**3.2 The lattice distortion effect**

Unlike conventional alloys with a single element solvent matrix, the HEAs comprises of a random solute matrix (RSM). The assumption, that RSM comprises of constituting elements arranged with complete disordered fashion and hence result in a SS, is mostly true. There is occasional occurrence of some intermetallic phases in the alloy which could be due to (a) enthalpy of formation of intermetallic is very high compared to the enthalpy of mixture; (b) the difference in atomic size is less (1.1-1.6) which supports formation of ordered structure; (c) the incorporation of elements with very small atoms in the alloy, which forms interstitial compounds. The Lattice Distortion Effect (LDE) is theoretically estimated by polydisparity or atomic mismatch factor δ given by Eq. (4) [7].

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*Tici*

$$\delta = \sqrt{\sum\_{i=1}^{n} c\_i \left(1 - \frac{r\_i}{\sum\_{j=1}^{n} c\_j r\_j}\right)^2} \tag{4}$$

The LDE tends to increase the hardness of HEAs because of solution hardening.

#### **3.3 The sluggish diffusion hypothesis**

Studies suggest that MPEAs exhibit a diffusion rate slower than any regular alloys and which could be responsible for the lower number of phases present in MPEAs [8]. This slow diffusion behaviour is claimed as sluggish diffusion. The HEAs studied at the beginning exhibited this behavior, though the sluggish


*Magnesium Alloys Structure and Properties*


**Table 1**

*Value of empirical parameters of Mg containing HEAs.*
