Thermal Aging efects on martensitic transformation in shape memory CuAlNi alloys

m SUMMARY Ph.D Thesis THERMAL AGING EFFECTS ON MARTENSITIC TRANSFORMATION IN SHAPE MEMORY CuAlNi ALLOYS AyseAYDOGDU Fırat University Graduate School of Natural and Applied Sciences Department of Physics 1995, Page: 121 Metastable |3-phases of copper-based shape memory alloys are very sensitive to the thermal aging effects and applied thermal treatments can change the relative stability of both martensite and parent phases. Two CuAlNi alloys investigated in the present study exhibit thermoelastic martensite transformation and have ordered D03 and layered 1 8R crystal structure in parent and product phase conditions, respectively. Aging and thermal cycling effects and martensite stabilization on two Cu-ll%Al-3,82%Ni and Cu-ll%Ai-3,38%Ni ( in weight ) have been investigated, using x-ray difractogram, hardness measurements, electron diffraction, differential scanning calorimetry ( DSC ) technique and metallographic observations. X-ray powder diffractograms and electron diffraction patterns reveal that both alloys have M18R martensite structure. Stabilization is directly related to disordering in martensitic state and the spacing differences ( Ad ) between some selected pairs of diffraction planes of type (h\kxT) and (h2k20 satisfying the relation (/»f ~ A|)/3 = (*| ~ *f)/4 for 18R martensites reflect the degree of ordering in martensite. Reverse transition starting ( Ag ) and finishing ( Af ) temperatures change depending on the ordering degree of martensite and aging effects. It has been observed that Ad values for the used alloys have decreased at the earlier stages of aging and reached a constant value. This means that the alloy reaches stabilization after a short aging duration. Vickers hardness numbers have also decreased at the earlier stages of aging and reached a constant value. Key Words : CuAlNi alloys, thermoelastic martensitic transformation, shape memory effect, ageing effects, transformation temperatures, interplane distance difference (Ad), Vickers hardness.