Transformation Mechanism of Preprecipitation NiMn Nanostructures
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interfacial dislocation dipoles lying on {011}a planes and Burgers vectors of a0 011 a type. During further aging, the b-NiMn clusters transformed martensitically into fct NiMn (h phase) precipitates as envisaged by the characteristic microtwinning texture. Considering the anomalous elastic softness of b-NiMn alloy, the transformation was plausibly assumed to take place by the localized soft-mode mechanism in which the structural instability of clusters is stimulated by the strain field of the interfacial dislocations. Computerized calculations in accordance with the phenomenological theory of martensite crystallography indicated that microtwinning of martensitic h precipitates occurs in a self-accommodating manner. Transformation of very small b clusters at earlier stages of aging is thermodynamically hampered by the interfacial energy barrier. DOI: 10.1007/s11661-013-2010-6 The Minerals, Metals & Materials Society and ASM International 2013
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INTRODUCTION
AGING of supersaturated solid solutions has been known as a strengthening mechanism of metallic materials for a long time.[1–3] The aging reactions initiate with a ‘‘preprecipitation’’ stage recognized with the formation of nanostructured solute clusters. The clusters have traditionally been referred to as Guinier–Preston (GP) zones. A GP zone is defined as a region of high solute concentration and usually of local lattice distortions, which has identical crystal lattice and coherent interface with the surrounding matrix phase.[2] GP zones are transient and often undergo sequential transformations during further aging, e.g., GP fi c¢¢ fi c¢ fi c transformation of Be-rich zones in Cu-Be alloys.[4,5] Motivating insights are still prompting the precipitation reactions[6,7] and, more overwhelmingly, the phenomenon
MOHAMAD REZA MOVAGHAR GARABAGH, Ph. D. Student, and HABIB HAMED ZARGARI, Instructor, are with the Advanced Materials Research Center, Department of Materials Engineering, Sahand University of Technology, P. O. Box: 51335-1996, Tabriz, Iran. SYAMAK HOSSEIN NEDJAD, Associate Professor, is with the Advanced Materials Research Center, Department of Materials Engineering, Sahand University of Technology, and also with the Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran, P. O. Box: 14395-731, Tehran, Iran. Contact e-mail: [email protected] MAHMOUD NILI AHMADABADI, Professor, is with the Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran. Manuscript submitted February 5, 2013. Article published online September 28, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
is currently adopted as the fabrication method of embedded nanostructures.[8] An important class of ultrahigh strength, precipitation hardening alloys is concerned with maraging steels.[9,10] Commercialized maraging steels contain high Ni, Co, Mo, Ti, etc., and are aged in the martensitic condition. A variety of meta-stable precipitates such as Ni3Mo (orthorhom
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