Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 2: The role of grain size and premelting on ice deformation at high homologous temperature

The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine-grained ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser-grained ice with a partial (great circle) girdle or multi-maxima CPO. In this study, the grain-size-sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting (liquid-like layer along the grain boundaries) on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine-grained layers are about an order of magnitude higher than in the much coarser-grained layers. The dominant deformation mechanisms, based on the flow relation of Goldsby and Kohlstedt (2001), between the layers is also different, with basal slip rate limited by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the finer-grained layers, while GBS-limited creep and dislocation creep (basal slip rate limited by non-basal slip) contribute both roughly equally to bulk strain in the coarse-grained layers. Due to the large difference in microstructure between finer-grained ice and the coarse-grained ice at premelting temperatures (T>262 K), it is expected that the fine-grained layers deform at high strain rates, while the coarse-grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between impurity-rich and low-impurity ice can have important consequences for ice dynamics close to the bedrock.

144 Monensin effects on beef calves receiving limited creep-feeding supplementation

Monensin is a feed additive used to increase performance of beef cattle; however, the effects on cattle grazing warm-season perennial forages with limited levels of concentrate are inconsistent. The objective of this study was to test the effects of monensin on performance of beef calves receiving limited creep-feeding supplementation. The experiment was conducted in Ona, FL, from April to August (112 d) 2018. The creep-feeding supplementation level was 400 g of soybean meal/d. Calves were supplemented 3 days per week, Monday, Wednesday, and Friday, thus the daily supplementation level was multiplied by 7 d and divided by 3 feeding days. Treatments were monensin (20 g/kg of the expected DMI) or control (no monensin), distributed in a randomized complete block design with four replicates. Twenty four cow-calf pairs (415±29 and 162±35 kg, respectively) were randomly distributed in eight limpograss (Hemarthria altissima) pastures (experimental units; 1.0 ha/pasture; 3 cow-calf pairs/pasture). Pastures were grazed with a continuous and fixed stocking rate. There was no difference in herbage mass (mean = 2,250 kg/ha, P =0.52, SE = 120) and herbage allowance (HA, mean = 1.2 kg DM/kg LW, P = 0.52, SE = 0.04) between treatments. In addition, there was no effect of monensin on calf average daily gain (mean = 0.77 kg/d, P = 0.27, SE = 0.04), cow body condition score (BCS, mean = 5.1, P = 0.15, SE = 0.15), and calf coccidia infestation (mean = 0.18 log egg count, P = 0.98, SE = 0.08). Calves receiving monensin consumed a lesser (P < 0.01) proportion of the supplement than control in the first 13 h after supplementation (76 vs. 95%). Monensin was not effective to increase performance of beef calves receiving limited creep-feeding supplementation.

Using a composite flow law to model deformation in the NEEM deep ice core, Greenland: Part 2 the role of grain size and premelting on ice deformation at high homologous temperature

The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine grained glacial ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser grained Eemian ice with a partial girdle type of CPO. In this study, the grain size sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine grained glacial layers are about an order of magnitude higher than in the much coarser grained Eemian layers. The dominant deformation mechanisms between the layers is also different with basal slip accommodated by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the glacial layers, while GBS-limited creep and dislocation creep (basal slip accommodated by non-basal slip) contribute both roughly equally to bulk strain in the coarse grained layers. Due to the large difference in microstructure between the impurity-rich glacial ice and the impurity-depleted Eemian ice at premelting temperatures (T>262 K), it is expected that the fine grained layers deform mainly by simple shear at high strain rates, while the coarse grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between glacial and interglacial ice at temperatures just below the melting point can have important consequences for ice dynamics close to the bedrock.

Study of Creep Strength for P91 Steels after Short Term Overheating above Ac3 Temperature

Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M23C6 precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental overshoot in temperature during PWHT. The short excursion to high temperature beyond Ac3 would have resulted in the formation of deleterious phases, for example, soft α-ferrite which has poor creep strength and hard martensite which has a low toughness. In this study, the degraded specimens with soft α ferrite as a result of cooling transformation from 900°C are proven to have a limited creep rupture life where the creep rupture strength dropped remarkably after 1000 hours. As the peak temperature increased to 950°C and 1000°C, hard and brittle martensite was formed on cooling. The creep specimens were found to exhibit better creep strength; most probably the creep behavior was improved by the tempering effect at 600°C during creep tests. Nevertheless, despite the tempering which might have improved the toughness slightly, the high temperature creep rupture stress still had dropped approximately 40%, as compared to the virgin alloys in the range of rupture time from 1,000 hours to 10,000 hours.

Creep Characteristics of Mudstone under Devastating or Integrated States by Uniaxial Tests

Based on general instantaneous strength test and uniaxial creep tests under devastating or integrated states, strain characteristics of mudstone in different stages have been determined. Analyzing strain features of mudstone developing in every stage of different tests, evolving creep law of mudstone under uniaxial devastating state may be validated, which axial limited creep strain under the decided loading standard is equal to that value at uniaxial complete strain-stress curve rearwards ultimate strength. Relationship between designed loading stress and its corresponding creep strain can be linear in uniaxial creep test of mudstone under integrated state, while must not be a determinate secant of uniaxial complete strain-stress curve. Studying results present that terminal creep strain of rock with certain loading levels and under devastating state can be only corresponded with an exclusive point of traditional uniaxial complete strain-stress curve rearwards the ultimate strength, and the extended limited creep courses of mudstone will answer for Boltzmann function.

Effect of SPS processing temperature on the microstructure and properties of a Ti49Al47Cr2Nb2 alloy

The sintering of TiAl alloys by the Spar Plasma Sintering (SPS) technique is investigated in the present paper. Compactions are conducted between 1100°C and 1225°C on a Ti49Al47Cr2Nb2 powder. Single phased and lamellar microstructures are generated at low and high temperatures, respectively. The former exhibits enhanced tensile properties at room temperature but at the expense of a limited creep resistance. On the other hand, the latter suffers from a poor ductility.The deformation systems which are activated are determined by post-mortem transmission electron microscopy. The behavior of the single phased-alloy is analyzed in terms of the Hall-Petch law.