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Evidence for superplasticity in a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion
Shahmir, Hamed,He, Junyang,Lu, Zhaoping,Kawasaki, Megumi,Langdon, Terence G. Elsevier 2017 Materials science & engineering. properties, micro Vol.685 No.-
<P><B>Abstract</B></P> <P>A CoCrFeNiMn high-entropy alloy was processed by high-pressure torsion to produce a grain size of ~10nm and then tested in tension at elevated temperatures from 773 to 1073K using strain rates in the range from 1.0×10<SUP>−3</SUP> to 1.0×10<SUP>−1</SUP> s<SUP>−1</SUP>. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of >600% at a testing temperature of 973K. 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.</P>
Shahmir, Hamed,Nili-Ahmadabadi, Mahmoud,Huang, Yi,Jung, Jai Myun,Kim, Hyoung Seop,Langdon, Terence G. Elsevier 2018 Materials science & engineering. properties, micro Vol.734 No.-
<P><B>Abstract</B></P> <P>A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19' phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19' phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition.</P>
Shahmir, Hamed,Mousavi, Tayebeh,He, Junyang,Lu, Zhaoping,Kawasaki, Megumi,Langdon, Terence G. Elsevier 2017 Materials science & engineering. properties, micro Vol.705 No.-
<P><B>Abstract</B></P> <P>A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673K and the results show that the strength increases gradually with increasing straining up to ~ 1GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073K for 60min and it is shown that the hardness increases slightly due to precipitation to 773K and then decreases to 1073K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle <I>σ</I>-phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773K produces a very high yield strength of ~ 1015MPa and an ultimate strength of ~ 1080MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.</P>
Shahmir, H.,He, J.,Lu, Z.,Kawasaki, M.,Langdon, T.G. Elsevier Sequoia 2016 Materials science & engineering. properties, micro Vol.676 No.-
<P>A CoCrFeNiMn high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) under 6.0 GPa pressure up to 10 turns at room temperature. It is shown that there is a gradual evolution in hardness with increasing numbers of turns but full homogeneity is not achieved even after 10 turns. Micro hardness measurements reveal that the material reaches a saturation hardness value of similar to 4.41 GPa and in this condition the microstructure shows exceptional grain refinement with a grain size of similar to 10 nm. An ultimate strength value of similar to 1.75 GPa and an elongation to fracture of similar to 4% were obtained in a sample processed for 5 turns. The nanostructured HEA was subjected to post-deformation annealing (PDA) at 473-1173 K and it is shown that the hardness increases slightly to 773 K due to precipitation and then decreases up to 1173 K due to a combination of recrystallization, grain growth and a dissolution of the precipitates. The formation of brittle precipitates, especially sigma-phase, at 873 and 973 K significantly reduces the ductility. Short-term annealing for 10 min at 1073 K prevents grain growth and leads to a combination of high strength and good ductility including an ultimate tensile strength of similar to 830 MPa and an elongation to failure of similar to 65%. (C) 2016 Elsevier B.V. All rights reserved.</P>
Shape memory effect in nanocrystalline NiTi alloy processed by high-pressure torsion
Shahmir, H.,Nili-Ahmadabadi, M.,Huang, Y.,Myun Jung, J.,Seop Kim, H.,Langdon, T.G. Elsevier Sequoia 2015 Materials science & engineering. properties, micro Vol.626 No.-
A NiTi alloy was processed by high-pressure torsion for 10 turns followed by post-deformation annealing at 673K for various times. An anneal for 60min gave a nanocrystalline microstructure with a superior shape memory effect and an improvement of more than 40% over the initial state.
Ramzan Muhammad,Shahmir Nazia,Alotaibi Hammad,Ghazwani Hassan Ali S,Muhammad Taseer 한국CDE학회 2022 Journal of computational design and engineering Vol.9 No.1
This exploration aims to study the comparison of heat transfer capabilities of two nanofluid oblique, steady stagnation-point flow combinations comprising single-walled carbon/water (SWCNHs/water) and multiwalled carbon nanotube/water (MWCNTs/water) toward a stretching surface influenced by nonlinear thermal radiation employing the Xue model. This envisaged comparison model is inimitable and still scarce in the literature. Relying on the Tiwari–Das nanofluid model, a mathematical framework is constructed. The system of partial differential equations is converted using suitable transformations into an ordinary differential system of three equations, which is evaluated numerically using the bvp4c method. The physical behavior of significant parameters and their graphical representation are thoroughly examined. The results show that the SWCNHs/water nanofluid outperforms the MWCNTs/water nanofluid. It is further witnessed that SWCNH nanoparticle contained nanofluid has considerably greater thermal radiation than MWCNT nanoparticles. The envisaged model is also validated by comparing it with a published study.