Introduction
The high-temperature deformation alloys, called Fe-based alloys, have been widely utilized as structural materials of a variety of industrial components, due to their good mechanical properties and resistance to oxidation. However, in many applications these alloys are exposed to high temperature and high strain rate, which cause the material to reach the yield point, a point at which further strain generates plastic deformation. To overcome this problem, a method known as time-temperature superposition (TTS) has been used to enhance the time-temperature bond strength of Fe-based alloys.
TTS invloves the use of two components at high temperature to increase the strength of the alloy. The first component is a low-temperature alloying element, such as chromium or nickel, which is added to the alloy prior to high-temperature processing. The second component is a high-temperature alloying element, such as titanium or zirconium, which is added to the alloy during the high-temperature processing stage. Aside from enhancing the time-temperature bond strength of iron-based alloys, TTS also has other benefits, such as improving hardness, reducing fatigue, and increasing creep resistance.
Time-Temperature Superposition
The concept of time-temperature superposition (TTS) is based upon the physical phenomenon of time-temperature equivalence. In this phenomenon, a material’s mechanical properties at a given temperature can be determined by simultaneously applying both a given temperature and a given strain rate. This phenomenon is shown experimentally through a series of mechanical tests at different temperature and strain rates. In TTS, the same mechanical properties at a given strain rate can be replicated by varying the temperature and strain rate of the same material. This can be accomplished by adding a low-temperature alloying element to the alloy prior to high-temperature processing, and a high-temperature alloying element to the alloy during high-temperature processing.
The low-temperature alloying element, such as chromium or nickel, allows for a low strain rate during cold working of the material. The high-temperature alloying element, such as titanium or zirconium, allows for a high strain rate during hot working of the material. The addition of these two alloying elements increases the strain rate at both the low and high temperatures, effectively time-temperature superpositioning the material and allowing for the development of desired mechanical properties at both low and high temperatures. This process is known as time-temperature superposition (TTS).
Benefits of Time-Temperature Superposition
Time-temperature superposition (TTS) allows for the development of desired mechanical properties of Fe-based alloys at low and high temperatures. TTS also has other benefits, such as improving hardness, reducing fatigue, and increasing creep resistance. When TTS is used, the alloy’s microstructure and mechanical properties are improved due to the increased strain rate. This improved microstructure and mechanical properties result in increased wear resistance, higher fatigue threshold, and better creep resistance.
Additionally, TTS increases wear resistance due to reduced grain growth, sharper grain boundaries, and the tighter packing of atoms within the alloy. This is because the increased strain results in a decreased activation energy for grain boundary motion and dislocation motion, which increases the rate of grain boundary migration. The decreased rate of grain boundary migration prevents large grain growth and increases the number of smaller grains with sharper grain boundaries, resulting in improved wear resistance.
Conclusion
Time-temperature superposition (TTS) is a method used to enhance the time-temperature bond strength of Fe-based alloys. TTS involves the addition of a low-temperature alloying element prior to high-temperature processing and a high-temperature alloying element during high-temperature processing. The addition of these two alloying elements increases the strain rate at both the low and high temperatures, effectively time-temperature superpositioning the material and allowing for the development of desired mechanical properties at both low and high temperatures. TTS also has other benefits, such as improving hardness, reducing fatigue, and increasing creep resistance. The improved microstructure and mechanical properties resulting from the time-temperature superposition process ultimately result in increased wear resistance, higher fatigue threshold, and better creep resistance.
