Equiatomic TiNi (Nitinol) has the unique ability to rapidly and reversibly change from its martensitic and austenitic phases with the application of stress. This phase change allows the intermetallic to absorb significant amounts of mechanical energy without permanently deforming. This binary alloy can be fabricated to contain a network of interconnected open pores, typically on the order of several hundred microns in size, using a process known as Self-propagating High-temperature Synthesis (SHS). This porous morphology augments the alloy’s inherent energy absorbing capability to produce a material with a very high shock mitigation capacity. This makes porous TiNi very attractive in armor applications. Click here to see the mechanical and shock damping properties of porous TiNi.
BONE IMPLANTS
Equiatomic nickel-titanium alloys have been studied extensively for use as implants due their mechanical behavior and biocompatibility.
Nickel-titanium implants show promise as substitutes for bone because the mechanical properties of TiNi are very similar to osseous tissue. Not only is the elastic modulus of TiNi comparable, but, like bone, TiNi has a high recoverable strain, up to 8%. This property of Nitinol is called superelasticity and is unique among metals. In comparison, the elastic strain of other biocompatible metals like stainless steel and titanium is less than 1%. Bone has a recoverable strain of 2%, this is what allows it to absorb a lot of the energy without deforming or fracturing.
It is very important for an osteoimplant to mimic the stiffness and strength of the tissue it replaces. If the implant is too weak, it will fail prematurely. If it is too strong, the bone adjacent the implant will be shielded from stress. The body will compensate for this by absorbing the surrounding osseous tissue reducing the contact between the implant and the bone resulting in poor integration of the prosthesis. Incorporation of the implant in the body can be enhanced by making TiNi porous. This allows osteoblasts to infiltrate the bioimplant and form bone within it. For optimum osseointegration, pore sizes must range from 50 to 500 mm. Also, since SHS is a net-shape process with the right mold any bone of any shape can be replaced. Click here to see paper on porous TiNi implants.
Self-Propagating High-Temperature Synthesis (SHS)
Shape Change Technologies has developed a Self-Propagating High-Temperature Synthesis
(SHS) process to produce our net shape TiNi components. SHS is a rapid means of making high quality material
with consistent properties. Our process
requires relatively simple and inexpensive equipment and materials compared
to other powder metallurgy alloying techniques and creates high quality porous
material with very little deleterious impurities. We fabricate parts
in minutes compared to the hours at extreme temperatures required
to sinter. Furthermore, the process is scalable to a wide range
of sample shapes and sizes. Click here to see video of our SHS process.
