In the scope of this study, TiNi foams with porosities in the range of 39-64 vol% were processed from prealloyed powders by Mg space holder technique. Porous TiNi alloys displayed homogeneously distributed spherical pores with interconnections, which is suitable for bone ingrowth. Porous Ti-50.8 at%Ni alloys were processed by sintering at 1200 &deg / C for 2 h to analyze the microstructure as well as mechanical behavior. SEM, TEM and XRD studies were conducted for the characterization of microstructure and phase analyses in addition to the mechanical characterization performed by monotonic and superelasticity compression tests as well as compressive fatigue tests. It was observed that stress required to trigger martensitic transformation was decreased via increasing porosity. The monotonic compression test results also indicated that altering the porosity content of TiNi foams leads to different monotonic compression behaviors. It was observed that the foams display more bulk deformation like behavior as a composite structure composed of TiNi and macropores when the porosity content was low. As the porosity content has increased, the struts became more effective and deformation proceeds by the collapse of favorable struts. On the other hand, cyclic superelasticity tests results indicated that maximum achieved and recovered strain values at the end of fifth cycle increase while the fraction of strain recovered at the end of fifth cycle decreases with decreasing porosity content. Furthermore, the fatigue lives of the processed foams were observed to vary within a band which has a width decreasing with decreasing &sigma / max / &sigma / y yielding an endurance limit ranging in between 26-89 MPa or 0.5-0.6 &sigma / y. 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-cracks aligned at 45o with respect to the loading axis. In addition to the mentioned characterization studies, the effects of sintering temperature and time on TiNi foams with 58 vol% porosity as well as heat treatment on the microstructure and the mechanical behavior of TiNi foams with 49 vol% porosity were analyzed with SEM and compression tests. Aging of TiNi foams with 49 vol% porosity at 450 &deg / C for 1.5 h has shown that the presence of Ti3Ni4 precipitates improve the superelastic response.

Porous TiNi alloys (Ti-50.4 at. %Ni and Ti-50.6 at. %Ni) with porosities in the range 21%-81% were prepared successfully applying a new powder metallurgy fabrication route in which magnesium was used as space holder resulting in either single austenite phase or a mixture of austenite and martensite phases dictated by the composition of the starting prealloyed powders but entirely free from secondary brittle intermetallics, oxides, nitrides and carbonitrides. Magnesium vapor do not only prevents secondary phase formation and contamination but also provides higher temperature sintering opportunity preventing liquid phase formation at the eutectic temperature, 1118 &deg / C resulting from Ni enrichment due to oxidation. By two step sintering processing (holding the sample at 1100 &deg / C for 30 minutes and subsequently sintering at temperatures higher than the eutectic temperature, 1118 &deg / C) magnesium may allow sintering probably up to the melting point of TiNi. The processed alloys exhibited interconnected (partially or completely depending on porosity content) open macro-pores spherical in shape and irregular micro-pores in the cell walls resulting from incomplete sintering. It has been found that porosity content of the foams have no influence on the phase transformation temperatures while deformation and oxidation are severely influential. Porous TiNi alloys displayed excellent superelasticity and shape memory behavior. Space holder technique seems to be a promising method for production of porous TiNi alloys. Desired porosity level, pore shape and accordingly mechanical properties were found to be easily adjustable.

Thesis (MScEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: In this study, commercially pure titanium foam was produced using space holder powder metallurgy techniques. Titanium foam is attractive as a scaffolding material for bone replacement and implants in the body. The porous morphology of the foam promotes osteogenesis, while the mechanical behaviour of the foam is closer to that of bone, which has an elastic moduli range of 5 - 40 GPa. Titanium foam was manufactured from powder mixtures of commercially pure titanium (CPTi) powder mixed with 41.4 wt% ammonium bicarbonate (ABC) powder and 1.45 wt% polyethyl glycol (PEG) powder. In this study, two CPTi powders with different particle size distributions, < 75 μm (-200 mesh, designated TiAA) and < 200 μm (-100 mesh, designated TiG), were mixed with the space holder ABC powder, that had been sieved into specified particle size ranges. The size ranges of space holder material studied were: 0 - 710, 250 - 425, 425 - 560, and 560 - 710 μm. This allowed foams with different large or macropore distributions to be produced from the different mixtures. The mixtures were uniaxially compacted at 100 MPa into transverse rupture bars. The ABC and PEG was then removed by thermal debinding in air for 5 hours at 100 °C and 1 hour at 330 °C each, consecutively. The debound samples are then sintered under high (10-6 mbar) vacuum on yttria-stabilised zirconia substrates, heating at 5 °C/min to 1200 °C, with a 2 hour hold at temperature. The microstructures of the different foams were evaluated by examining the polished samples using light optical microscopy. Three point bend tests were conducted on the sintered bars in order to determine the flexural strength and flexural modulus of the different foams. The produced foams had a relative density range between 37.5 - 62.5 % and average macro pore size range between 300 - 500 μm. The foams were found to have an elastic modulus similar to that of bone, 2 - 7 GPa. Finally, the mechanical properties of the foams were compared to known open foam mechanical models and other research projects. It was found that: (i) changes in either metal or space holder powder influences the sintering behaviour of metal foams, (ii) sintered titanium foams with similar densities but different macro/micropore size distributions have different mechanical responses to stress and (iii) the Ashby-Gibson model, based on foam density alone, gives a rough estimate of mechanical properties for the titanium foams studied, but does not capture variations due to pore size distribution. / AFRIKAANSE OPSOMMING: In hierdie studie is kommersiële suiwer titaanskuim geproduseer met behulp van ruimtehouer poeier metallurgie tegnieke. Titaanskuim is aantreklik as 'n raamwerkmateriaal vir beenvervanging en -inplantings in die liggaam. Die poreuse morfologie van die skuim bevorder osteogenese, terwyl die meganiese gedrag van die skuim naby aan dié van been is, met ‘n elasticiteitsmodulus tussen 5 - 40 GPa. Titaanskuim is vervaardig van ‘n poeier mengsel van kommersiële suiwer titaan (CPTi) poeier gemeng met 41,4 gew% ammonium bikarbonaat (ABC) poeier en 1.45 gew% poli-etileenglikol (PEG) poeier. In hierdie studie is twee tipes CPTi poeiers met verskillende deeltjiegrootteverspreiding, < 75 μm (-200 stofdigtheid, TiAA genoem) en <200 μm (-100 stofdigtheid, TiG genoem), met die ruimtehouer ABC-poeier, wat in bepaalde deeltjiegroottereekse gesif is, gemeng. Die wisselende groottes van ruimtehouer wat bestudeer is, was: 0 - 710, 250 - 425, 425 - 560, 560 - 710 μm. Dit het die vervaardiging van skuim met verskillende groot of macroporeuse vanaf die verskillende mengsels toegelaat. Die mengsel is teen 100 MPa in een rigting gekompakteer. Die ABC en PEG is dan verwyder word deur termiese ontbinding in lug vir 5 uur by 100 °C en 1 uur by 330 °C elk, onderskeidelik. Die ontbinde monsters is dan onder hoë (10-6 mbar) leemte op yttrium-gestabiliseer zirconia-substraat, met verwarming teen 5 °C/min tot 1200 °C met 'n verdere 2 uur by 1200 °C, gesinterd. Die mikrostrukture van die verskillende skuim is geëvalueer deur gepoleerde monsters met behulp van ‘n ligmikroskopie te ondersoek . Driepunt draaitoetse is op die gesinterd stawe uitgevoer om die buigsterkte en buigmodulus van die verskillende skuime te bepaal. Die vervaardigde skuime se relatiewe digtheid het tussen 37,5 - 62,5 % gewissel en die gemiddelde makroporiegrootte tussen 300 - 500 μm gewissel. Die skuim het 'n elastisiteitsmodulus soortgelyk aan dié van been getoon, 2 – 7 GPa. Ten slotte is die meganiese eienskappe van die skuim met bekende oop skuim meganiese modelle en ander navorsingsprojekte vergelyk. Daar is bevind dat: (i) veranderinge in óf metaal of ruimtehouer poeier beïnvloed die sinteringgedrag van metaalskuime, (ii) gesinterd titaniumskuim met soortgelyke digthede, maar verskillende makro / mikroporeuse verdelings, toon verskillende meganiese reaksies op stres en die Ashby-Gibson model, gebaseer op die skuimdigtheid alleen, (iii) wat 'n rowwe skatting van die meganiese eienskappe vir die bestudeerde titaniumskuime gee, maar nie die variasies ingrootteverspreiding van porieë ondervang nie.