Microstructure And Strain Recovery Properties Of Fe-doped Ni-Mn-Ga Glass-coating Microwires

In this thesis,Fe-doped Ni-Mn-Ga based microwires were fabricated using the modified Taylor-wire method and the influences of processing parameters on wire diameter,phase structure and crystalline morphology were investigated.The structure evolution,phase transition temperature and strain recovery properties at room temperature of different Fe-doped microwires were analysed.Subsequent stress-relief annealing and mechanical cycling treatments at room temperature were applied to the as-cast wires in order to tailor their strain recovery properties.The attributes for Ni-Mn-Ga based glass-coating microwires were influenced by wire drawing speed and the cooling distance.The drawing speed directly affects the diameter,i.e.,faster speed results in a smaller microwire diameter and a lower crystallization degree.The cooling distance has profound influence on the microwire structure:the grain size has been restricted to a large extent as the distance of the cooling water decreases.Further analysis indicates that it is also possible for the Ni-Mn-Ga based microwires to form an amorphous structure at extremely small cooling distances.The effect of Fe-doping on the microstructure,phase structure,phase transition temperature and mechanical properties of Ni-Mn-Ga based microwires hase been systematically explored.With the increase of Fe content,the phase structure changes from austenite to martensite at room temperature and the corresponding transformation temperature increases.A remarkable increase of fracture strain of wires is obtained after a small portion of Fe is introduced.Ni55.07Mn15.76Ga23.4Fe5.77 microwires are featured with a stress platform corresponding to the martensite transformation at room temperature,e.g.,an increase in the Fe content leads to a"rubber-like behaviour" at room temperature,with,nevertheless,rather limited strain recovery performance.On the other hand,superelasticity is absent in non-doped Ni-Mn-Ga microwire that is attributed to the low transformation temperature and fracture strength.Annealing treatment is helpful in improving the strain recovery ratio of Ni55.07Mn15.76Ga23.4Fe5.77 microwire,which displays excellent superelasticity.The loading-unloading cycling training decreases the phase transformation stress to a stable value,and an improved strain recovery rate was attained with an absorbed energy around 4.3 MJ/m3 per cycle.These properties warrant its application as functional filler for composite materials with tailored eng:ineering properties.