Resistivity and Transmission Electron Microscopy Investigations of Ordering Transformation in Stoichiometric Ni 2 (Cr 0.
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TRODUCTION
THE Ni-Cr-Mo–based alloys, such as HASTELLOYS C-4, C-22, and C-276, HASTELLOY* S, *HASTELLOY and HAYNES are trademarks of Stoody Deloro Stellite, Inc., Industry, CA.
HAYNES* 242, Alloy 625, and Alloy 686, are used for applications requiring very high corrosion and oxidation resistance.[1,2] Some of these alloys are considered as candidate materials for long-term storage applications of radioactive waste up to a period of 10,000 years in geological repository sites at temperatures varying in the range of 323 K to 573 K (50 C to 300 C).[3,4] These alloys are normally used in solid solution condition and are generally considered nonhardenable by conventional aging treatments.[2] Prolonged aging of these alloys in the temperature range of A. VERMA, formerly Guest Scientist, with the Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, Institut fu¨r angewandte Materia1 forschung, D-14109 Berlin, Germany, is now Scientist, with the Mechanical Metallurgy Division, Bhabha Atomic Research Center, Mumbai 400 085, India. Contact e-mail: barc.amit@gmail. com JUNG B. SINGH, Scientist, is with the Mechanical Metallrugy Division, Bhabha Atomic Research Center. M. SUNDARARAMAN, formerly with the Mechanical Metallurgy Division, Bhabha Atomic Research Center, is now Professor, School of Engineering Sciences and Technology, University of Hyderabad, Gachibowli, Hyderabad 500046, India. NELIA WANDERKA, Scientist, is with the HelmholtzZentrum Berlin fu¨r Materialien und Energie GmbH, Institut fu¨r angewandte Materia1 forschung. Manuscript submitted October 14, 2011. Article published online June 6, 2012 3078—VOLUME 43A, SEPTEMBER 2012
673 K to 973 K (400 C to 700 C) results in the precipitation of a long-range ordered Ni2(Cr, Mo) phase (which has Pt2Mo prototype oI6 structure) from the disordered face-centered-cubic (fcc) matrix.[5–10] This precipitation reaction occurs homogeneously with no preferential nucleation at grain boundaries or twin boundaries. In the temperature range of 1023 K to 1173 K (750 C to 900 C), various carbides and topologically-close-packed phases precipitate in the matrix as well as at grain boundaries. Their evolution has been extensively reported.[3,11–13] The formation of carbides leads to sensitization of grain boundaries promoting intergranular failure. The continuous evolution of the long-range order (LRO) from the short-range order (SRO) state attracted numerous researchers to investigate Ni-Cr-Mo–based alloy systems.[9–16] All these researchers used transmission electron microscopy (TEM), X-ray diffraction, and differential scanning calorimetry[9–11,14–16] for their investigations on the formation and dissolution of ordered Ni2(Cr, Mo) precipitates in Ni-Cr-Mo alloys. Resistivity measurements were also employed in the past for the investigations of SRO and LRO in alloy systems other than Ni-Cr-Mo.[17–23] Karmazin’s group carried out experimental work to determine the solvus temperature and the details of phase boundary of Ni2(Cr, Mo) structure in binary Ni-Cr and ternary Ni-Cr-Mo (
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