Dogan, Ebubekir (2010-08). CoNiGa High Temperature Shape Memory Alloys. Master's Thesis. Thesis uri icon


  • Shape memory alloys (SMAs) are an important class of smart materials that have
    the ability to remember a shape. Current practical uses of SMAs are limited to below
    100 degrees C which is the limit for the transformation temperatures of most commercially
    successful SMAs such as NiTi and Cu-based alloys. In recent years, the CoNiGa system
    has emerged as a new ferromagnetic shape memory alloy with some compositions
    exhibiting high martensitic transformation temperatures which makes CoNiGa a
    potential high temperature shape memory alloy (HTSMA). In this study, the
    microstructural evolution and martensitic transformation characteristics of CoNiGa
    (mainly Co46Ni27Ga27 and Co44Ni26Ga30 in at.percent) HTSMAs were investigated in as-cast
    and hot-rolled conditions as a function of different heat treatments. Heat treatment
    conditions were selected to introduce single, two, and three phase structures, where two
    precipitate phases (ductile Y and hard Y') do not martensitically transform. Calorimetry,
    X-ray analysis, scanning and transmission electron microscopy, thermo-mechanical
    process and cycling techniques are applied to understand the structural and chemical
    factors influencing the thermal stability and transformation characteristics. The main findings include improvement of ductility, most cyclically stable compositions with
    narrow transformation hysteresis (<40 degrees C) and transformation temperatures in the range
    of 100 degrees C to 250 degrees C, formation of new phases and their effects, and associated
    compositional changes in the matrix, on the transformation temperatures and on the
    microstructural evolution. In addition, Ms temperature depends linearly on the valence
    electron concentration (e/a) of the matrix, only if the Ga content is constant, and the
    samples with narrow transformation hysteresis demonstrate reversible martensitic
    transformation in constant-stress thermal cycling experiments.

publication date

  • August 2010