G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... 435–442 CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY PRODUCED WITH SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS KARAKTERIZACIJA POROZNE NIKELJ-TITANOVE ZLITINE, IZDELANE S SPONTANO NAPREDUJO^O VISOKOTEMPERATURNO SINTEZO Gul Tosun 1 , Musa Kilic 2 , Latif Ozler 3 , Nihat Tosun 3 1 Fýrat University, Technology Faculty, Mechanical Engineering Department, 23119 Elazig, Turkey 2 Batman University, Technology Faculty, Mechanical and Manufacturing Engineering, 72060 Batman, Turkey 3 Fýrat University, Engineering Faculty, Mechanical Engineering Department, 23119 Elazig, Turkey gultosun@firat.edu.tr Prejem rokopisa – received: 2017-09-24; sprejem za objavo – accepted for publication: 2018-01-25 doi:10.17222/mit.2017.156 The porous NiTi shape-memory alloy (SMA) is a promising biomaterial due to its attractive mechanical property and proper biocompatibility. In this research, the effects of the production parameters on the porosity of the above SMA were investigated using self-propagating high-temperature synthesis (SHS). Powders containing 50.5 atomic percent of Ni and 49.5 atomic percent of Ti were blended for 24 h and cold pressed at pressures of (100, 150 and 200) MPa. After that, NiTi-alloy green compacts were synthesized with the SHS process at different preheating temperatures of (200, 250 and 300) °C, while some of these samples were sintered. The effects of the production parameters, namely, the heat treatment, pressure and preheating temperature on the microstructure and porosity were examined. Microhardness tests were done in order to evaluate the mechanical properties. In addition to NiTi, there were other secondary intermetallic compounds (NiTi2,Ni 3Ti and Ni4Ti3)asthe base phases of the microstructure. The porosity was changed depending on the process parameters. The porosities of the synthesized products were obtained in a range of 38.6–56 volume percent. It was observed that the preheating temperature had an important effect on the pore-size distribution of the NiTi product. The sintering heat treatment changed the distribution of the phases in the microstructure. The main phase of the porous NiTi alloy was the NiTi phase together with the Ni3Ti2 and Ni2Ti secondary phases. Keywords: biomaterials, high-temperature synthesis (SHS), powder metallurgy, porosity, microstructure, hardness Porozne Ni-Ti zlitine z oblikovnim spominom (SMA; angl.: Shape Memory Alloys) so zaradi privla~nih mehanskih lastnosti in primerne biokompatibilnosti obetaven biomaterial. V ~lanku avtorji opisujejo raziskavo vpliva tehnolo{kih parametrov na poroznost SMA, ki so jo izdelali s pomo~jo spontano napredujo~e visokotemperaturne sinteze (SHS; angl.: self-propagating high-temperatures synthesis). Me{anico prahov Ni (50,5 %) in Ti (49,5 %) so me{ali 24 ur in jo nato hladno stisnili pri tlakih (100, 150 in 200) MPa. Potem so surovce iz Ni-Ti zlitine konsolidirali s SHS procesom pri razli~nih temperaturah predgrevanja: (200, 250 in 300) °C. Del vzorcev pa je bil zgo{~en s sintranjem. Nato so analizirali vpliv tehnolo{kih parametrov sinteze (temperature predgretja in toplotne obdelave, tlaka) na mikrostrukturo in poroznost. Mehanske lastnosti so ovrednotili z meritvami mikrotrdote. Poleg NiTi kot osnovne faze so se v mikrostrukturi zlitine pojavljale {e sekundarne faze, kot so intermetalne spojine NiTi2,Ni 3Ti in Ni4Ti3. Poroznost je bila odvisna od procesnih parametrov. Poroznosti sintetizirane zlitine so se gibale med 38,6 in 56,0 prostorninskih dele`ev. Ugotovili so, da ima temperatura predgrevanja pomemben vpliv na velikostno porazdelitev por Ni-Ti produkta. Sintranje je spremenilo porazdelitev faz v mikrostrukturi. V sintetiziranih poroznih Ni-Ti zlitinah je kot glavna faza nastopala faza NiTi s sekundarnima fazama Ni3Ti2 in Ni2Ti. Klju~ne besede: biomateriali, visokotemperaturna sinteza (SHS), metalurgija prahov, poroznost, mikrostruktura, trdota 1 INTRODUCTION Discovering a cost-efficient production procedure for innovative metal alloys is one of the most significant im- provement in scientific disciplines. 1 The nickel-titanium alloy with a near-equiatomic concentration is well known for its shape-memory effect (SME). 2 NiTi SMAs have been successfully used in many industries such as engineering, medical industry, space researches, automo- tive, micro-electromechanical and orthopedic applica- tions. As they have superior mechanical features, for example, superelasticity (SE), SME, biocompatibility, they are appropriate for surgical operations and brackets, hard-tissue replacements and corrosion-resistant im- plants. 3–7 Highly porous NiTi is commonly produced with several techniques such as melting, casting, reactive sintering, conventional sintering, SHS, HIP (hot ýsostatic pressing) with gas entrapment, capsule-free hot isostatic pressing, spark plasma sintering and HIP with space holders. 8,9 These techniques, however, only entail rounded/sponge-like pore geometries. 10,11 Furthermore, the NiTi produced with the SHS treatment has a larger porosity and linear channels. 12 The SHS procedure for manufacturing porous intermetallic compounds has several advantages such as time, energy savings and purity of the reaction product. 13 To form a compact from powders, routes such as powder blending, cold pressing and sintering are usually followed. 14 The Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 435 UDK 54.057:669.018.2:539.217:669.245:669.295 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)435(2018) following phenomena generally occur during the SHS of NiTi SMAs; preheating, ignition, propagation of the combustion wave front and cooling. 13 Several phenomena cause porosity during the SHS process: a modified volume because of a crystal-lattice re-ordering, the Kirkendall effect, thermal migration, gas porosity or residual porosity after the pressing. 14 The sintering temperature is first reached outside and later inside; consequently, NiTi is first produced in the ectotheca of a sample. Furthermore, regional melting/ collapsing does not happen simply because of a direct contact under an inert atmosphere (argon) with high pressure during the process. At the increased sintering temperature, exothermic reactions take place inside the sample of Ni and Ti: Ni+Ti NiTi + 67 kJ/mol (1) Ni+Ti Ti 2 Ni + 83 kJ/mol (2) Ni+Ti Ni 3 Ti + 140 kJ/mol (3) Ni+Ti Ni 4 Ti 3 +heat (4) The reactions cause local melting due to the local overheating in these regions. Consequently, the pores expand and coalesce and the interior small pores convert into one or a few great pores when the sintering tem- perature is enhanced. 12 The present work focuses on the influence of produc- tion parameters and heat treatment on the microstructure and porosity of porous NiTi made with SHS. 2 EXPERIMENTAL PART The powders of Ti (99.5 %) and Ni (99.8 %) supplied by Alfa Inc., USA, were used to produce NiTi (Table 1). The Ni and Ti powders with a combination of Ni:Ti = 50:50 % were blended in an argon-gas sealed container using a spherical-ball mill for 24 h. Cylindrical green bulks had a diameter of 12 mm and a height of approximately 10 mm. They were produced via uniaxial cold pressing at pressures of (100, 150 and 200) MPa using a hydraulic press. During the experiments, the preheating temperatures were (200, 250 and 300) °C under an atmosphere of inert gas (Figure 1). The pre- heated green bodies were ignited at the endpoint via a tungsten coil in a furnace. Afterwards, the synthesizing reaction was actualized; we waited until the temperature of the specimens was equal to room temperature in the reaction chamber. Some samples were sintered at a heating rate of 15 °C/min, a temperature of 850 °C and a duration of3hinaProtherm sintering furnace. During the experiments, the ignition temperatures were measured in a range of 607–1238 °C depending on the process parameters. The microstructure investigations were done with a scanning electron microscope (SEM) and an optical microscope. The total porosity of the samples ( )w a s determined with the following formula: =− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ × 1 100 0 (5) where is the density of the sample and 0 is the corresponding theoretical density of the sample. The sample density was calculated by measuring the weight and dimensions of the specimens. The theoretical density of the NiTi alloy with x = 50.5 % Ni was 6.21 g/cm 3 . The porosity and density of sintered and non- sintered samples were measured with the Archimedes method as specified in the ASTMB962-08 standard. After polishing, etching processes of the NiTi speci- mens were done with a mixture of 10 % HF and 5 % HNO 3 in water. The optical microscope (Alltion- NIM100) and SEM (LEO Evo-40VP) were used to analyze the pore characteristics of the NiTi products and the microstructure. The chemical composition of the samples was specified with an energy-dispersive-spec- troscopy (EDS) analysis. Table 1: Features of Ti and Ni powders Features of the material Nickel Titanium Purity (%) 99.8 99.5 Specific gravity (g/mol) 58.71 47.9 Powder dimension (mesh) -325 -325 Melting temperature (°C) 1453 1680 Specific weight (g/cm 3 ) 8.9 4.507 Boiling temperature (°C) 2832 3260 Ignition temperature (°C) Not determined 250 3 RESULTS AND DISCUSSION No distortion or surface cracks were observed on the produced compacts. Figure 2 shows an optical macro- graph of the porous structure produced by synthesizing. Ignition channels occurred as vertical circles on the electrode axis. As can be seen, the porous NiTi SMA produced in this research exhibits a homogeneous pore distribution. The pores in the produced NiTi are almost G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... 436 Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 Figure 1: Experimental set-up three-dimensionally interdependent, and the sample has an open porous structure. 15 The pores are homogeneous- ly distributed on the cross-section and their edges are not sharp. The rounded shape reduces the stress concentra- tion at the edges of the pores, increasing the mechanical strength and improving the shape-memory behavior. Therefore, to check the pore character and enhance the mechanical properties, the superelasticity and biocompa- tibility of these SMAs are of key importance. 2 In the microstructure, the single phase instrumental for the necessary superelasticity is NiTi. Therefore, any unwanted phases and unreacted Ni need to be removed to develop its superelasticity for improving its biomecha- nical compatibility. Mechanical properties should be improved without reducing the porosity because a reduced porosity affects the ingrowth of the bone tissue. Commonly, if the sintering temperature, or the com- bustion temperature, is high enough, a product with a single phase is obtained. 16 When the microstructures of the produced samples are investigated, the NiTi phase is the main phase, as seen in Figure 3. In addition, secondary phases such as Ni 2 Ti, Ni 3 Ti or, in some cases, even elemental Ni in variable quantities are observed (Figure 4). 6,15,17,18 In the microstructure images of the samples, the NiTi matrix is grey, the Ni 3 Ti 2 or Ni 4 Ti 3 phases are light grey and the NiTi 2 phase is corner-shaped and dark grey. 14 Micrographs in Figures 3, 4 and 5, show the effect of the cold-compaction pressure on the microstructure. The combustion properties, mostly associated with the forma- tion of the NiTi intermetallic obtained with the SHS procedure, are the preheating temperature and the initial specimen density. It was found that in the specimens with a low green density and at a cold-compaction pressure of 100 MPa, the main phase was NiTi and the secondary phase was Ni 3 Ti 2 (Figure 3a). The NiTi phase and uniform Ni 3 Ti 2 and Ni 2 Ti phase islands were also seen when the green density, or the cold-compaction pressure, was increased. When the pressure was in- creased to 150 MPa (Figure 3b), interconnected islands disappeared. When the pressure became 200 MPa, interconnected islands re-occurred (Figure 3c). When the cold-compaction pressure was increased to 200 MPa, the NiTi phase was dominant while the amount of Ni 3 Ti 2 increased and the amount of Ni 2 Ti decreased. Figure 4 shows a SEM graph and EDX analysis of interconnected islands for the samples produced at 100 MPa. At the preheating temperature of 250 °C, it was seen that the main phase was Ni 3 Ti 2 and the secondary phases were NiTi and Ni 2 Ti in the specimens with a low green density at the 100 MPa cold-compaction pressure (Fig- ure 5a). An increased amount of the NiTi phase and a decreased amount of the uniform Ni 3 Ti 2 and Ni 2 Ti phase G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 437 Figure 2: Macrograph of a synthesized porous, solid, cylindrical NiTi sample Figure 3: Specimens at the preheating temperature of 200 °C, non- sintered and at pressures of: a) 100 MPa, b) 150 MPa, c) 200 MPa islands were also seen when the green density, or the cold-compaction pressure, was increased. When the pressure was increased to 150 MPa (Figure 5b), inter- connected islands decreased. When the pressure became 200 MPa, interconnected islands re-increased (Fig- ure 5c). When the cold-compaction pressure was in- creased to 200 MPa, the NiTi phase was dominant and the amounts of Ni 3 Ti 2 and Ni 2 Ti increased. The optical micrographs in Figure 6 show that the amount of the NiTi main phase of the SHS + sintering samples increases with the increasing preheating tempe- rature. 19 It is seen that the porous products contain very small NiTi, NiTi 2 and Ni 3 Ti 2 phases at the low preheating G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... 438 Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 Figure 5: Specimens at the preheating temperature of 250 °C, non- sintered and at pressures of: a) 100 MPa, b) 150 MPa, c) 200 MPa Figure 4: a) pressure of 100 MPa, preheating temperature of 300 °C, a non-sintered specimen, b) e1 EDX, c) r1 EDX temperature (Figure 6a) and the NiTi phase is sur- rounded by pores. There is not any homogeneity in the microstructure. When the preheating temperature is increased to 250 °C (Figure 6b), the amounts of NiTi and NiTi 2 are increased, and Ni 3 Ti 2 is decreased. When the preheating temperature is increased to 300 °C (Figure 6c), the amount of NiTi is decreased, Ni 3 Ti 2 is increased and NiTi 2 disappears. The effect of the preheating temperature on the spe- cimens at the 200 MPa cold-compaction pressure was different. It was seen that the porous products contained the NiTi, NiTi 2 and Windsmatten-type Ni 3 Ti 2 phases at a low preheating temperature (Figure 7a) When the pre- heating temperature was increased to 250 °C (Figure 7b), the amount of NiTi increased and NiTi 2 disappeared. The Windsmatten-type Ni 3 Ti 2 phase was seen. When the preheating temperature was increased to 300 °C (Fig- ure 7c), the Windsmatten-type Ni 3 Ti 2 phase disappeared. When the preheating temperature was increased during SHS, the molten fraction of NiTi increased. The increase of the molten fraction caused the formation of both the micropores with a small size and the macro- pores with a large size. Consequently, it was observed that the preheating temperature had an important effect on the pore-size distribution of a NiTi product. 15 G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 439 Figure 7: Specimens at the pressure of 200 MPa and preheating temperatures of: a) 200 °C, b) 250 °C, c) 300 °C Figure 6: Specimens obtained with SHS + sintering at the pressure of 150 MPa and preheating temperatures of: a) 200 °C, b) 250 °C, c) 300 °C The sintering heat treatment changes the distribution of the phases in the microstructure. The amount of the NiTi phase is high and the amounts of the Ni 2 Ti and Ni 3 Ti 2 phase are low in the non-sintered samples. On the other hand, the amount of the NiTi phase decreases and the amounts of the Ni 2 Ti and Ni 3 Ti 2 phase increase in the sintered samples (Figures 6 and 7). The combustion channels are oriented perpendicul- arly to the combustion wave. The pore orientation de- pends on the point of ignition during SHS. 2,3,19 It seems that some transient liquid phases are created due to locally high temperatures at the combustion front. As a result of a high temperature gradient along the combus- tion wave, the transient liquid phases extend perpen- dicularly to the combustion wave, resulting in elongated pores. 3,19 There are sharp edges of the channels of the samples produced at the pressure of 100 MPa and a low number of micropores (Figure 8). However, when the preheating temperature is increased, the edges of the channels are round, and the width and number of the micropores increase. Mechanical properties change de- pending on the orientation, shape and width of the channels. 2,3 Because of this, the mechanical properties will probably improve. In addition, the micropores increase and the combustion channels become wide with the increasing pressure (Figure 8). Wide combustion channels are desired because the tissue progression into medical implants takes place in the combustion channels. An important structural feature of a porous solid is the porosity (or, in other words, the relative density). Porosity greatly affects its mechanical properties. 2,20 Figure 9 shows the porosity of non-sintered and sintered SHS specimens. The porosity changes depend- ing on the process parameters. Bone-implant materials with a porosity in the range of 30–90 % and the optimum pore size of 100–400 μm are ideal according to the refe- rences. 21,22 Consequently, the porous NiTi alloy produced using microwave sintering and SHS have the appropriate porosity and pore size and will hopefully be used for bone implants. 21,22 In our study, its porosity is in a range of 38.61–56.05 %. In addition, the preheating temperature affects the porosity. The sintering process after the SHS negatively affects the porosity at the preheating temperature of 200 °C. The porosity of the non-sintered samples is higher than that of the samples sintered at the 200 °C preheating temperature. On the contrary, the porosity of the non-sintered samples is lower than that of the sam- ples sintered at the preheating temperatures of 250 °C and 300 °C. When the preheating temperature is in- creased to 250 °C, the porosity of the SHS samples decreases. When the preheating temperature is increased to 300 °C, the porosity increases, except at 100 MPa (Figure 9). When the preheating temperature is in- G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... 440 Materiali in tehnologije / Materials and technology 52 (2018) 4, 435–442 Figure 9: Porosity of the samples produced with SHS+sintering and SHS Figure 8: Specimens at the preheating temperature of 200 °C, non- sintered, pressures of: a) 100 MPa, b) 200 MPa creased to 250 °C, the porosity increases for the SHS + sintering samples except at 100 MPa. When the preheating temperature is increased to 300 °C, the poro- sity increases for the SHS + sintering samples. The hardness versus preheating temperature graphs for the SHS and SHS + sintering samples are shown in Figure 10. When the preheating temperature is increased to 250 °C, the microhardness of the SHS samples in- creases. When the preheating temperature is increased to 300 °C, the microhardness of the SHS samples increases, except at 200 MPa. When the preheating temperature is increased to 250 °C, the microhardness increases, except for the SHS+sintering samples at 150 MPa. When the preheating temperature is increased to 300 °C, the poro- sity increases, except for the SHS + sintering samples at 100 MPa. The hardness decrease in this condition is attributed to the secondary phases (Ni 2 Ti and Ni 3 Ti 2 )ob - served with the optical microscope (Figures 6 and 7). 4 CONCLUSIONS Porous NiTi specimens were manufactured with the SHS procedure. It is seen that the NiTi specimens pro- duced with different manufacturing techniques and those produced with SHS show different amounts of porosity. The variance in the porosity and microstructure of the porous NiTi was examined by changing the production parameters and sintering heat treatment. The porosities of the NiTi alloys change from 38.61 % to 56.05 % depending on the production para- meters. The porous NiTi alloys consisted of a nearly single NiTi phase with Ni 3 Ti 2 and Ni 2 Ti secondary phases. For the non-sintered samples, it was observed that the interconnected Ni 3 Ti 2 islands disappear at the pressure of 150 MPa, the NiTi phase is dominant and the amount of Ni 3 Ti 2 increases at the pressure of 200 MPa. It can be shown that with the increasing preheating temperature, the amount of the NiTi main phase in the SHS + sinter- ing samples increases. It was observed that the preheat- ing temperature has an important effect on the pore-size distribution of a NiTi product. The sintering heat treat- ment changes the distribution of the phases in the micro- structure. The amount of the NiTi phase is high and the amounts of the Ni 2 Ti and Ni 3 Ti 2 phases are low in the non-sintered samples. On the other hand, the amount of the NiTi phase decreases and the amounts of the Ni 2 Ti and Ni 3 Ti 2 phases increase for the sintered samples. The edges of the channels are round; their width and the number of micropores increase when the preheating temperature is increased. The porosity changes depending on the process parameters. In our study, the porosity is in the range of 38.61–56.05 %. The preheating temperature affects the porosity. The sintering process after the SHS negatively affects the porosity at the preheating temperature of 200 °C. The porosity of the non-sintered samples is higher than that of the sintered samples at the preheating temperature of 200 °C. On the contrary, the porosity of the non-sintered samples is lower than that of the samples sintered the at the preheating temperatures of 250 °C and 300 °C. When the preheating temperature is increased to 250 °C, the porosity of the SHS samples decreases. When the preheating temperature is increased to 300 °C, the porosity increases, except at 100 MPa. When the preheating temperature is increased to 250 °C, the porosity of the SHS + sintering samples increases, except at 100 MPa. When the preheating temperature is increased to 300 °C, the porosity of the SHS+sintering samples increases. When the preheating temperature is increased to 250 °C, the microhardness of the SHS samples increases. When the preheating temperature is increased to 300 °C, the microhardness of the SHS samples increases, except at 200 MPa. When the preheating temperature is in- creased to 250 °C, the microhardness of the SHS + sint- ering samples increases, except at 150 MPa. When the preheating temperature is increased to 300 °C, the microhardness increases, except for the SHS + sintering samples at 100 MPa. G. TOSUN et al.: CHARACTERIZATION OF A POROUS NICKEL-TITANIUM ALLOY ... 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