| Abstrakt: | This PhD thesis describes the influence of y-phase particles on martensitic transformation temperatures, compression strength, hardness and Young modulus of Ni-Co-Mn-In ferromagnetic shape memory alloys. The presented alloys were prepared by induction melting, annealed at 1173 K for 24 hours and then slowly cooled with the fumace. Finally, the alloys were heated up to 1173 K, kept 20 minutes and quenched into
iced water. The prepared alloys differed in chemical composition. Changes in nickel and cobalt content allowed obtaining varied values of electron concentration ratio e/a in a range from 7.9 to 8.1. The structure of the as cast A, B and C alloys was determined as 14M modulated martensite. The D alloy with the highest value of the valence electron concentration ratio e/a revealed existence of about 18% of the Ni- and Co-rich and
In-depleted phase, which was indexed by the x-ray technique as y-phase. The applied heat treatment caused additionally precipitation of the y-phase particles in B and C alloys. The amount of y-phase particles depended on the applied heat treatment and alloy chemical composition. Slow cooling with the fumace caused precipitation of the large amount of y-phase particles. The quantitative analysis results showed that the amount of y-phase
in B alloy was around 11%, while for C alloys was about 15%. For D alloy the amount of y-phase increased from 18% to 25%. Quenching at 1173 K caused dissolving the part of y-phase particles and following fast cooling did not allow to y-phase precipitation. In FSMA the phase transformation temperatures strongly depend on valence electron concentration ratio (e/a). Increasing the e/a value caused increase in the martensitic
transformation temperatures which was confirmed in all studied alloys. However, the y-phase particles had also an influence on martensitic transformation temperatures shifting them to lower values. Total transformation suppression was observed for alloy with more than 18% of y-phase. Moreover, the existence of large number of the y-phase particles caused increasing of the compression strength, from about 400 MPa for single-phase alloy
to of about 1000 MPa for D alloy containing about 25% of y-phase. The indentation results showed that the values of Young modulus and hardness of parent phase and martensite were similar to each other, however these parameters had significantly higher values for y-phase particles. The observed pile ups for y-phase particles indicated their better plasticity in comparison to the matrix (either parent phase or martensite). Therefore, the alloy s containing the y-phase particles are less brittle in comparison to single phase alloys which
may improve the practical applications of the FSMA. |