Hig entropy alloys
Report on High Entropy Alloys(HEA)
Introduction:
Alloys can be defined as a mixture of two or more elements containing one or more elements containing one metal at the least. From bronze age , alloys have been developed based on one principal element. Later metal matrix composites and inter metallic compounds gained attention. Development in technology and innovation lead to the idea of multicomponent system. These systems consists of multiple principal elements. This was attained by alloying elements with equimolar or near-equimolar ratios. The concept of using multiple principal elements was initially discouraged for it was believed , by conventional metallurgical knowledge, there will be several complex phases and inter-metallic compounds which result in difficulty in micro structural analysis and deterioration of mechanical properties.
However , these beliefs were disproved by recent discoveries and synthesis of high entropy alloys.
High entropy alloys:
As the name indicates, these alloys possess high entropy (>1.6R J/K-mol). This large entropy is rendered by entropy of mixing due to equimolar ratio of elements in the alloy. The ratio or percentage of elements in the alloy can vary from 5 at% to 35 at%. The estimated value of the entropy can be calculated by assuming n elements in exactly equimolar ratio by
Boltzmann's hypothesis:
When we substitute the values of n=3,5,6,9,13 we get the values of mixing entropy
ΔSmix = 1.1R , 1.61R ,1.79R, 2.20R, 2.57R respectively. By Richard's rule , entropy changes in most of the metals are empirically equal to R at their melting points. Considering this ΔSmix of the 5 element alloy is far greater than R, hence the name high entropy alloys has been given(Since, high entropy alloys usually has at least 5 elements).
Stability:
High entropy alloys derive their strength by formation of random solid solution with a simple structure. But solid solution is not always formed. Sometimes the alloy can
Introduction:
Alloys can be defined as a mixture of two or more elements containing one or more elements containing one metal at the least. From bronze age , alloys have been developed based on one principal element. Later metal matrix composites and inter metallic compounds gained attention. Development in technology and innovation lead to the idea of multicomponent system. These systems consists of multiple principal elements. This was attained by alloying elements with equimolar or near-equimolar ratios. The concept of using multiple principal elements was initially discouraged for it was believed , by conventional metallurgical knowledge, there will be several complex phases and inter-metallic compounds which result in difficulty in micro structural analysis and deterioration of mechanical properties.
However , these beliefs were disproved by recent discoveries and synthesis of high entropy alloys.
High entropy alloys:
As the name indicates, these alloys possess high entropy (>1.6R J/K-mol). This large entropy is rendered by entropy of mixing due to equimolar ratio of elements in the alloy. The ratio or percentage of elements in the alloy can vary from 5 at% to 35 at%. The estimated value of the entropy can be calculated by assuming n elements in exactly equimolar ratio by
Boltzmann's hypothesis:
When we substitute the values of n=3,5,6,9,13 we get the values of mixing entropy
ΔSmix = 1.1R , 1.61R ,1.79R, 2.20R, 2.57R respectively. By Richard's rule , entropy changes in most of the metals are empirically equal to R at their melting points. Considering this ΔSmix of the 5 element alloy is far greater than R, hence the name high entropy alloys has been given(Since, high entropy alloys usually has at least 5 elements).
Stability:
High entropy alloys derive their strength by formation of random solid solution with a simple structure. But solid solution is not always formed. Sometimes the alloy can