B. Kockar, I. Karaman, Texas A&M University, College Station, TX; C. Yu, E. Rosenzweig, Naval Air System Command, PAX River, MD; J. Sharp, Marlow Industries, Inc., Dallas, TX
Among many shape memory alloys (SMAs), NiTi SMAs are the most utilized due to their large recoverable transformation strain and actuation stress; however, their applications have been still limited due to three main reasons, i.e.: 1) transformation temperatures lower than 100°C, 2) poor cyclic stability, and 3) wide thermal hysteresis. The first issue is usually tackled with the addition of one of Pd, Pt, Zr or Hf to NiTi. 16 at.%Pd substitution for Ni in the equiatomic NiTi alloy increases the transformation temperatures above 100°C and decreases thermal hysteresis, however Pd addition degrades thermal cyclic response due to the decrease in critical shear stress (CSS) for slip. Therefore, to overcome the second problem above in both binary and ternary alloys, our aim in this study is to refine grain size down to nanometer range using severe plastic deformation. This is expected to significantly increase CSS for slip. For this purpose, near equiatomic binary alloy is initially deformed via equal channel angular extrusion (ECAE) at 400°C, 425°C and 450°C to multiple ECAE passes. NiTi-16at.%Pd alloy is also processed via ECAE at 600°C, 500°C and 400°C. These temperatures were selected to be able to process the materials up to four ECAE passes, with the aim of creating a stable microstructure. Thermomechanical response of the alloys during thermal cycling under constant tensile stresses is investigated before and after ECAE. The variations in transformation and irrecoverable strains, transformation temperatures and thermal hysteresis are revealed. The evolution of the microstructure is examined via transmission electron microscopy to compare the extent of grain refinement after different ECAE processes. The effect of grain refinement on the shape memory properties is discussed. It is observed that ECAE processing significantly improves cyclic stability and TEM observations revealed that this improvement is caused by ultrafine and nano-scale grains.
Summary: Among many shape memory alloys (SMAs), NiTi SMAs are the most utilized due to their large recoverable transformation strain and actuation stress levels; however, their applications have been still limited due to three main reasons. These are: 1) transformation temperatures lower than 100°C, 2) poor cyclic stability, and 3) wide thermal hysteresis.
The first issue is usually tackled with the addition of one of Pd, Pt, Zr or Hf elements to NiTi. 16 at. % Pd substitution for Ni in the equiatomic NiTi alloy increases the transformation temperatures above 100°C and decreases thermal hysteresis, however Pd addition degrades thermal cyclic response under stress due to the decrease in critical shear stress (CSS) for slip. It is known that CSS for slip can be significantly increased by refining the grain size to nanometer range. To overcome the second problem above, i.e. poor cyclic stability in both binary and ternary alloys, our aim in this study is to refine grain size down to nanometer range using severe plastic deformation. For this purpose, near equiatomic binary alloy is initially deformed via equal channel angular extrusion (ECAE) at 400°C, 425°C and 450°C to multiple ECAE passes. NiTi-16at. %Pd alloy is also processed via ECAE at 600°C, 500°C and 400°C. These temperatures were selected to be able to process the materials up to four ECAE passes, with the hope of creating a stable microstructure.
Thermomechanical response of the alloys during thermal cycling under constant tensile load levels is investigated before and after ECAE. The variations in transformation and irrecoverable strains, transformation temperatures and thermal hysteresis are revealed. The evolution of the microstructure is examined via transmission electron microscopy to compare the extent of grain refinement after different ECAE processes. The effect of grain size refinement on the shape memory properties is discussed. It is observed that ECAE processing significantly improves thermal cyclic stability and TEM observations revealed that this improvement is caused by ultrafine and nano-scale grains.
