Şakir Bor

Superelasticity and compression behavior of porous TiNi alloys produced using Mg spacers

In the scope of the present study, Ni-rich TiNi (Ti-50.6 at %Ni) foams with porosities in the range 38-59% were produced by space holder technique using spherical magnesium powders as space formers. Single phase porous TiNi alloys produced with spherical pores were subjected to loading-unloading cycles in compression up to 250 MPa stress levels at different temperatures in as-processed and aged conditions. It has been observed that strength, elastic modulus and critical stress for inducing martensite decrease with increasing porosity. Partial superelasticity was observed for all porosity levels at different test temperatures and conditions employed. Irrecoverable strain was found to decrease with pre-straining and with increasing test temperature. Unlike in bulk TiNi alloys a constant stress plateau has not been observed during the compression testing of porous TiNi alloys. Instead linear superelasticity with a quite steep slope allowing 5% applied strain to be recovered after pre-straining or aging was observed. Even at test temperatures higher than austenite finish temperature in as-sintered and aged condition, strain applied could not be recovered fully due to martensite stabilization resulting from heavy deformation of macro-pore walls and sintering necks. TiNi foams produced with porosities in the range of 38-51% meet the main requirements of biomaterials in terms of mechanical properties for use as bone implant.

Fatigue behavior of TiNi foams processed by the magnesium space holder technique

While the wide range of applications of TiNi alloys makes them highly appealing due to their shape memory and superelasticity properties, the production of TiNi in the porous form further enlarges their application fields. Porous TiNi alloys have been studied extensively for biomedical applications since their elastic modulus is similar to that of bone. Accordingly, TiNi foams have been widely characterized in terms of their various mechanical properties; however, their fatigue properties have not been well studied, even though this is of vital importance in structural applications such as medical implants. In the scope of this study, TiNi foams processed from prealloyed powders by the magnesium space holder technique were mechanically characterized by monotonic and cyclic compression tests. TiNi foams with a porosity range of 49-64 vol.%, which is suitable for bone ingrowth, were determined to have a compressive strength varying in the range 93.27-273.45 MPa. Moreover, the wide range of elastic modulus values obtained (2.93-8.71 GPa) is promising for fulfilling various requirements of different implant applications without causing stress shielding. On the other hand, the endurance limit of TiNi foams was determined to be 0.6σy, where σy is the yield strength, independent of the porosity content. Fractography studies on the failed foams after fatigue testing revealed that the failure occurs by the coalescence of micro-cracks initiated from pore walls leading to macro-crack formation aligned at 45 ° with respect to the loading axis.

Characterization of Ti6Al7Nb alloy foams surface treated in aqueous NaOH and CaCl2 solutions.

Ti6Al7Nb alloy foams having 53-73% porosity were manufactured via evaporation of magnesium space holders. A bioactive 1µm thick sodium hydrogel titanate layer, NaxH2-xTiyO2y+1, formed after 5M NaOH treatment, was converted to crystalline sodium titanate, Na2TiyO2y+1, as a result of post-heat treatment. On the other hand, subsequent CaCl2 treatment of NaOH treated specimens induced calcium titanate formation. However, heat treatment of NaOH-CaCl2 treated specimens led to the loss of calcium and disappearance of the titanate phase. All of the aforementioned surface treatments reduced yield strengths due to the oxidation of the cell walls of the foams, while elastic moduli remained mostly unchanged. Accordingly, equiaxed dimples seen on the fracture surfaces of as-manufactured foams turned into relatively flat and featureless fracture surfaces after surface treatments. On the other hand, Ca- and Na-rich coating preserved their mechanical stabilities and did not spall during fracture. The relation between mechanical properties of foams and macro-porosity fraction were found to obey a power law. The foams with 63 and 73% porosity met the desired biocompatibility requirements with fully open pore structures and elastic moduli similar to that of bone. In vitro tests conducted in simulated body fluid (SBF) showed that NaOH-heat treated surfaces exhibit the highest bioactivity and allow the formation of Ca-P rich phases having Ca/P ratio of 1.3 to form within 5 days. Although Ca-P rich phases formed only after 15 days on NaOH-CaCl2 treated specimens, the Ca/P ratio was closer to that of apatite found in bone.

The Processing and Fatigue Characterization of TiNi and Ti-6Al-4V Porous Materials

Porous titanium alloys have been extensively studied in biomedical applications due to their elastic moduli similar to that of bone compared to other implant materials. Accordingly, TiNi and Ti-6Al-4V foams have been widely characterized in terms of their various mechanical properties; however, their fatigue properties have not been well studied, even though, it has a vital importance in structural applications such as medical implants. In this study, porous titanium alloys were processed via sintering at 1200 °C for 2 hours employing Mg space holder technique. TiNi and Ti-6Al-4V alloys with a porosity of 49 and 51 vol.%, respectively, were mechanically characterized by monotonic and cyclic compression tests. The compressive strength was determined to be 148 MPa for TiNi foams whereas 172 MPa for Ti-6Al-4V foams with homogenously distributed pores having diameters in the range of 250-600 µm. Endurance limit values were determined relative to the yield strength of each porous alloy in order to enable the comparison of fatigue behavior. The fatigue tests applied with a frequency of 5 Hz and a constant stress ratio (σmin/σmax) of 0.1 have revealed that porous TiNi alloys have an endurance limit of approximately 0.6 σy whereas porous Ti-6Al-4V alloys have an endurance limit of approximately 0.75 σy. The differences and similarities in the microstructure and their effect on mechanical behavior of the two alloys were also studied by employing scanning electron microscope (SEM).

Fatigue Behavior of 51 Vol.% Porous Ti-6Al-4V Alloy

Porous titanium alloys are widely used as implant materials due to their mechanical behavior similar to that of bone. In addition, fatigue properties of implant materials are especially important since medical implants mostly exposed to cyclic compressive loading conditions. In this study, porous Ti-6Al-4V alloy has been produced via sintering at 12000C for 2 hours employing magnesium space holder technique. Porosity of the produced foams were measured according to Archimedes’ principle and calculated to be in the range of 51 ± 1 vol.%. Mechanical properties of the foams were characterized by monotonic compressive and compression-compression mode fatigue tests. The compressive strength and elastic modulus of the foams were determined to be 167 ± 18 MPa and 12 ± 1 GPa respectively. Fatigue tests conducted with a frequency of 5 Hz and a constant stress ratio (σmin/σmax) of 0.1 revealed that porous Ti-6Al-4V alloys have a fatigue limit of approximately 135 MPa. Furthermore fracture surfaces of the foams were characterized by field emission scanning electron microscopy (FESEM).

Fatigue and Fracture Behavior of Porous TiNi Alloys

While the wide range of applications of TiNi alloys make them highly appealing due to their shape memory and superelasticity properties, production of TiNi in the porous form further enlarges their application fields. Porous TiNi alloys have been studied extensively for biomedical applications due to their elastic modulus similar to that of bone. Accordingly, TiNi foams have been widely characterized in terms of their various mechanical properties; however, their fatigue properties have not been well studied, even though, it has a vital importance in structural applications such as medical implants. In the scope of this study, fatigue behavior of TiNi foams, which were produced from prealloyed powders by Mg space holder technique, was examined via load controlled cyclic compression-compression tests. The endurance limit of the tested foams was taken as the stress level at which the specimens sustain their integrity without showing any sign of failure beyond 106 cycles. TiNi foams with porosity contents in the range of 39-64 vol%, which is suitable for bone ingrowth, were determined to have an endurance limit ranging in between 26-89 MPa. On the other hand, fractography studies on the failed foams after fatigue testing revealed that the failure occurs by the coalescence of micro-cracks initiated from pore walls leading to macro-crack formation aligned at 45o with respect to the loading axis.

Recrystallization of AZ31 Alloy

Magnesium alloys are extensively used in electronics, automotive and aerospace industries due to their low densities and high specific strengths. However, limited deformability of magnesium alloys at room temperature restricts the applications. Grain refinement and texture weakening as a result of recrystallization can be used to enhance the deformability of these alloys. In this study, recrystallization behavior of cold rolled and swaged AZ31 alloy is investigated at different temperatures in the range of 200-300°C. Effects of unidirectional and complex deformation modes on recrystallization behavior and microstructure development are studied. Microstructural analyses consisted of the examination of the grain structure and twinned regions by using optical and scanning electron microscopes. The volume fraction of recrystallized grains is determined by image analysis and supported by the hardness measurements.