Диаграммы состояния трехкомпонентной систем с двойными химическими соединениями, плавящуюся инконгруэнтно.

 

Точка 1 находится вне поля кристаллизации этого соединения и соединительная линия не пересекает линии резорбции (растворения). Каждый элемент треугольника должен располагать своей трехлучевой звездой в которой заканчивается кристаллизация. Для треугольника АСAmBn такой трехлучевой звездой будет Pn. Для треугольника AmBnСВ - Е₄ . Поэтому все составы расположенные в треугольнике АСAmBn заканчивают кристаллизоваться в точке Pn, а все составы треугольника AmBnСВ – в точке Е₄. Линию UP называют линией резорбции, эта линия является характерной на всем протяжении и на ней происходит растворение ранее выделившихся кристаллов А из всех составов которые расположены в поле компонента А. Одновременно с резорбцией кристалла А происходит образование кристаллов AmBn. Точка Pn не является эвтектической и втоже время служит конечным пунктом кристаллов составов треугольника АС AmBn и называется точкой двойного подъема

 

Lecture № 29 Theme:

Definition of the way of crystallization at immiscibles liquid. Definition of character of boundary curves. Definition of character of process occurring at change of temperature along boundary curves. Definition of direction of fall of temperature on boundary curves (rule of the temperature maximum).

Boundary curves and fields primary crysallization.

All field of a triangle of concentration is divided by boundary curves (for example, ah, be, ef, dg and etc, fig. 2) on line of sites named as fields primary crystallization (for example, field k-n-е-b-а-k, b-е-f-с-d-b, d-с-g-d and etc).

To each chemical connection in the given system there corresponds the certain field primary crystallization (in figures an accessory of fields primary crystallization to those or other connections is designated by the formula, taken in a circle, this connection). After beginning crystallization within the limits of any field primary crystallization in balance there are two phases - liquid and crystals of that connection, which possesses this field. For example, the field d-с-g-d in a fig. 2 is a field primary crystallization of connection ABC and, hence, in it a field in balance with a liquid there are crystals of this connection. The system means by a rule of phases within the limits of fields primary crystallization due-variants. If crystallization is not completed, all points of fields primary crystallization show structure of a liquid phase (alloy), taking place in balance with firm phase - crystals of connection, which belongs this field primary crystallization (if crystallization is completed, point place primary crystallization characterize total chemical structure of firm phases).

Fig. 3. The mutual arrangement points of structures and fields primary crystallization for connections, melting cogrous (а), uncogrous (b, c) and decomposed in firm condition (b)

 

The arrangement on the diagram of a condition of a point of structure of the given connection and its field primary crystallization determines character of process occurring at heating of this connection:

1) if the point of structure of double or threefold connection lays in an own field primary crystallization, this connection melting without decomposition, i.e. cogrous (for example, connection АС and ABC, fig. 3, а);

2) if the point of structure of double connection lays outside of a field by its primary crystallization and the field adjoins to the party of a triangle, on which the point of structure of this connection, it melting with decomposition, i.e. uncogrous (for example, connection АС, fig. 3, b);

3) if the point of structure of double connection lays outside of a field by its primary crystallization, and last is located inside a triangle of concentration, not adjoining to the party of a triangle, on which the point of structure of this double connection lays, it at heating is decomposed in firm condition (for example, connection ВС, fig. 3, b);

4) if the point of structure of threefold connection lays outside of a field by its primary crystallization, this connection melting with decomposition, i.e. uncogrous (for example, connection ABC, fig. 3, b).

On boundary curves, each of which divides two fields primary crystallization, in balance there are three phases - liquid and crystals of two connections, which fields divides this curve (for example, on a boundary curve ab in a fig. 2 in balance with a liquid there are crystals of connections АВ and АС). The system by a rule of phases in this case is mono-variants. All points of boundary curves in process crystallization show structure of a liquid phase which is taking place in balance with crystals of the appropriate connections. Boundary curves the arrows usually underline direction of fall of temperature. Depending on character of process occurring in system at change of temperature along boundary curves, they are divided on cogrous and uncogrous. On cogrous of boundary curves there is a physical process crystallization (at downturn of temperature) or melting (at increase of temperature). Uncogrous curves as against cogrous are curves, on which there is a chemical reaction accompanying with disappearance one and occurrence of other phases in system. Cogrous and uncogrous boundary curves differ also by that the way crystallization from first never goes, and from second can (though and not necessarily) leave, having left uncogrous curve.