Question

Explain how alloys based on the base material titanium and nickel can be cured.

What curing mechanisms can be used and what are the most important mechanism (creating the most stretch limit increase) that can be created through heat treatments and how do they disrupt the mechanism in each system?

Answer 1

Sol:-

Ni-Ti alloy (also known as Nitinol) is an alloy with a near-equiatomic composition (i.e., 49%–51%) of nickel and titanium. Ni-Ti belongs to the class of shape memory alloys that can be deformed at a low temperature and are able to recover their original, permanent shape when exposed to a high temperature. The shape memory effect of Nitinol is related to its martensitic transformation. In fact, at high temperature, the atoms are arranged in an orderly manner, in a BCC crystal structure (i.e., austenitic phase). In this phase, the alloy is resistant to twisting and bending. At low temperature, Ni-Ti alloy exhibits a monoclinic distorted crystal structure, that is, a martensitic phase. At this stage, the alloy can be easily deformed. The temperature at which the austenitic phase transforms into a martensitic one is called the transformation temperature. The atomic percentage composition is important in determining the shape memory effect at body temperature; in fact, the presence of a higher content of nickel (even by 0.1%) can induce a decrease in the transformation temperature and an increase in yield strength of the alloy. Moreover, contaminants, such as carbon and oxygen, can vary the transformation temperature, affecting the mechanical properties. Hence, attention should be taken to minimize the concentration of the contaminants.The shape memory effect can be used in different biomedical devices; among them, self-expanding vascular stents represent the main example. Nitinol stents have a small diameter at room temperature so they can be crimped onto the delivery system and inserted into the human body via catheter, using a minimally invasive approach. Once it reaches the correct site, it can recover its permanent shape (i.e., larger diameter) at body temperature, as the stent can remember the shape in its austenitic phase at elevated temperature.

Multistage heat treatments are given to powder metallurgy superalloy products to develop alloy microstructures appropriate for the application. Heat treatment involves a sequence of a solution anneal followed by one or more precipitation aging treatments. The solution anneal is performed to allow carbides and precipitated constituents to go into solid solution. This anneal is usually followed by some type of rapid cooling to prevent dissolution and reprecipitation. Directional heat treatments can be used for ODS alloys to promote the formation of very coarse, elongated grains. These anneals and treatments prepare the alloy for later heat treatments where control of temperature, time, and cooling rate promotes selective precipitation of the various phases to the desired locations in the microstructure. Single or multistage aging treatments are then used to develop the desired precipitate size and size distributions.

Strengthening mechanisms in metals

• Work hardening.
• Solid solution strengthening and alloying.
• Precipitation hardening.
• Grain boundary strengthening.
• Transformation hardening.
• Polymer.
• Glass.
• Fiber reinforcement.     Effect of heat treatment on each mechanism
• Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.
• Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal's microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does.
• Precipitation hardening is also known as age hardening. It creates uniformity in a metal's grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 900 degrees Fahrenheit to 1,150 degrees Fahrenheit. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.
• Commonly used in steelmaking today, tempering is a heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals.
• Stress relieving is a heat treatment process that decreases stress in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.
• Normalizing is a form of heat treatment that eliminates impurities and improves strength and hardness by altering the grain size to be more uniform throughout the metal. This is achieved by cooling the metal by air after it has been heated to a precise temperature.
• When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal stress of the metal. Next, the metal part is maintained at a temperature of roughly minus 190 degrees Celsius for about a day. When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately 149 degrees Celsius. This helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.