Tseng, Li-Wei (2015-05). Microstructure and Superelastic Response of Iron-Based Shape Memory Alloys. Doctoral Dissertation. Thesis uri icon

abstract

  • Fe43.5Mn34Al15Ni7.5 shape memory alloys have attracted attention as potential superelasticity alloys in server environments, capable of exhibiting good superelastic behavior with up to 5% recoverable strain over a wide temperature range from -196?C to 240?C. The objective of this work is to investigate the effects of martensite variants, aging heat treatment, crystal orientations and grain size on the superelastic properties. Although the theoretical transformation strain of single crystal with the <100> orientation in tension (26.5%) is higher than it is in compression (10.5%), the compression sample shows a better recoverable strain than that in the tension sample from the experimental results. The differences in reversibility and the shape of the stress-strain curves under tension and compression are attributed to the lower number of martensite variants activated under tension as compared to compression. The effect of aging has profound influences on the size and volume fraction of precipitates, transformation temperatures, stress-temperature phase diagram, recoverable strain and stress hysteresis. Increasing the aging times improves the superelastic strain in compression and decreases it in tension. The reason for these is related to the selection of martensite variants. The crystal orientations affect the superelastic properties such as the stress-temperature phase diagram and transformation strain. The differences in the slope of the stress-temperature curves and critical stress in different orientation are attributed to the transformation strain and resolved shear stress factors, respectively. Aging heat treatment decreases the orientation effect on slope of stress-temperature curve. The sheet is brittle (high hardness values) after being hot-rolled which causes to appear after the cold-rolled experiment. The hot-rolled sheet was annealing at 900?C for 1 hour to obtain two phases (bcc+fcc) and ductility (small hardness values). Therefore, cold-rolling of the annealing sheet is practical up to 76%. The large grain size can be obtained by repetition of the heat treatment. Increasing the grain sizes improves the superelastic strain due to reduction of grain boundary constraints.
  • Fe43.5Mn34Al15Ni7.5 shape memory alloys have attracted attention as potential superelasticity alloys in server environments, capable of exhibiting good superelastic behavior with up to 5% recoverable strain over a wide temperature range from -196?C to 240?C. The objective of this work is to investigate the effects of martensite variants, aging heat treatment, crystal orientations and grain size on the superelastic properties.

    Although the theoretical transformation strain of single crystal with the <100> orientation in tension (26.5%) is higher than it is in compression (10.5%), the compression sample shows a better recoverable strain than that in the tension sample from the experimental results. The differences in reversibility and the shape of the stress-strain curves under tension and compression are attributed to the lower number of martensite variants activated under tension as compared to compression.

    The effect of aging has profound influences on the size and volume fraction of precipitates, transformation temperatures, stress-temperature phase diagram, recoverable strain and stress hysteresis. Increasing the aging times improves the superelastic strain in compression and decreases it in tension. The reason for these is related to the selection of martensite variants.

    The crystal orientations affect the superelastic properties such as the stress-temperature phase diagram and transformation strain. The differences in the slope of the stress-temperature curves and critical stress in different orientation are attributed to the transformation strain and resolved shear stress factors, respectively. Aging heat treatment decreases the orientation effect on slope of stress-temperature curve.

    The sheet is brittle (high hardness values) after being hot-rolled which causes to appear after the cold-rolled experiment. The hot-rolled sheet was annealing at 900?C for 1 hour to obtain two phases (bcc+fcc) and ductility (small hardness values). Therefore, cold-rolling of the annealing sheet is practical up to 76%. The large grain size can be obtained by repetition of the heat treatment. Increasing the grain sizes improves the superelastic strain due to reduction of grain boundary constraints.

publication date

  • May 2015