Title page for ETD etd-12142016-092226


Type of Document Master's Thesis
Author Khalfallah, Ibrahim
Author's Email Address ibrahk1@vt.edu
URN etd-12142016-092226
Title Designing Microstructure through Reverse Peritectoid Phase Transformation in Ni3Mo Alloy
Degree Master of Science
Department Materials Science and Engineering
Advisory Committee
Advisor Name Title
Alexander O. Aning Committee Chair
Carlos T. A. Suchicital Committee Member
William T. Reynolds Committee Member
Keywords
  • Ni3Mo Alloy
  • Age Hardening
  • Reverse Peritectoid
  • Bulk Processing
Date of Defense 2016-12-07
Availability unrestricted
Abstract
High-energy ball milling and powder metallurgy methods were used to produce a partially alloyed nickel and molybdenum of γ-Ni3Mo composition (Ni-25at.%Mo). Milled powders were cold-compacted, sintered/solutionized at 1300°C for 100h sintering followed by quenching. Three transformation studies were performed. First, the intermetallic γ-Ni3Mo was formed from the supersaturated solution at temperatures ranging between 600°C and 900°C for up to 100h. The 100% stable γ-Ni3Mo phase was formed at 600°C after 100h, while aging at temperatures ranging between 650°C and 850°C for 25h was not sufficient to complete the transformation. The δ-NiMo phase was observed only at 900°C as cellular and basket strands precipitates.

Second, the reversed peritectoid transformation from γ-Ni3Mo to α-Ni and δ-NiMo was performed. Supersaturated solid solution samples were first aged at 600C for 100h followed by quenching to form the equilibrium γ-Ni3Mo phase. After that, the samples were heat treated between 910°C and 1050°C for up to 10h followed by quenching. Regardless of heat-treatment temperature, samples heat-treated for shorter times exhibited small precipitates of δ-NiMo along and within grain boundaries of α-Ni phase, and it coarsened with time. Third, the transformation from the supersaturated solution α-Ni to the peritectoid two-phase region was performed. The samples were aged between 910°C and 1050°C for up to 10h followed by quenching. Precipitates of δ-NiMo were observed in the α-Ni matrix as small particles and then coarsened with aging time. In all three cases, hardness values increased and peaked in a way similar to that of traditional aging, except that the peak occurred much rapidly in the second and third cases. In the first case, hardness increased by about 113.6% due to the development of the new phases, while the hardness increased by 90.5% and 77.2% in the second and third cases, respectively.

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