Crystal Structure Defects in Titanium Nickelide after Abc Pressing at Lowered Temperature

Area of Science:

• Materials Science
• Metallurgy
• Condensed Matter Physics

Background:

• The microstructure and defect evolution of shape memory alloys under severe plastic deformation are critical for their functional properties.
• Understanding the interplay between processing parameters and defect structures is essential for tailoring alloy performance.

Purpose of the Study:

• To investigate the impact of warm (573 K) abc pressing on the microstructure and crystal structure defects of Ti49.8Ni50.2 alloy.
• To quantify the changes in grain size, dislocation density, and vacancy concentration as a function of true strain (e).

Main Methods:

• Experimental investigation using warm abc pressing up to a true strain of 9.55.
• Microstructural analysis including grain and subgrain size measurements.
• X-ray diffraction for phase analysis and dislocation density determination.
• Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) for defect characterization.

Main Results:

• A two-level microstructure (grains-subgrains) was observed, with average grain and subgrain sizes decreasing with increasing strain.
• Dislocation density increased significantly, from 10^14 m^-2 to 2x10^15 m^-2, with increasing true strain.
• Positron annihilation spectroscopy revealed that dislocations became the dominant defect type, and monovacancies present in the initial state were eliminated after pressing.

Conclusions:

• Warm abc pressing effectively refines the microstructure of TiNi alloy and introduces a high density of dislocations.
• Dislocations are the primary defects responsible for the observed changes after severe plastic deformation, superseding vacancies.
• The study provides insights into defect engineering in TiNi alloys through severe plastic deformation.