Characterization of Ni–Ti shape memory alloys prepared by powder metallurgy Adnan S. Jabur a,⇑ , Jafar T. Al-Haidary b , Emad S. Al-Hasani b a Department of Mechanical Engineering, University of Kerbala, Kerbala, Iraq b Department of Materials Engineering, University of Technology, Baghdad, Iraq article info Article history: Received 10 March 2013 Received in revised form 3 May 2013 Accepted 6 May 2013 Available online 14 May 2013 Keywords: Powder metallurgy Biocompatibility Pseudoelasticity Shape memory effect Ni–Ti alloy abstract In this research, study of the effect of compacting pressure and Cr and Al additions on the properties of Ni–Ti shape memory alloys prepared by powder metallurgy compared with the commercial cast- deformed reference Archwires were conducted. It was found that; the transformation temperatures, phase structures and hardness of the samples prepared by powder metallurgy resemble to that of com- mercial Thermal reference Archwire especially in containing Martensite and Austenite structure. Also, the compacting pressure and additions of Cr and Al showed negligible effect on the transformation temper- atures, Vickers hardness values increased with increasing of Cr and Al additions, porosity percentages decreased with increasing of compacting pressure and addition of Cr and Al and finally corrosion rate decreased with increasing of Cr and Al additions. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Ni–Ti alloys combine the shape memory effect and super elas- ticity with excellent corrosion and mechanical properties and good biocompatibility. The use for medical applications was first re- ported in the 1970s [1]. The present manufacturing technology for these alloys is mostly the casting under vacuum followed by deformation processes. Zheng et al. [2] showed that the martens- itic transformation temperatures change slightly with deformation at 147 °C, while they increase 18 °C with deformation at room tem- perature. Khaleghi et al. [3] found that annealing of cold drawn Ni– Ti wires at 700 °C for 3 s leads to produce of an ultra fine grained structure and the best superelastic behavior. Ock and Kim [4] sta- ted that the martensitic and reverse transformation temperatures increase with decreasing hot working rate and increasing anneal- ing temperature. On the other hand, powder metallurgy has the advantage to produce porous structures that needed in biomateri- als. Mentz et al. [5], improved the mechanical properties of powder metallurgically produced Ni–Ti alloys by reducing the impurity contents of oxygen and carbon. Neves et al. [6] processed a set of Ni–Ti powder mixtures through mechanical alloying. The results indicated that milling time affects significantly the enthalpy of the high temperature reaction while the milling speed has a lower effect. Zhao et al. [7] used the liquid phase diffusion process for fabricating porous Ni–Ti samples. The results showed that a poros- ity of around 10% and pore size of within 20 lm is obtained when the sample is heated above 1000 °C for 30 min. Parvizi et al. [8] showed that hot compaction of mechanical alloyed Ni and Ti pow- ders resulted in yield and tensile strength enhancement with re- spect to conventionally produced specimens. Kim et al. [9] prepared Ti–Ni–Cu alloy porous specimens by spark plasma sinter- ing. The microstructure of as-solidified powders exhibited a cellu- lar structure and the transformation temperatures were strongly dependent on the Cu-content. This research aims to study of the ef- fect of compacting pressure and the Cr and Al additives on the properties of Ni–Ti shape memory alloys prepared by powder met- allurgy compared with the commercial cast-deformed Archwires. 2. Experimental The test samples were prepared by powder metallurgy from Ni and Ti powders. Powders were produced by Aldrich Chemical Company. The purity and particle size of these powders are shown in Table 1. Ni–Ti powder (master mixture; 55 wt% Ni with 45 wt% Ti) was mixed by ball mill for two hours. This mixture was used to pre- pare four groups of samples as shown in Table 2. The samples with Cr and Al addi- tives were additionally mixed for two hours. After mixing, six master samples of 5 g weight, as discs of 15 mm diameter and about 5 mm height were compacted at 300, 400, 500, 600, 700 and 800 MPa by tool steel die. The same procedure was repeated with other sample groups which were compacted at 800 MPa only, since this pres- sure has given the best result. All samples were sintered at 950 °C for 9 h under con- trolled atmosphere of argon. Two reference Archwires from the International Orthodontic Services (USA) were used to compare with the prepared samples by powder metallurgy. First type is Thermal (with both martensitic and austenitic phases) and the second is Austen- itic Archwire. Both were produced by casting and forming, with cross section of 0.45  0.55 mm. An average of 10 readings of Vickers microhardness was taken for every sample. The porosity was measured on the basis of density [10], where: 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.029 ⇑ Corresponding author. Tel.: +964 07802443867. E-mail address: ednan909@yahoo.com (A.S. Jabur). Journal of Alloys and Compounds 578 (2013) 136–142 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom