Chapter 2-3 Solid structure and basic crystal theory

Chapter 2-3 Solid structure and basic crystal theory

Chapter 2: Solid structure and basic crystal theory(Foundation of crystallography )

Chapter 2-3 Solid structure and basic crystal theory

2.3 Alloy phase structureIntroduction

Alloy an alloy is a partial or complete solid solution of one ormore elements in a metallic matrix. Constituent element: the fundamental unit that composed of the alloy. It can be metals, nonmetals and compounds.

Structure the morphology image of the materials. It can beobserved by the naked eye , the magnifier or microscope. Macro-structure: the morphology image that can be observed by the naked eye or a 30 times magnifer . Micro-structure: the morphology images under the microscope.

Phase Uniform parts in the alloy having the same condensedstate, same chemical component, same crystal structure and properties as well as being separated each other by the interface.

Chapter 2-3 Solid structure and basic crystal theory

Single phase alloys:alloys composed with one phase Multi phase alloys: alloys composed with several different phases

The properties of the alloy are determined by the following factors:(a) Electrochemistry (electronegativity or chemical appetency) (b) the size of the atom (c) the atomic value

Phase classification : solid solution and interphase

Chapter 2-3 Solid structure and basic crystal theory

1. Solid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. The distinct property of the solid solution is keeping the crystal structure of the solvent.

Chapter 2-3 Solid structure and basic crystal theory

a). Classification of the solid solution

According to the location of the solute atoms in the lattice:Substitutional solid solution Interstitial solid solution

According to the solubility of the solute atoms in the solvent:Limited solid solution Unlimitted solid solution

Chapter 2-3 Solid structure and basic crystal theory

According to distribution of the solute atoms in the solvent:Non-ordered solid solution Ordered solid solution

According to the body type :Primary solid solution Secondary solid solution

Chapter 2-3 Solid structure and basic crystal theory

Two types of the solid solution (substitution or interstice)

Chapter 2-3 Solid structure and basic crystal theory

Ordered solid solution-short range

Chapter 2-3 Solid structure and basic crystal theory

Ordered solid solution-long range

Chapter 2-3 Solid structure and basic crystal theory

Ordered solid solution-segregation

Chapter 2-3 Solid structure and basic crystal theory

b). Substitution solid solutionSubstitution solid solution in general is formed among the metal elements.

The same crystal structure is the necessary condition of forming the unlimited solid solution. When the size of the atom △r<15% or r(solute)/r(solvent)>0.85, the solubility of the solid solution will be large under other conditions is nearly same and vice versa.

For example: iron-based alloys, unlimited solid solution △r<8%。 copper-based alloys, unlimited solid solution △r<10%

Electronegativity Electronic density

Large-interphaseSmall-solid solution Large-compound Small-solid solution

Chapter 2-3 Solid structure and basic crystal theory

The sketch map of the substitution solid solution

Chapter 2-3 Solid structure and basic crystal theory

Lattice aberrance induced by the large and small solute ato

ms in the substitution solid solution

Chapter 2-3 Solid structure and basic crystal theory

c). interstitial solid solution

The solute atoms are some nonmetal elements with the radius less than 0.1 nm (C, N, O, H, B) Formation condition: △r>41% or r(solute)/r(solvent)> <0.59

interstitial solid solution can only be limitted solid solution and its solubility is smallFor example: C atoms in α-Fe(max.Wt=0.0218%) C atoms inγ-Fe(max.Wt=2.11%)

Chapter 2-3 Solid structure and basic crystal theory

The sketch map of the interstitial solid solution (1)

Chapter 2-3 Solid structure and basic crystal theory

The sketch map of the interstitial solid solution (2)

Chapter 2-3 Solid structure and basic crystal theory

Lattice aberrance induced by the large and small solute atoms in the interstitial solid solution

Chapter 2-3 Solid structure and basic crystal theory

d). The microcosmic nonuniform of the solid solution

The distribution of the solute atoms in the solid solution is not all unordered. At given conditions, solute atoms and solvent atoms align in an order and form the ordered solid solution. The ratio of the solute atoms with the solvent atoms is a fixed value and it can be expressed by a chemical molecular formula.The ordered solid solution structure can be named as superlattice. For example: for Cu-Al alloy, Cu:Al=1:1 or 3:1 ordered superlattice CuAl or Cu3Al

Chapter 2-3 Solid structure and basic crystal theory

e). Properties of the solid solution

Changes of lattice parameters Substitution solid solution : rsolute>rsolvent,an increase;rsolute<rsolvent,a decrease. Interstitial solid solution : a increases with the dissolving of the solute. Strengthening of the solid solution. changes of the physical and chemical properties.