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Terence G Langdon - One of the best experts on this subject based on the ideXlab platform.
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evidence for superplasticity in a cocrfenimn high entropy alloy processed by high pressure torsion
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Hamed Shahmir, M Kawasaki, Junyang He, Z P Lu, Terence G LangdonAbstract:Abstract A CoCrFeNiMn high-entropy alloy was processed by high-pressure torsion to produce a grain size of ~10 nm and then tested in tension at elevated temperatures from 773 to 1073 K using strain rates in the range from 1.0×10−3 to 1.0×10−1 s−1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic Elongations with a maximum Elongation of >600% at a testing temperature of 973 K. It is concluded that the formation of precipitates and the sluggish diffusion in the HEA inhibit grain growth and contribute to a reasonable stability of the fine-grained structure at elevated temperatures. The results show the activation energy for flow matches the anticipated value for grain boundary diffusion in nickel but the strain rate sensitivity is low due to the occurrence of some grain growth at these high testing temperatures.
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developing superplasticity and a deformation mechanism map for the zn al eutectoid alloy processed by high pressure torsion
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011Co-Authors: M Kawasaki, Terence G LangdonAbstract:Abstract A Zn–22% Al eutectoid alloy was processed by high-pressure torsion (HPT) for 1, 3 and 5 turns at room temperature to produce an ultrafine grain size of ∼350 nm. Tensile testing at a temperature of 473 K gave excellent superplastic properties with Elongations to failure up to a maximum of 1800% at an imposed strain rate of 1.0 × 10 −1 s −1 : this is within the range of high strain rate superplasticity and represents the highest Elongation recorded to date for a specimen processed by HPT. It is shown that the experimental data are in excellent agreement with a deformation mechanism map constructed for a temperature of 473 K.
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developing grain refinement and superplasticity in a magnesium alloy processed by high pressure torsion
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008Co-Authors: Zenji Horita, Terence G LangdonAbstract:Experiments were conducted on a Mg–9% Al alloy to evaluate the microstructural characteristics and the tensile properties at elevated temperatures after processing by high-pressure torsion (HPT) at room temperature and at 423 K. Ultrafine grain sizes were achieved by processing samples in both an extruded and a cast condition. The results demonstrate the viability of using HPT as a processing technique for achieving significant grain refinement in magnesium alloys which are not processed easily by equal-channel angular pressing (ECAP). Superplastic ductilities were achieved in tensile testing at a temperature of 473 K with a maximum measured Elongation of 810%. In general, higher superplastic Elongations were achieved after processing by HPT at 423 K because of the development of some limited internal cracking when processing at room temperature.
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achieving exceptional superplasticity in a bulk aluminum alloy processed by high pressure torsion
Scripta Materialia, 2008Co-Authors: Zenji Horita, Terence G LangdonAbstract:Bulk specimens of an aluminum–magnesium–scandium alloy, in the form of small cylinders, were processed by high-pressure torsion at room temperature. Following processing, the microstructure was inhomogeneous with larger grains in the center and ultrafine grains of ∼130 nm at the periphery. Tensile testing after processing revealed the potential for achieving exceptional superplastic Elongations but the measured Elongations depended upon the positions of the specimens within the cylinder. The highest tensile Elongation recorded in these experiments was 1600%.
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microstructural and mechanical characteristics of az61 magnesium alloy processed by high pressure torsion
Materials Transactions, 2008Co-Authors: Yosuke Harai, Zenji Horita, Terence G Langdon, Kenji KanekoAbstract:Experiments were conducted on an AZ61 magnesium alloy to evaluate the microstructural characteristics and the mechanical properties after processing by High-Pressure Torsion (HPT). The results show that processing by HPT produces excellent grain refinement with average grain sizes of ?0.22 and ?0.11 ?m after processing at 423 K and room temperature, respectively. Tensile testing after HPT revealed the potential for achieving superplastic Elongations with a maximum recorded Elongation of 620% when testing at a temperature of 473 K. Using microhardness measurements, it is demonstrated that the the microstructure gradually evolves with increasing torsional straining in HPT so that ultimately there is a reasonably homogeneous structure across the disk.
David H Price - One of the best experts on this subject based on the ideXlab platform.
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rna polymerase ii Elongation control
Annual Review of Biochemistry, 2012Co-Authors: Qiang Zhou, David H PriceAbstract:Regulation of the Elongation phase of transcription by RNA Polymerase II (Pol II) is utilized extensively to generate the pattern of mRNAs needed to specify cell types and to respond to environmental changes. After Pol II initiates, negative Elongation factors cause it to pause in a promoter proximal position. These polymerases are poised to respond to the positive transcription Elongation factor, P-TEFb, and then enter productive Elongation only under the appropriate set of signals to generate full length properly processed mRNAs. Recent global analyses of Pol II and Elongation factors, mechanisms that regulate P-TEFb involving the 7SK snRNP, factors that control both the negative and positive Elongation properties of Pol II and the mRNA processing events that are coupled with Elongation are discussed.
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controlling the Elongation phase of transcription with p tefb
Molecular Cell, 2006Co-Authors: Matija B Peterlin, David H PriceAbstract:The positive transcription Elongation factor b (P-TEFb) is a cyclin-dependent kinase that controls the Elongation phase of transcription by RNA polymerase II (RNAPII). This process is made possible by the reversal of effects of negative Elongation factors that include NELF and DSIF. In complex organisms, Elongation control is critical for the regulated expression of most genes. In those organisms, the function of P-TEFb is influenced negatively by HEXIM proteins and 7SK snRNA and positively by a variety of recruiting factors. Phylogenetic analyses of the components of the human Elongation control machinery indicate that the number of mechanisms utilized to regulate P-TEFb function increased as organisms developed more complex developmental patterns.
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p tefb a cyclin dependent kinase controlling Elongation by rna polymerase ii
Molecular and Cellular Biology, 2000Co-Authors: David H PriceAbstract:The Elongation phase of transcription by RNA polymerase II is one of the many steps during the generation of mature mRNAs that is subject to regulation. Shortly after initiation, RNA polymerase II comes under the control of negative transcription Elongation factors, generally termed N-TEFs, and enters abortive Elongation (51). During this postinitiation process, only short transcripts are generated that may be prematurely terminated. These short transcripts arise from transcription of many genes, including c-myb, c-myc, c-fos, HSP70, and the human immunodeficiency virus (HIV) long terminal repeat (LTR), and are normally subject to rapid degradation (3, 63). Escape from the action of N-TEF requires the action of at least one positive transcription Elongation factor (P-TEF), eventually identified as P-TEFb (52). P-TEFb allows the transition into productive Elongation, producing long transcripts from which mRNAs are derived. In this way, the fraction of initiating RNA polymerase II molecules that produce full-length transcripts is controlled by a selection process that occurs early in the Elongation phase of the transcription cycle. After the transition is made into productive Elongation, the efficiency of Elongation may be influenced by additional factors, including S-II, TFIIF, ELL, and elongin (62, 65).
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p tefb a cyclin dependent kinase controlling Elongation by rna polymerase ii
Molecular and Cellular Biology, 2000Co-Authors: David H PriceAbstract:The Elongation phase of transcription by RNA polymerase II is one of the many steps during the generation of mature mRNAs that is subject to regulation. Shortly after initiation, RNA polymerase II comes under the control of negative transcription Elongation factors, generally termed N-TEFs, and enters abortive Elongation (51). During this postinitiation process, only short transcripts are generated that may be prematurely terminated. These short transcripts arise from transcription of many genes, including c-myb, c-myc, c-fos, HSP70, and the human immunodeficiency virus (HIV) long terminal repeat (LTR), and are normally subject to rapid degradation (3, 63). Escape from the action of N-TEF requires the action of at least one positive transcription Elongation factor (P-TEF), eventually identified as P-TEFb (52). P-TEFb allows the transition into productive Elongation, producing long transcripts from which mRNAs are derived. In this way, the fraction of initiating RNA polymerase II molecules that produce full-length transcripts is controlled by a selection process that occurs early in the Elongation phase of the transcription cycle. After the transition is made into productive Elongation, the efficiency of Elongation may be influenced by additional factors, including S-II, TFIIF, ELL, and elongin (62, 65).
Zenji Horita - One of the best experts on this subject based on the ideXlab platform.
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developing grain refinement and superplasticity in a magnesium alloy processed by high pressure torsion
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008Co-Authors: Zenji Horita, Terence G LangdonAbstract:Experiments were conducted on a Mg–9% Al alloy to evaluate the microstructural characteristics and the tensile properties at elevated temperatures after processing by high-pressure torsion (HPT) at room temperature and at 423 K. Ultrafine grain sizes were achieved by processing samples in both an extruded and a cast condition. The results demonstrate the viability of using HPT as a processing technique for achieving significant grain refinement in magnesium alloys which are not processed easily by equal-channel angular pressing (ECAP). Superplastic ductilities were achieved in tensile testing at a temperature of 473 K with a maximum measured Elongation of 810%. In general, higher superplastic Elongations were achieved after processing by HPT at 423 K because of the development of some limited internal cracking when processing at room temperature.
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achieving exceptional superplasticity in a bulk aluminum alloy processed by high pressure torsion
Scripta Materialia, 2008Co-Authors: Zenji Horita, Terence G LangdonAbstract:Bulk specimens of an aluminum–magnesium–scandium alloy, in the form of small cylinders, were processed by high-pressure torsion at room temperature. Following processing, the microstructure was inhomogeneous with larger grains in the center and ultrafine grains of ∼130 nm at the periphery. Tensile testing after processing revealed the potential for achieving exceptional superplastic Elongations but the measured Elongations depended upon the positions of the specimens within the cylinder. The highest tensile Elongation recorded in these experiments was 1600%.
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microstructural and mechanical characteristics of az61 magnesium alloy processed by high pressure torsion
Materials Transactions, 2008Co-Authors: Yosuke Harai, Zenji Horita, Terence G Langdon, Kenji KanekoAbstract:Experiments were conducted on an AZ61 magnesium alloy to evaluate the microstructural characteristics and the mechanical properties after processing by High-Pressure Torsion (HPT). The results show that processing by HPT produces excellent grain refinement with average grain sizes of ?0.22 and ?0.11 ?m after processing at 423 K and room temperature, respectively. Tensile testing after HPT revealed the potential for achieving superplastic Elongations with a maximum recorded Elongation of 620% when testing at a temperature of 473 K. Using microhardness measurements, it is demonstrated that the the microstructure gradually evolves with increasing torsional straining in HPT so that ultimately there is a reasonably homogeneous structure across the disk.
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superplastic forming at high strain rates after severe plastic deformation
Acta Materialia, 2000Co-Authors: Zenji Horita, Minoru Furukawa, Minoru Nemoto, A J Barnes, Terence G LangdonAbstract:An Al-3% Mg-0.2% Sc alloy was fabricated by casting and subjected to severe plastic deformation through equal-channel angular pressing to a strain of ~8. The grain size after pressing was ~0.2 ?m and increased to ?1.1 ?m when holding at 673 K for 10 min. Very high tensile Elongations were recorded at 673 K with a maximum Elongation of ~2280% when testing with an initial strain rate of 3.3 × 10?2 s?1. The strain rate sensitivity was measured as ~0.5 at strain rates in the vicinity of 10?2 s?1. Small disks were cut from the rods after pressing and these disks were successfully formed into domes at 673 K using a biaxial gas-pressure forming facility and forming times up to a maximum of 60 s. Measurements of the local thicknesses at selected points around the domes revealed reasonably uniform thinning which is consistent with the high strain rate sensitivity of this alloy.
Joh T Lis - One of the best experts on this subject based on the ideXlab platform.
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getting up to speed with transcription Elongation by rna polymerase ii
Nature Reviews Molecular Cell Biology, 2015Co-Authors: Iris Jonkers, Joh T LisAbstract:Recent advances in sequencing techniques that measure nascent transcripts and that reveal the positioning of RNA polymerase II (Pol II) have shown that the pausing of Pol II in promoter-proximal regions and its release to initiate a phase of productive Elongation are key steps in transcription regulation. Moreover, after the release of Pol II from the promoter-proximal region, Elongation rates are highly dynamic throughout the transcription of a gene, and vary on a gene-by-gene basis. Interestingly, Pol II Elongation rates affect co-transcriptional processes such as splicing, termination and genome stability. Increasing numbers of factors and regulatory mechanisms have been associated with the steps of transcription Elongation by Pol II, revealing that Elongation is a highly complex process. Elongation is thus now recognized as a key phase in the regulation of transcription by Pol II.
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breaking barriers to transcription Elongation
Nature Reviews Molecular Cell Biology, 2006Co-Authors: Abbie Saunders, Leighto J Core, Joh T LisAbstract:Recent studies have challenged the view that transcription is predominantly regulated at the level of RNA polymerase II recruitment to promoters. Transcription is also regulated at the level of Elongation by factors that act directly upon RNA polymerase II or that manipulate the chromatin environment. Hundreds of protein factors participate in transcription and its regulation in eukaryotes. Many of these proteins regulate specific genes by targeting upstream promoter regions, whereas a smaller but mechanistically diverse set of factors functions at most genes during RNA polymerase II (Pol II) Elongation. These Elongation factors can affect mRNA production at particular stages and in different ways during transcription. Some factors act directly on Pol II, whereas others manipulate the chromatin environment.
John T Lis - One of the best experts on this subject based on the ideXlab platform.
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control of transcriptional Elongation
Annual Review of Genetics, 2013Co-Authors: Hojoong Kwak, John T LisAbstract:Elongation is becoming increasingly recognized as a critical step in eukaryotic transcriptional regulation. Although traditional genetic and biochemical studies have identified major players of transcriptional Elongation, our understanding of the importance and roles of these factors is evolving rapidly through the recent advances in genome-wide and single-molecule technologies. Here, we focus on how Elongation can modulate the transcriptional outcome through the rate-liming step of RNA polymerase II (Pol II) pausing near promoters and how the participating factors were identified. Among the factors we describe are the pausing factors--NELF (negative Elongation factor) and DSIF (DRB sensitivity-inducing factor)--and P-TEFb (positive Elongation factor b), which is the key player in pause release. We also describe the high-resolution view of Pol II pausing and propose nonexclusive models for how pausing is achieved. We then discuss Pol II Elongation through the bodies of genes and the roles of FACT and SPT6, factors that allow Pol II to move through nucleosomes.
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breaking barriers to transcription Elongation
Nature Reviews Molecular Cell Biology, 2006Co-Authors: Abbie Saunders, Leighto J Core, John T LisAbstract:Recent studies have challenged the view that transcription is predominantly regulated at the level of RNA polymerase II recruitment to promoters. Transcription is also regulated at the level of Elongation by factors that act directly upon RNA polymerase II or that manipulate the chromatin environment.
