Coincident with the occurrence of negative strain-rate sensitivities was the observation of serrated stress-strain curves in the CPNiAl and NiAl-Si alloys. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main specie responsible for strain aging in NiAl is C but indicate that residual Si impurities can enhance the strain aging effects. The corresponding dislocation microstructures at low temperatures (300 to 600 K) were composed of well-defined cells. At intermediate temperatures (600 to 900 K), either poorly defined cells or coarse bands of localized slip, reminiscent of the vein structures observed in low-cycle fatigue specimens deformed in the DSA regime, were observed in conventional purity, Si-doped, and in Mo-doped alloys. In contrast, a well-defined cell structure persisted in the low interstitial, high-purity alloy. At elevated temperatures (greater than or equal to 1000 K), more uniformly distributed dislocations and sub-boundaries were observed in all alloys. These observations are consistent with the occurrence of DSA in NiAl single-crystal alloys at intermediate temperatures.

1Sir Christopher Wren: Scientist, Scholar and Architect

By Lawrence Weaver

This 1923 monograph on the life and work of Sir Christopher Wren was written by Lawrence Weaver, an English architectural writer and editor. Though this recording naturally omits the illustrations and photographs of the printed edition, the text is clear, informative, and interesting. Wren comes to life and his work is knowledgeably described by an expert in architecture. This recording omits the three appendices and the section on authorities and acknowledgments. The text refers many times to a book, Parentalia, published by Wren’s son. This is: Parentalia: Or, Memoirs Of The Family Of The Wrens… The biographies of Sir Christopher Wren and two earlier members of the Wren family are followed by four short tracts by Sir Christopher on the aesthetics and origins of architecture, and by notes on the temple of Diana at Ephesus and on the 'temple of peace' (i.e. basilica of Constantine) and temple of Mars Ultor at Rome. - Summary by david wales