*2.1. The Methods of Zr addition*

The direct addition of pure Zr metal into Mg melt is very difficult, because Zr has a much higher melting point (~1852 ◦C) than Mg (~650 ◦C) and a stable oxide film that normally presents on Zr particle surfaces [9,12]. Before the popular use of the Mg-Zr master alloy, many approaches were tried, such as alloying with pure Zr powder, ZrO2, or Zr-rich salt mixtures [6,13–15]. Table 1 shows some examples of early patents claiming methods for alloying Zr in Mg alloy melt [16–19]. For example, Sauerwald et al. tried using the Zr powder as a source [16]. However, Zr powder is prone to ignite, and the oxide film inhibits the diffusion of Zr in the Mg melt. In addition, the direct addition of Zr-containing chloride or fluoride salt mixtures into Mg alloy melt has also been tried, which showed that the residuals MgCl2 or MgF2 are difficult to remove from the melt [6]. In a word, early methods were not satisfactory due to their complicated processes and the contamination of the melt. Recent research has attempted the traditional Zr-rich salts method again, through the addition of K2ZrF6-NaCl-KCl [20,21], KZrF5-LiCl-KCl-CaF2 [22] or ZrCl4 [23] salts to Mg-RE alloys, further confirming that good grain refinement effects can be achieved. Nevertheless, the salt inclusions are really difficult to remove.

**Table 1.** Early patents of Zr alloying methods in Mg alloy.


In addition to the direct use of Zr-rich salt mixtures, earlier researchers have also tried to prepare a Mg-Zr master alloy. For example, Robert [24] tried three different routes to produce Mg-Zr master alloys: (1) Distillation of Mg out of low-Zr Mg-Zr alloy to obtain a high-Zr Mg-Zr alloy. Mg could either be sublimated at pressures of mercury below about 3 mm, or boiled at higher pressures. However, the alloys showed serious compositional segregation. (2) Fusing compacts of ZrH2 powder and Mg shavings. The reason ZrH2 was used is that ZrH2 is not pyrophoric, unlike pure Zr powder. A compact of ZrH2 powder and Mg shavings was formed using a hydraulic press. Then, the compact was heated to a temperature above 900 ◦C for a couple of hours in an electric vacuum furnace in order to remove the H2 and form Mg-Zr alloy. However, several trials only demonstrated limited success. (3) Melting compacts of Zr and Mg shavings in a sealed steel "bomb". The steel bomb was heated in an electric muffle furnace at a temperature below 500 ◦C for a couple of hours. After being taken out of the furnace, it was shaken vigorously and then quenched in water. The results showed that the Mg-Zr alloy was of sufficient quality. However, due to their complex processes and low production capacity, these methods have been abandoned.

In terms of the development of a production method for Mg-Zr master alloys, the chemical reduction of Zr-rich fluoride or chloride salt with molten Mg has become a suitable route [14]. During the production process of the Mg-Zr master alloy, the Zr particles were formed in-situ within the Mg matrix, and thus the particle surfaces were not contaminated with O2 or other impurities. This kind of Zr particle is clean, active, and quickly diluted in Mg melt, helping to easily achieve the saturation of Zr content [6]. Therefore, since about 1960, only Mg-Zr master alloy has been widely used as a satisfactory Zr refiner in Mg alloys. A famous commercial Mg-Zr product named Zirmax, developed by Magnesium Elektron in the UK, has become one of the popular products [6]. This product contains nominally more than 30% Zr in a relatively homogeneous distribution of Zr particles.
