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Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel
Yao, M.J.,Welsch, E.,Ponge, D.,Haghighat, S.M.H.,Sandlö,bes, S.,Choi, P.,Herbig, M.,Bleskov, I.,Hickel, T.,Lipinska-Chwalek, M.,Shanthraj, P.,Scheu, C.,Zaefferer, S.,Gault, B.,Raabe, D. Elsevier 2017 ACTA MATERIALIA Vol.140 No.-
<P>We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 degrees C, nano-sized kappa-carbides form,, as characterized by TEM arid APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate.& para;& para;In comparison to the as-quenched kappa-free state, the precipitation of kappa-carbides leads to a significant increase in yield strength (similar to 480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the kappa-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111 }<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to under stand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of kappa-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, kappa-free state. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved</P>
Yao, M.J.,Dey, P.,Seol, J.B.,Choi, P.,Herbig, M.,Marceau, R.K.W.,Hickel, T.,Neugebauer, J.,Raabe, D. Elsevier Science 2016 Acta materialia Vol.106 No.-
<P>We report on the investigation of the off-stoichiometry and site-occupancy of kappa-carbide precipitates within an austenitic (gamma), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the kappa-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L'12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the kappa/gamma mismatch, which facilitates occupation of Al sites in kappa-carbide by Mn atoms (Mn-Al(gamma) anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
Mattern, N.,Han, J.H.,Pradeep, K.G.,Kim, K.C.,Park, E.M.,Kim, D.H.,Yokoyama, Y.,Raabe, D.,Eckert, J. Elsevier Sequoia 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.607 No.-
<P>The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)(100-x)Ag-x melts was investigated (x = 0-40 at.%). Fully glassy alloys were obtained for 0 <= x <= 20 at.% Ag, which are characterized by a homogeneous microstructure without any indication of phase separation. For 30 <= x <= 40 at.% Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for 30 <= x <= 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid phase of the ternary Ag-Cu-Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the microstructure of ternary (Cu0.5Zr0.5)(100-x)Ag-x glasses (x = 10, 20 at.%). (C) 2014 Elsevier B.V. All rights reserved.</P>