X-Ray Diffraction Study on the Strain Anisotropy and Dislocation Structure of Deformed Lath Martensite
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DUCTION
THE process of nanoscale structural refinement, occurrence of the late stage (stages IV and V) cold work hardening, enhancement of superplasticity, and monotonic increase of ductility with strength at large strain plastic deformation of metals are concerned with the tremendous storage and annihilation of dislocations.[1–3] Line profile analysis (LPA) of X-ray diffraction (XRD) patterns obtained from the severely deformed metals was used for determination of lattice distortions, mean size of coherently scattering crystalline domains (apparent grain size), total dislocation density, fraction of edge and screw dislocations, screening action of the strain fields associated with dislocations, effective outer cut-off radius of dislocations, etc.[4,5] Some of the present authors previously reported LPAs of XRD patterns obtained from 18Ni (300) maraging steel in the solution-annealed and equal-channel angular-pressed conditions.[6,7] The steel exhibits heavily dislocated martensitic structure in the solution-annealed condition and evolves overwhelmingly to nanostructure by cold deformation. This article was pursued to understand, in detail, the strain anisotropy and evolution of the dislocation structure of the 18Ni (300) maraging steel by making the use of LPA in accordance with the classical and modified Williamson–Hall equations. S. HOSSEIN NEDJAD, Associate Professor, and M.R. MOVAGHAR GARABAGH and S.R. DAMADI, Doctoral Students, are with the Faculty of Materials Engineering, Sahand University of Technology, P.O. Box: 51335-1996, Tabriz, Iran. Contact e-mail: [email protected] F. HOSSEINI NASAB, Doctoral Student, formerly with the Faculty of Materials Engineering, Sahand University of Technology, is now with the Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran. M. NILI AHMADABADI, Professor, is with the School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box: 14395-731, Tehran, Iran. Manuscript submitted June 19, 2010. Article published online February 3, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
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MATERIALS AND METHODS
An 18Ni (300) maraging steel with a chemical composition of Fe-17.95Ni-8.78Co-5.01Mo-0.65Ti (wt pct) was solution annealed for 3.6 ks at 1223 K (950 °C) in a vacuum furnace and then cooled in air. Bars cut from the solution-annealed steel were equalchannel angular pressed at ambient temperature using a die with an inner angle of 90 deg and an outer angle of 0 deg according to route Bc in which the sample is consecutively rotated counterclockwise around its longitudinal axis by 90 deg after each pass. Four passes of angular pressing were performed, storing an equivalent von-Mises strain of 4.62 on the solution-annealed structure. Henceforth, the solution-annealed and equal-channel angular-pressed steels will be referred to as ‘‘LM’’ and ‘‘equal-channel angular pressing’’ (ECAP) steels, respectively. XRD patterns were obtained from the electropolished surfaces of the LM and ECAP steels in a Bruker AX
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