Intermetallic Phase Evolution of Cold-Sprayed Ni-Ti Composite Coatings: Influence of As-Sprayed Chemical Composition

Area of Science:

• Materials Science
• Metallurgy
• Surface Engineering

Background:

• Cold spray (CS) is a deposition method offering advantages for metallic and composite coatings.
• Achieving desired coating composition from blended powders is a challenge in CS.
• Coating composition impacts intermetallic phase evolution during post-spray heat treatment.

Purpose of the Study:

• To investigate the cold spray of composite Nickel-Titanium (NiTi) coatings.
• To examine the influence of feedstock powder composition on as-sprayed coating composition.
• To study the phase evolution of intermetallics during post-spray heat treatment of NiTi coatings.

Main Methods:

• Spraying of three different Nickel-Titanium powder mixtures onto titanium substrates using cold spray.
• Energy Dispersive Spectroscopy (EDS) analysis for as-sprayed coating composition.
• Post-spray annealing treatments at varying temperatures (400, 500, 900 °C) and times (1, 2 h).
• Microstructural investigation and thermal etching analysis.

Main Results:

• High-density, well-dispersed NiTi composite coatings were successfully produced.
• As-sprayed coatings exhibited varying Ni content (10.5, 35.9, 56.9 at.%).
• Ni-rich coatings formed Ni3Ti, Ti2Ni, and NiTi phases after annealing, with NiTi dominating near-equiatomic compositions.
• Ti-rich coatings primarily formed Ti2Ni after annealing.
• NiTi phase exhibited a gradient microstructure attributed to grain refinement and nucleation mechanisms.

Conclusions:

• Cold spray allows for the production of NiTi composite coatings with controllable compositions.
• Feedstock composition significantly influences the types and amounts of intermetallic phases formed post-heat treatment.
• Understanding these relationships is crucial for developing bulk NiTi materials via cold spray.