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A columnar-grain variant of single-crystal RENÉ N4 has been directionally solidified (DS) over a range of conditions in order to assess the possible benefits of the use of liquid metal-enhanced cooling for large cross-sectional castings. Castings were solidified at a rate of 2.5 mm/min using conventional radiation cooling and at rates between 2.5 and 8.5 mm/min using liquid-metal cooling (LMC) with tin as a cooling medium. Thermocouples inserted in the casting directly measured thermal gradients during solidification. The LMC process exhibited higher gradients at all withdrawal rates. The higher thermal gradients resulted in a refined structure measurable by the finer dendrite-arm spacing. Additionally, the conventionally cast material exhibited several freckle-type defects, while none were observed in the liquid-metal-cooled castings.

There have been numerous efforts to develop creep-resistant materials strengthened by incoherent particles at high temperatures and stresses in response to future energy needs for steam turbines in thermal-power plants. However, the microstructural instability of the incoherent-particle-strengthened ferritic steels limits their application to temperatures below 900 K. Here, we report a novel ferritic alloy with the excellent creep resistance enhanced by coherent hierarchical precipitates, using the integrated experimental (transmission-electron microscopy/scanning-transmission-electron microscopy, in-situ neutron diffraction, and atom-probe tomography) and theoretical (crystal-plasticity finite-element modeling) approaches. This alloy is strengthened by nano-scaled L21-Ni2TiAl (Heusler phase)-based precipitates, which themselves contain coherent nano-scaled B2 zones. These coherent hierarchical precipitates are uniformly distributed within the Fe matrix. Our hierarchical structure material exhibits the superior creep resistance at 973 K in terms of the minimal creep rate, which is four orders of magnitude lower than that of conventional ferritic steels. These results provide a new alloy-design strategy using the novel concept of hierarchical precipitates and the fundamental science for developing creep-resistant ferritic alloys. The present research will broaden the applications of ferritic alloys to higher temperatures.

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Collected from a Late Permian to Early Triassic sedimentary section in the ZhongliangMountain of Chongqing, Southwest China, sixty marine carbonate samples were measured for the 87 Sr/ 86 Sr ratios, and corresponding evolution curve was constructed. The concentrations of SiO 2 , CaO, MgO, Mn and Sr are used to evaluate reservation of strontium isotopic composition for original seawater and the credibility of the dissolution method for sample preparation. The results show that most of the samples (except seven samples with the Mn/Sr ratios higher than 2) contain the original geochemistry signatures of ancient seawater. Compared to the published 87 Sr/ 86 Sr ratios from the Late Permian to Early Triassic, our database reported here is the largest and the curve constructed is the most complete. The strontium isotopic curve from Late Permian to Early Triassic is consistent globally and exhibits a general trend of steady increase during this period. The minimum of 87 Sr/ 86 Sr ratios (0.707011) occurs in the Late Permian (30 m in thickness below the Permian-Triassic boundary), and the maximum (0.708281), near the Early-Middle Triassic boundary. The lack of land plants and the rapid continental weathering result in the increase of 87 Sr/ 86 Sr ratios during the interval. The Permian-Triassic boundary in Zhongliang Mountain Section has been accepted internationally. The 87 Sr/ 86 Sr ratios of six samples near the boundary vary from 0.70714 to 0.70715 with an average of 0.70714, which is consistent with the value of 0.70715 (samples are from articulate brachiopod shells) from Korte et al. published in 2006 (within the error range in experiment). Accordingly, the strontium isotope composition in the Permian-Triassic boundary in this paper is of global significance. It can be confirmed that the 87 Sr/ 86 Sr ratios of the seawater in the Permian-Triassic transition are in the range of 0.70714-0.70715.

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