Molybdenum-rhenium alloys commonly encountered by people working in the field of refractory metals are mainly used in infrared halogen lamp filaments, heating elements in chemical vapor deposition furnaces, thermocouple jackets, heat shields, spacecraft thrusters and other components operating at extremely high temperatures. More recently, traditional "Mo-50 Re" alloys (52.5 percent molybdenum and 47.5 percent rhenium) have been discovered for use at body temperature because of their mechanical and biological properties.
Most cardiovascular stents are made with solid tubes, which are made into mesh tubes through complex and sophisticated processing techniques such as micromachining or laser cutting. The mesh tube or stent is used to dilate the blocked artery and restore its blood flow. To place a stent, a surgeon makes a small incision in an artery in the arm or groin, inserts a catheter containing one or more stents into the artery, delivers it to the site of the vascular lesion, and then dilates the stent with a balloon. In the past, the support has been made of small-bore stainless steel, titanium, cobalt-based or nickel-titanium (Nitinol) alloy seamless tubing. The external diameter of the stent used for small arteries is generally less than 2 mm, while the external diameter of the stent used for large arteries is generally 2-5 mm.
Similar to common metals, cold working improves the strength of Mo-RE alloys. However, during cold working, Mo-Re produces an unusual change in deformation called twin-induced plasticity (TWIP). MoRe® implants benefit from a unique twIP-induced plasticity (TWIP) effect that gives the material both high strength and high ductility. The latest ASTM standard, Forging Molybd-47.5 Rhenium Alloy (UNS R03700) F3273-17 for Surgical Implants, specifies several different strength grades of Mo-RE alloys based on the amount of cold processing. The highest strength MoRe alloys have minimum yield strength and ultimate tensile strength in excess of 1300MPa and ductility percentages in the double digits. Using high-strength alloys, designers can design smaller and lighter implants that reduce the intrusion of surrounding bone tissue and also help avoid protrusions that blend better with the bone's natural shape.
For small and medium-sized surgical implants, the weight gain due to high implant density is small, while the higher density of Mo-RE alloy has little impact. In orthopaedic applications, however, the elastic modulus of the material is generally not expected to be too high because rigid implants reduce the load on the bone, a phenomenon known as "stress shielding." Because the human body carries less load, it will weaken the growth of bones. The effect becomes more pronounced the larger the implant, so designers must try to keep the size of the implant to a minimum. This means that the high modulus alloy must also have a high strength in order to have a thinner cross section size, and mo-RE has this advantage.
