1 Abstract— High temperature shape memory alloys Ti50Ni25Pd25 and Ti50Ni20Pd25Cu5 were developed and characterized for microstructural analysis and transformation temperatures. Both ternary and quaternary alloys were consisted of single martensite phase having second phase precipitates; Ti2Ni (Pd, Cu) randomly distributed along the grain ďouŶdaries. The aǀerage graiŶ size ϭϯμŵ of the terŶary alloy TiϱϬNiϮϱPdϮϱ ǁas iŶĐreased to ϭϲμŵ ďy replaĐiŶg Ni ǁith ϱ at% Cu. Martensite start temperatures of ternary Ti50Ni25Pd25 alloy was increased by 12.5 °C by substitution of Ni with 5 at% Cu. At the same time, transformation heat absorbed and released during forward and reverse martensitic transformation was also increased. The overall results suggested that by addition of 5 at% Cu in place of Ni in Ti50Ni25Pd25 alloy, the microstructure remained almost same, however the transformation temperatures was increased significantly. Index Terms— Phase Transformation Temperature; High Temperature Shape memory alloy; Transformation Heat; Second Phase Precipitate. I. INTRODUCTION Recently the application of NiTi alloys has been extended in the industries like power generation, automotive, oil and gas exploration and aerospace as solid state actuators [1-3]. However, in the mentioned applications, the actuators are to be operated at higher temperature due to high temperature environment (temperature greater than 100°C). Therefore it is needed for the NiTi shape memory alloys to raise their phase transformation temperatures. The transformation temperatures of the NiTi base alloys have been successfully increased by alloying with some elements like Pd, Pt, Au, Zr and Hf [4-9]. However, alloying of Pd and Pt has got relatively more attention as compared to other ternary alloying elements; due to its comparable properties of high work output and good workability like NiTi alloys [10-13]. In addition to these properties, TiNiPd has narrow thermal hysteresis which is desired for fast and active control of actuators [10]. As the actuators are exposed to high temperature environment during operation, therefore, for reliable and long-life performance, it is necessary for the high temperature SMAs to have microstructural stability and resistance to oxidation at elevated temperatures. It should have enough strength in the martensite phase to resist transformation induced plasticity [14]. Apart from that, it should also have high critical stress for slip deformation to resist against recovery, recrystallization and creep in the high temperature austenite phase. The proposed techniques for strengthening the alloy against plastic deformation, thermal driven mechanisms and enhancing the dimensional stability includes solid solution strengthening [15-18], precipitation hardening [19, 20], thermomechanical treatment [20] and annealing after cold working [21]. Alloying of different quaternary elements to TiNiPd has been investigated by very few researchers [10, 15-18] . Solid solution strengthening has been carried out by addition of 0.12 and 0.2 at.% boron in TiNiPd [15, 16]. It has been reported that the ductility of TiNiPd alloy at room and high temperature was improved due to formation of fine TiB2 precipitates. Due to addition of boron, grain size of the alloy was refined and resulted in improved ultimate Effect of Cu addition on microstructure and transformation temperatures of Ti50Ni25Pd25 high temperature shape memory alloys Saifur Rehman , Mushtaq Khan*, , Liaqat Ali, Syed Husain Imran Jaffery, Aamir Mubashar School of Mechanical and Manufacturing Engineering (SMME), National University of Sciences and Technology (NUST), Islamabad, Pakistan saifur.rehman@smme.nust.edu.pk, mkhan@smme.nust.edu.pk, Liaqat@smme.nust.edu.pk, Imran@smme.nust.edu.pk, Aamir@smme.nust.edu.pk