High porosity and large pore size shape memory alloys fabricated by using pore-forming agent (NH4HCO3) and capsule-free hot isostatic pressing

Shape-memory NiTi foams produced by replication of NaCl space-holders

“…Closed porosity of $2-3% is present as small pores ($10 lm in size), owing to incomplete densification between NiTi powders and possibly trapped NaCl vapor which prevents full densification. This amount is small compared with the microporosity observed (from micrographs) in other studies that rely only on pressureless sintering with NaCl space-holders [13] or on the use of gas-decomposable space-holders [12].…”

“…One concern in the microstructure of the present foams is that the fenestrations connecting adjacent pores might be much smaller than the pore size, thus restricting the ingrowth of bone tissue for foams used as implants, unlike the larger fenestrations in the porous NiTi (42% porosity) produced by capsule-free hot isostatic pressing [12]. Narrow fenestrations are indicated by arrows in Fig.…”

“…The effective stiffness of porous NiTi may further be reduced by the superelastic effect, which produces near-linear, recoverable strains by a reversible phase transformation, while maintaining a high strength. Indeed, porous NiTi with 30-80% porosity shows average stiffness values as low as that of cortical bone (12)(13)(14)(15)(16)(17) or even cancellous bone (<3 GPa) [3]). Moreover, the superelastic effect (and the related shape-memory effect) can be used to deploy a NiTi foam in the implant location (similarly to deployable stents and staple [4]).…”

“…The above methods, however, do not allow independent control over pore size, shape and connectivity. This issue can be solved by the use of a temporary space-holder phase during the initial cold compaction of metallic powders into preform which is then removed prior to or during the densification of the elemental or pre-alloyed NiTi powders, e.g., ammonium bicarbonate (NH 4 HCO 3 ) in capsule-free hot isostatic pressing [12] and sodium chloride (NaCl) with a polymeric binder in metal injection molding (MIM) and sintering [13].…”

Phase Transformation Behavior of Porous TiNi Alloys Produced by Powder Metallurgy Using Magnesium as a Space Holder

“…Powder metallurgy techniques, on the other hand, provide the flexibility of low-temperature processing as well as close composition control, as well as mechanical and physical property modifications via adjustment of the processing parameters and characteristics of the powders used. Accordingly, various powder metallurgy methods, such as self-propagating high-temperature synthesis (SHS), [9][10][11][12][13][14] hot isostatic pressing (HIP), [2][3][4][5]15,16] conventional sintering, [4,[17][18][19][20] spark plasma sintering, [21] space holder technique, [6,7,[22][23][24] and metal injection molding (MIM) [25,26] were employed intensively to fabricate TiNi foams. However, incomplete diffusion when elemental powders were used and contamination during processing even when prealloyed powders were used by these methods led to formation of secondary intermetallics (Ti 2 Ni, TiNi 3 , and Ti 3 Ni 4 ) and oxides (Ti 4 Ni 2 O) or carbonitrides, all of which deteriorate the shape memory and superelasticity characteristics.…”