Processing and Characterization of NiTi Porous SMA by Elevated Pressure Sintering

Abstract

Currently, three methods are commonly used for producing porous NiTi shape memory alloys (SMAs) from elemental powders. These include conventional sintering, Self-propagating High temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press (HIP). Conventional sintering requires long heating times and samples are limited in shape and pore size. SHS is initiated by a thermal explosion ignited at one end of the specimen, which then propagates through the specimen in a self-sustaining manner. One of the difficulties with SHS is the inability to control intermetallic phases. This work will focus on the fabrication and characterization of porous NiTi SMA material produced from elemental powders via HIPping.Porous NiTi SMA was produced from elemental Ni and Ti powders at elevated temperature and pressure using a Hot Isostatic Press (HIP). Small and large pore specimens containing average pore sizes ranging from 20 microns up to 1mm have been produced by slightly varying the HIPping sintering temperatures and times.Quasi-static and dynamic loading experiments are conducted on various samples produced using the presented methodology and their shape recovery and energy absorption characteristics are measured during the forward and reverse phase transformation and detwinning. Their phase transformation characteristics were found using calorimetric measurements and their composition has been studied using optical and electron microscopy and microprobe x-ray analysis.

Powder Metallurgical Near‐Net‐Shape Fabrication of Porous NiTi Shape Memory Alloys for Use as Long‐Term Implants by the Combination of the Metal Injection Molding Process with the Space‐Holder Technique

“…Conventional sintering, [11,12] capsule-free hot isostatic pressing (HIP), [13,14] self-propagating high-temperature synthesis (SHS), [15][16][17][18][19] HIP with gas entrapment, [20,21] and spark plasma sintering [22] are discussed in the literature. The disadvantages of these methods are the lack of control of the pore size and pore volume fraction as well as chemical inhomogeneities.…”

Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry

“…Porous NiTi alloys have been fabricated by various methods such as conventional sintering, 3) combustion synthesis 4) and the HIP process 5) etc., in addition to the ordinary powder metallurgical method using a polymeric foamprecursor. 6) Generally in metal foams, the plateau regions in stress-strain curves do not appear until the porosity is above 80%, but hitherto only a few NiTi foams have attained this level, so the shape memory characteristics of highly porous NiTi alloys have not yet been investigated.…”

Manufacturing and Post Treatment of SMA Components

“…The use of higher temperatures (generally joule heating) and pressures causes better diffusion of constituents due to partial melting of components [9,10]. A follow up sintering procedure on the resultant product at reduced pressures can force the trapped Air/Argon bubbles thus resulting in near spherical pores [11]. HIP technique allows superior control over surface finish, final geometry, microstructure homogeneity and porosity of the NiTi samples compared to conventional cintering techniques [9].…”

“…HIP technique allows superior control over surface finish, final geometry, microstructure homogeneity and porosity of the NiTi samples compared to conventional cintering techniques [9]. However, NiTi 2 or Ni 3 Ti precipitates in the resultant product cannot be avoided with such a technique [1,11].…”