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Feasibility Study of Adding Manganese to Nitinol Shape Memory Alloy by Diffusion Coating in Manganese, Manganese-Nickel, and Manganese-Titanium Powder Beds and Evaluation of its Effect on Microstructure, Phase Composition, Chemical Behavior, and Mechanical Properties

Mohammadi Varmazyar, Reza | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58239 (07)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Sadrnezhaad, Khatiboleslam
  7. Abstract:
  8. The addition of manganese to Nitinol alloys can lead to enhanced mechanical properties and improved corrosion resistance. However, conventional melt-based alloying methods face significant challenges, primarily the high vapor pressure of manganese and its strong tendency toward oxidation. This research, for the first time, investigates the feasibility and optimization of a solid-state manganese diffusion coating process to improve the surface properties of Nitinol and overcome these challenges. To this end, three distinct powder packs were utilized: pure manganese, a manganese-nickel mixture (70:30 wt.%), and a manganese-titanium mixture (same ratio). Wire samples were subjected to an optimized heat treatment cycle (heating to 1050°C over 180 min, with holding stages at 1050°C for 45 min, 975°C for 105 min, and 650°C for 120 min) under an inert argon atmosphere. The binary powders were prepared via mechanical alloying, which resulted in physical homogenization and particle size reduction without the formation of new phases. The coated samples were characterized using XRD, FESEM-EDS, ICP-OES, and Vickers microhardness testing.ICP-OES results confirmed the successful diffusion of manganese in all three beds; the highest manganese content (approx. 1.9 wt.%) was observed in the manganese-nickel pack, and the lowest (0.14 wt.%) in the manganese-titanium pack. XRD analysis revealed the formation of MnNi and MnTi intermetallic phases in all coated samples. In the manganese-nickel pack, an MnO oxide phase was also identified, whereas in the manganese-titanium pack, a thick and dominant layer of TiO₂ alongside a highly crystalline MnTi phase (indicated by sharp and well-defined diffraction peaks) was formed. The coating obtained from the manganese-titanium pack, with a thickness of about 207 µm, exhibited the highest structural uniformity and the highest surface hardness (455 HV, an increase of over 120% compared to the pristine sample). The pure manganese pack created a coating with a thickness of about 52 µm and a hardness of 402 HV and had the most significant effect on the hardness of the wire's central region. The manganese-nickel pack, despite high manganese diffusion, showed the lowest surface hardness (349 HV) among the coated samples due to structural heterogeneity and the presence of oxide phases.This research demonstrates that powder-pack diffusion coating is an effective method for the surface alloying of Nitinol with manganese, and the manganese-titanium pack composition offers superior results in terms of coating hardness and uniformity
  9. Keywords:
  10. Nitinol Alloy ; Diffusion Coating ; Manganese ; Microstructure ; Mechanical Properties ; Shape Memory Alloy ; Powder Pack

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