Microstructural evolution during heating of CNT/Metal Matrix Composites processed by Severe Plastic Deformation.

Area of Science:

• Materials Science
• Metallurgy
• Nanotechnology

Background:

• Nickel matrix composites reinforced with carbon nanotubes (Ni/CNT) are of interest for advanced applications.
• Severe plastic deformation (SPD) techniques like high pressure torsion (HPT) can significantly alter material properties.
• Understanding the thermal stability of these composites is crucial for their practical use.

Purpose of the Study:

• To investigate the microstructural evolution and thermal stability of Ni/CNT composites after HPT.
• To examine the influence of carbon nanotube (CNT) composition on the composite's properties.
• To study the formation and dissolution of nickel carbide phases.

Main Methods:

• Fabrication of Ni/CNT composites via solid-state processing.
• Application of severe plastic deformation using high pressure torsion (HPT).
• Microstructural and thermal analysis using differential scanning calorimetry (DSC), high-temperature X-ray diffraction (HT-XRD), and electron backscattered diffraction (EBSD).

Main Results:

• Irreversible damage to CNTs during HPT led to the formation and dissolution of a metastable nickel carbide (Ni₃C) phase.
• Ni/CNT composites exhibited improved thermal stability compared to pure nickel processed under identical conditions.
• The final grain size was found to be dependent on the CNT volume fraction (V_CNT) following a V_CNT^-1/3 relationship, resulting in ultrafine grains.

Conclusions:

• Severe plastic deformation induces changes in Ni/CNT composites, including CNT damage and carbide formation.
• The presence of CNTs enhances the thermal stability of nickel matrix composites.
• CNT volume fraction is a critical factor in controlling the final grain size and achieving ultrafine microstructures.