The internal structure of a material plays an important part on its mechanical properties.!

Transcription

1 Phase Diagrams

2 The internal structure of a material plays an important part on its mechanical properties.! There is a strong correlation between micro structure and mechanical properties.

3 Definitions Component! A component is the pure metals and or compounds of which an alloy is composed.! Brass is copper and Zinc mixed together to form an alloy

4 Brass Copper is the solute, it is the least common and fills in substitutional or interstitial positions.! Zinc is the solvent. The Zinc forms the major lattice points.

5 Solid Solution Consists of atoms of a least two different types.! Made up of Solute (atoms) and solvent (atoms).

6 Solubility For most systems at a specific temperature there is a maximum number of solute atoms the fit (or dissolve) in to the solvent this is the solubility.

7 Example of Solubility Solubility limit 200 Two regions! 1.) Liquid! 2.) Liquid + solid Temperature ( C) Liquid solution (syrup) Liquid solution + solid sugar Temperature ( F) The amount of sugar! which dissolves depends! on concentration and! temperature The solubility limit is! shown 50 Sugar Water Composition (wt%)

8 Solubility limit Temperature ( C) Liquid solution (syrup) Liquid solution + solid sugar Temperature ( F) We can see that we can! change the temperature! or the composition! of the system to alter! the properties of the syste Sugar Water Composition (wt%)

9 Definitions Phases! A phase is defined as a homogeneous portion of a system that has uniform physical and chemical characteristics! Every pure material is a phase, also considered phases are the solid, liquid and gasses of the material.! In our previous example, Liquid is one phase and the solid sugar another.

10 If more than one phase is present in a given system it will have its own district properties.! Phases don t need to have both different chemical and physical characteristics (just one)

11 Microstructure Simply the observed structures, studied using either optical or electron microscopy Brass

12 Phase equilibrium Exists in systems with more than one phase and is characterised by time consistency between phases.! In our sugar example at 20 degrees centigrade, with 65% sugar, some will be solid, some in solution. The amount moving from one state to the other is equal to the amount moving in the opposite direction.! Upon heating however, the system will not be in equilibrium until the two directions once again are equal.

13 Metastable states Very slow movements from one state to the other are said to me metastable.!

14 Phase diagrams Much of the information about the phase structure of a system is conveniently described in whats called a phase diagram! Defined by three parameters which affect phase structure! Temperature, Pressure and composition

15 One component phase diagrams (Unary) Single component! So we can change! Temp and Pressure

16 Binary Isomorphous systems Composition (at% Ni) Liquid 1453 C Temperature ( C) 1300 Liquidus line B + L Solidus line 2400 Temperature ( F) Nickel Copper alloy C A 2000 In binary systems we can (Cu) Composition (wt% Ni) (a) (Ni)

17 Measurements Composition (at% Ni) Temperature ( C) C Liquid Liquidus line B + L A 1453 C Solidus line Temperature ( F) The phases present! The composition of the phases! The percentage fraction of those phases Liquid Phase! Solid (alpha) phase! Mixed (range of conditions where liquid! and solid are in equilibrium)! (Cu) Composition (wt% Ni) (a) (Ni)

18 Phases Present Composition (at% Ni) Examples Liquid 1453 C % Ni at 1100 degrees centigrade! A Temperature ( C) Liquidus line B + L Solidus line Temperature ( F) 35% Ni at 1250 degrees centigrade! B C A (Cu) Composition (wt% Ni) (a) (Ni)

19 Determination of the Phase compositions Composition (at% Ni) Temperature ( C) Temperature ( C) Liquid Liquid + Liquid Tie line B Liquidus line B R + L 1453 C + Liquid Solidus line A C S Temperature ( F) Examples! 60% at 1100 C! has phase (alpha) Examples! 35% at 1250 C! has phase (alpha+liquid) (Cu) 20 C L C 0 C Composition (wt% Ni) (b) Composition (wt% Ni) 100 (Ni) (a)

20 Determination of phase amounts 1300 Liquid Temperature ( C) Tie line B + Liquid Liquid R S C L C 0 C Composition (wt% Ni) (b)

21 Question: A copper-nickel alloy of composition 70 wt% Ni-30 wt% Cu is slowly heated from a temperature of 1300C. (a) At what temperature does the first liquid phase form? (b) What is the composition of this liquid phase? (c) At what temperature does complete melting of the alloy occur? (d) What is the composition of the last solid remaining prior to complete melting?