Solid solution alloys

Solid solution alloys

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  Solid Solution alloys C.Karthika AP/Physics KonguEngineering College
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  Alloy ❖ Metals intheir pure form are seldom used in engineering applications. Most of the useful metallic materials are combinations of metals known as alloys. ❖ An alloy is any combination of two or more elements that results in a substance possessing metallic properties. ❖ Elements may combine in different ways to form alloys. The components of the alloys are usually completely soluble in the liquid state. ❖ In solid state the elements may form mechanical mixtures, solid solutions or intermediate phases.
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  ❖Solvent represents theelement or compound that is present in the greatest amount ❖Solute is used to denote an element or compound present in a minor concentration. ❖In Alloys: The element which is present in the largest proportion is called the base metal (solvent), and all the other elements present are called alloying elements (solute). Solution
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  mechanical mixtures
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  Mechanical mixtures ❖Mechanical mixturesare formed when the two elements are completely insoluble in the solid state. ❖For example, lead is insoluble in iron. The alloy of lead and iron is an intimate mechanical mixture of the components where each component retains its own identity, properties and crystal structure.
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  Intermetallic compounds ❖ Intermediatestructures formed between two metals are neither the parent metals nor like an alloy ❖ If two elements have high difference in electronegativity, they tend to from a system called intermetallic compound. ❖ Intermetallic compounds like MgSe, PbSe, Mg2Si, Cu2S are cubic ❖ whereas NiAs, MnSe, CuSn are hexagonal
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  Solid solution alloy ❖A solid solution is a solid-state solution of one or more solutes in a solvent ❖ A solid solution forms when, as the solute (impurity) atoms are added to the host material, the crystal structure is maintained and no new structures (phases) are formed. ❖ This is analogy with a liquid produced as the molecules intermix, and its composition is homogeneous throughout. ❖ A solid solution is also compositionally homogeneous; the impurity atoms are randomly and uniformly dispersed within the solid.
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  Types of Solidsolution Depending upon the the places occupied by the solute atoms, solid solutions could be either substitutional or interstitial -Point defects
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  ➢ Interstitial solidsolution Interstitial solid solutions form for relatively small impurity atoms that occupy interstitial sites (voids) among the host atoms. ⚫ For metallic materials that have relatively high atomic packing factors, these interstitial positions are relatively small. ⚫ Consequently, the atomic diameter of an interstitial impurity must be substantially smaller than that of the host atoms. Normally, the maximum allowable concentration of interstitial impurity atoms is low (less than 10%). Even very small impurity atoms are ordinarily larger than the interstitial sites, and as a consequence they introduce some lattice strains on the adjacent host atoms. ⚫ Carbon forms an interstitial solid solution when added to iron (steel); the maximum concentration of carbon is about 2%. The atomic radius of the carbon atom is much less than that for iron: 0.071 nm versus 0.124 nm
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  Hume-Rothery rules (Interstitial solidsolution) 1. Atomic radius: Solute atoms should have a smaller radius than 40% of the radius of solvent atoms. 2. Electronegativity: The solute and solvent should have similar electronegativity. 3. Valence factor: two elements should have the same valence. The greater the difference in valence between solute and solvent atoms, the lower the solubility.
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  Substitutional solid solution Soluteor impurity atoms replace or substitute for the host atoms.
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  Hume-Rothery rules (Substitutionalsolid solution) Example : copper & nickel Atomic radii for Cu and Ni are 0.128 and 0.125 nm, respectively; both have the FCC crystal structure; and their electronegativities are 1.9 and 1.8. Finally, the most common valences are 1 for Cu (although it sometimes can be 2) and 2 for Ni. 1. Atomic size factor - Appreciable quantities of a solute may be accommodated in this type of solid solution only when the difference in atomic radii between the solute and solvent atom types is less than 15% Otherwise the solute atoms will create substantial lattice distortions and a new phase will form. 2. Crystal structure – The crystal structures of solute and solvent must be similar. 3. Electronegativity - The solute and solvent should have similar electronegativity. If the electronegativity difference is too great, the metals tend to form intermetallic compounds instead of solid solutions. 4. Valences – Complete solubility occurs when the solvent and solute have the same valency. The solubility of a metal with higher valence in a solvent of lower valence is more compare to the reverse. Zn is more soluble in Cu than Cu soluble in Zn.
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  Ordered Solid solution-atoms of solute occupy certain preferred sites in the lattice of the solvent. In Cu-Au systems, Cu atoms occupying face Centred sites and Au atoms occupying the corner sites of the fcc unit cell. Ordered & disordered Substitutional solid solution Disordered Solid solution - the atoms of the solute are present randomly in the lattice of the solute
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  Is alloy acompound? What is the difference between solid solution and mechanical mixture? No. In alloys metals and non metals do not chemically react with each other. Alloy is just a physical mixture. Alloy retains the properties of the constituent elements Solid solution: Completely dissolved - homogeneous mixture Mechanical mixture: Just mixed not completely dissolved - heterogeneous mixture Questions
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  Why steel isso strong? Usually materials deforms by the movement of dislocations. The carbon interstitials make Steel Stronger by fully or partially blocking the movement of dislocations What are the advantages of solid solution ? Formation of solid solution usually cause increase of electrical resistance and mechanical strength and decreases Brass, Bronze and Steel - Classify them based on solid solution type. ⮚ Brass, Bronze- substitutional solid solution ⮚ Steel- interstitial solid solution
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  Thank you………..