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Hume-Rothery rules:-

HUME-ROTHERY'S RULES:- 

             In the course of an alloy development, it is frequently desirable to increase the strength of the alloy by adding a metal that will form a solid solution. In the choice of such alloying elements, a number of rules govern the for- mation of substitutional work of Hume-Rothery. Unfortunately, if an alloying element is chosen at random, intermediate phase instead of a solid solution. 

Hume-Rothery's Rules are described below. is likely to form an objectionable.

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CHEMICAL AFFINITY FACTOR 

             The greater the chemical affinity of two metals the more restricted is their solid solubility. When their chemical affinity is great, two metals tend to form an intermediate phase rather than a solid solution.

             Chemical Affinity Factor-The greater the chemical affinity of two metals, the more restricted is their solid solubility. When their chemical affinity is great, two metals tend to form an intermediate phase rather than a solid-solution. Generally, the farther apart the elements are in the periodic table, the greater is their chemical affinity.

RELATIVE VALENCY FACTOR 

              If the alloying element has a different valence from that of the base metal, the number of valence electrons per atom, called the electron ratio, will be changed by alloying. Crystal structures are more sensitive to a decrease in the electron ratio than to an increase. Therefore, a metal of high valence can dissolve only a small amount of lower valance metal, while the lower valence metal may have good solu- bility for a higher valence metal. 

              Relative Valence (Valency) Factor - It has been found that the metal of high valence can dissolve only a small amount of a lower valence metal, while the lower valence metal may have good solubility for the higher valence metal. For example in the Al-Ni alloy system both metals have FCC structure. The relative size factor is approximately 14%. However, Ni is lower n valence than Al and thus solid nickel dissolves 5% aluminium, but the higher valence Al dissolves only 0.04% Ni.

RELATIVE SIZE FACTOR 

            If the size of two metallic atoms (given approxima- tely by their constants) differs by less than 15 percent, the metals are said to have a favourable size factor for solid solution formation. So far as this factor is concerned, each of the metals will be able to dissolve apprecia- bly (to the order of l0%) in the other metal. If the size factor is greater than 15°, solid solution formation tends to be severely limited and is usually only a fraction of one percent.

               Relative Size Factor- If the two metals are to exhibit extensive solid solubility in each other, it is essential, that their atomic diameters shall be fairly similar, since atoms differing greatly in size cannot be accommodated readily in the same structure (as a substitutional solid solution) without producing excessive strain and corresponding instability. This is what referred as the term size factor. The extensive solid solubility is encountered only when the two different atoms differ in size by less than 15%, calleda favourable size factor (e.g. Cu-Ni, Au-Pt). If the relative size factor is between 8% and 15 % , the alloy system usually shows a minimum and if this is greater than 15 % , substitutional solid solution formation is very limited. 

LATTICE-TYPE FACTOR 

           Only metals that have the same type of lattice (FCC for example) can form a complete series of solid solutions. Also, for com- plete solid solubility, the size factor must usually be less than 8 percent. Copper-nickel and silver-gold-platinum are examples of binary and ter- nary systems exhibiting complete solid solubility.

              Crystal Structure Factor The crystal lattice structure of the two elements (metal) should be same (i.e. both should be of BCC, FCC, or HCP structure) for complete solubility, otherwise the two solutions would not merge into each other. Also for complete solid solubility the size must usually be less than 8%.

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Hume-Rothery's Rules are described below. is likely to form an objectionable.

रासायनिक प्रभाव क्षेत्र

             दो धातुओं का रासायनिक आत्मीयता जितना अधिक प्रतिबंधित है, उनकी ठोस घुलनशीलता है। जब उनकी रासायनिक आत्मीयता महान होती है, तो दो धातुएं एक ठोस समाधान के बजाय एक मध्यवर्ती चरण बनाती हैं।

रिश्ता वैधता का कारखाना

              यदि मिश्र धातु तत्व का आधार धातु से भिन्न भिन्नता है, तो प्रति परमाणुओं में वैलेंस इलेक्ट्रॉनों की संख्या, जिसे इलेक्ट्रॉन अनुपात कहा जाता है, को मिश्रधातु द्वारा बदल दिया जाएगा। वृद्धि की तुलना में क्रिस्टल संरचनाएं इलेक्ट्रॉन अनुपात में कमी के प्रति अधिक संवेदनशील हैं। इसलिए, उच्च वैलेंस की एक धातु केवल कम वैलेंस धातु को भंग कर सकती है, जबकि कम वैलेंस धातु में उच्च वैलेंस धातु के लिए अच्छा सॉल्यू-बाइट हो सकता है।

विश्वसनीय आकार फैक्टरी

            यदि दो धात्विक परमाणुओं का आकार (उनके स्थिरांक द्वारा अनुमानित रूप से दिया गया) 15 प्रतिशत से कम होता है, तो धातु को ठोस घोल बनाने के लिए अनुकूल आकार कारक कहा जाता है। जहाँ तक इस कारक का संबंध है, प्रत्येक धातु दूसरे धातु में apprecia- bly (l0% के क्रम में) को भंग करने में सक्षम होगी। यदि आकार का कारक 15 ° से अधिक है, तो ठोस समाधान गठन गंभीर रूप से सीमित हो जाता है और आमतौर पर केवल एक प्रतिशत का एक अंश होता है।

लेटिस-टाईप फैक्टर

           केवल धातुएँ जिनमें एक ही प्रकार की जाली होती है (उदाहरण के लिए FCC) ठोस समाधानों की एक पूरी श्रृंखला बना सकती है। इसके अलावा, कॉम-पिल सॉलिड सॉल्युबिलिटी के लिए, साइज फैक्टर आमतौर पर 8 प्रतिशत से कम होना चाहिए। कॉपर-निकेल और सिल्वर-गोल्ड-प्लैटिनम बाइनरी और टेर-नैरी सिस्टम के उदाहरण हैं जो पूर्ण ठोस घुलनशीलता प्रदर्शित करते हैं।

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CONTINUOUS CASTING PROCESS (Asarco Process)

Continuous casting process:-

In Continuous casting process the forming die is an integral part of the furnace and thus there is no problem of controlling the flow of metal . The metal is fed by gravity into the mould from the furnace as it is continuously solidified and withdrawn by the rolls below . The die is water cooled and self lubricating and thus has excellent resistance to thermal shocks . The upper end of the die is in molten metal and thus serves the function of riser and acts as path for dissipation of evolved gases .
      At the time of starting , a similar rod is put into the die till its upper end melts and forms Holding continuity with molten metal , and it is then Furnace withdrawn by rolls and the process becomes continuous . By this process , rounds , tubes , squares and special shapes can be conveniently produced and it is particularly suited to alloys like phosphorised copper and standard bronzes .

      The physical properties of castings Casting - produced are superior to other processes . In continuous casting , solidifying zone being relatively small , practically all problems encountered with feeding and shrinkage are overcome . Problems due to fast cooling in mould zone are faced segregation and cracking ) . A greater degree of control is required in comparison batch casting . Asarco process is shown n fig.

 Uses - Continuous casting process is specially suited to rounds , tubes , squares and special shapes , particularly made from alloys like phosphorized popper and standard bronzes .

Merits - ( i ) Physical properties of castings produced are superior to other processes , because metal is protected from contamination , while melting and being poured .

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