Transformation-Induced Relaxation and Stress Recovery of TiNi Shape Memory Alloy

Area of Science:

• Materials Science
• Metallurgy
• Solid Mechanics

Background:

• Shape memory alloys (SMAs), particularly TiNi, exhibit unique thermomechanical properties.
• Understanding stress relaxation and recovery is vital for SMA applications.
• Previous studies have focused on macroscopic behavior, with less attention to localized thermal effects.

Purpose of the Study:

• To investigate the transformation-induced stress relaxation and stress recovery in TiNi SMA under stress-controlled subloop loading.
• To analyze the influence of local temperature variations and transformation bands on SMA behavior.
• To provide insights for the design of SMA elements.

Main Methods:

• Experimental investigation using tension tests on TiNi SMA.
• Monitoring local temperature variations and transformation bands on the material surface.
• Applying stress-controlled subloop loading conditions.

Main Results:

• Stress relaxation occurs during loading due to exothermic martensitic transformation (MT) and subsequent temperature decrease at constant strain.
• Stress recovery is observed during unloading due to endothermic reverse transformation and subsequent temperature increase at constant strain.
• High stress rates lead to significant stress relaxation and recovery; stress changes are pronounced initially and gradual at constant strain.

Conclusions:

• TiNi SMA exhibits distinct stress relaxation and recovery mechanisms linked to phase transformations and temperature changes.
• The force output of SMA elements is sensitive to temperature fluctuations, even at constant atmospheric conditions.
• These findings underscore the importance of considering thermomechanical coupling in SMA component design.