Phase diagrams polymorphic metals

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

The phase diagram for all the lanthanoid sesquioxides is given in Fig. 3-17 and shows the different transformation temperatures. From the diagram it is obvious that promethium, Pm, was the first metal for which the existence of all three polymorphs and the transition temperatures could be established ]39]. For praseodymium, Pr, under atmospheric pressure the C-type structm-e is formed above 500 °C and transforms to A-type above 700 °G. Depending on the ionic radii only for the rare earth metals from Pm to Gd all five polymorphs will be formed. With increasing temperature the order of transition is C B—>A. 

Hardly any phase diagram studies were found to concentrate on the boron-rich corner of multicomponent systems and many confusing results exist about boron-rich transition metal borides, which more recently turned out to merely represent a solid solution of metal atoms in ) -rhombohedral boron. Only few reliable data exist on the solubility limits of metals in the different boron modifications as a function of temperature as well as on the (stabilizing) influence of T metals on the polymorphic transition itself, Thus metal solubilities in B indicated throughout this publication are the ss of M in f -rh. B . 

---