Microstructure, Texture, and Orientation-Dependent Flow Behavior of Binary Ni-16Cr and Ni-16Mo Solid Solution Alloys
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THE Ni-Cr and Ni-Mo binary alloys are subsets of several commercial Ni-based solid solution alloys such as Inconel 617, Inconel 625, Inconel 686, Hastelloy S, Hastelloy C, Hastelloy C-4, Hastelloy C-22, Hastelloy C-276, Nicrofer 5923, etc.[1,2] The Ni-Cr-Mo alloy systems are predominantly used in wrought forms such as plates, sheets, strips, extruded tubes, wires, and flanges due to excellent hot/cold deforming capability with moderate-to-high work-hardening rate and weldability. These wrought forms are produced by thermomechanical processing which involves hot working (forging, extrusion, rolling), cold deformation, and heat treatments. Thermomechanical processing is likely to introduce specific microstructure as well as texture which in turn governs the anisotropy of mechanical properties. Therefore, an understanding of binary alloys which are a subset of above-mentioned alloys in terms of composition, thermomechanical processing, and properties is quite important. This can effectively be utilized K.K. MEHTA, Senior Scientific Officer-I, is with the RDAQA (M), c/o Defence Research and Development Organization, Hyderabad 500058, India. PRANTIK MUKHOPADHYAY and A.K. SINGH, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500058, India. Contact e-mail: singh_ashok3@rediffmail. com R.K. MANDAL, Professor, is with the Department of Metallurgical Engineering IIT (BHU), Varanasi 221005 India. Manuscript submitted September 10, 2014. Article published online June 10, 2015 3656—VOLUME 46A, AUGUST 2015
to tailor the compositions and processing of multicomponent alloys with optimized properties. In addition to being a subset of several commercial alloys, the Ni-Cr and Ni-Mo alloys form ideal systems for solid solution strengthening. The contents of Cr and Mo lie in the range of 10 to 20 (wt pct) in all the alloys mentioned above apart from other alloying elements. The limits of solubility of Cr and Mo in Ni are 35 to 40 wt pct and about 20 wt pct, respectively.[3] In addition, the size difference between the Ni-Mo is quite large than that of the Ni-Cr since the atomic radii (A˚) of Ni, Cr, and Mo are 1.25, 1.28, and 1.36, respectively.[4] A linear relation between flow stress and lattice parameter change is obeyed for any single solute element (Cr, Mo, W, Fe, Co, and Cu) in nickel.[5] Fleischer has concluded that 20 wt pct of Cr in Ni imparts change in lattice parameter and flow stress around 0.033 and 157 MPa while only 4 wt pct of Mo in Ni causes change around 0.035 and 167 MPa.[6] Apart from this, the valency difference (Dz) between Ni-Cr is less than 1/2 to that of the Ni-Mo. As a result, the equal alloying in terms of weight percent of both the elements in Ni displays large difference in stacking fault energy.[7] Fleischer has also suggested that with increase in electron to atom (e/a) ratio the valency difference increases and causes decrease in SFE of face-centered cubic (fcc) alloys.[6] As a result, equal wt pct of Mo with higher e/a ratio as compared to Cr in Ni results in lower value of SFE. The S
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