Posibilities of strength-enhancing Advanced material and technologies, MSc 2017

Increase of strength Characters of metastable states They have a role in strength-enhancing. CHARACTER OF METASTABLE STATE EXAMPLES EXCESS ENERGY (RTm) EXCESS ENERGY J/mol) COMPOSITIONAL SUPERSATURATED SOLUTIONS  1 10 STRUCTURAL OVERCOOLED MELTS, AMORPHOUS METALS AND INTERMETALLIC PHASES  0.5 5 MORPHOLOGICAL OR TOPOLOGICAL NANO SIZED DISPERSION OF PHASES WITH HIGH SURFACES (TO VOLUME)  0.1 1

What kind of strength-enhancing mechanisms are there? Increase of strength What kind of strength-enhancing mechanisms are there? Strain hardening (work hardening) plastic deformation, increase of dislocation density (recently: radiation damages also) Solution hardening (properties of dissolved elements, connection with the Hume—Rothery-rules!) Precipitation hardening Dispersion hardening Quench hardening: high density of lattice defect, non-equiulibrium constituents Grain refinement 3

The lowest and highest value in hardness Increase of strength – strain hardening Changes of hardness (and Young-modulus) due to changes in dislocation density The lowest and highest value in hardness of metals due to dislocation density Single crystal → recrystallized (tempered) state → state with high plastic deformation (high density of dislocations)

Increase of strength – strain hardening I. phase: after the elastic range, the phenomenon of easy slip or single slip is dominant. phase: high slope, independent of T (slip lines are short, inhomogeneous deformation ranges). phase: parabolic (non-linear) range, less-known dislocation motion mechanism, start point and appearance depends on temperature. The shape of the flow (yield) stress - true strain curve depends largely on the crystal structure!

Increase of strength – strain hardening 1 – tempered, recrystallized state 2 – effect of plastic deformation 3 – effect of quenching 4 – effect of thermomechanical process

Hall―Petch-equation: Increase of strength – grain refinement Hall―Petch-equation:

Increase of strength – strain and solution hardening Increase of strength caused by interaction between dissolved atoms and dislocations: dissolved atoms are concentrated in the stress field of the dislocations, modifying the local binding force (critical shear stress) the stress field of the dislocation has interaction with electrostatic and mechanical stress field of the dissolved atom, thus making the movement of the dislocations difficult (increase local critical shear stress) → ageing

Increase of strength – strain and solution hardening

Increase of strength – solution hardening Which effect determines the solubility? Hume-Rothery: atomic size, electron configuration (difference in valence electron structure), electronegativity, type of lattice Zn As Sn

decrease of ability to plastic deformation, decrease of impact energy. Increase of strength – effect of ageing Irrespective of atomic mechanism and/or changes in phase relationships, aging at alloys means: increase of strenght, ReH near to Rm, decrease of ability to plastic deformation, decrease of impact energy. → embrittlement

Steps of precipitation hardening: Increase of strength – precipitation hardening Steps of precipitation hardening: 0. Heating and heat retention for complete dissolving: heating in a homogeneous, single phase range where the second phase dissolves. 1. Establish a supersaturated solid solution over equilibrium dissolution conditions: - generally with quenching (from a homogeneous region which has a higher solubility). 2. Appearance of fine precipitated phases → the system approaching the equilibrium dissolution and phase relationships. - „artificial aging” with heat treatment: in a lower temperature as the 1. step, - natural ageing. T t 0. 1. 2. mild state

What are the boundary conditions? Increase of strength – precipitation hardening Slope of T0 curves and maximum supersaturation, solidification without compositional partition, phenomenon of glass forming What are the boundary conditions? Forming of supersaturated, crystalline solid solutions. Forming of metallic amorphous states (glassy alloys)

Increase of strength – precipitation hardening Precipitation processes in one step concrete examples: in Fe-based alloys Ti, Mo, carbide precipitation main point: high thermodynamic driving force! ΔGV: free enthalpy change of phase transformation (supersaturated solid solution to phase of precipitate) ΔGe: the energy of the stress generated by the change in specific volume What should be regulated during the precipitation process? Amount of precipitates, average size and distance! With what kind of parameters? - concentration, - temperature of the heat treatment - time of the heat treatment ΔGe > 0

Example for precipitation process from supersaturated solid solution Increase of strength – precipitation hardening Example for precipitation process from supersaturated solid solution Precipitation hardening in Al-based alloys: in this case precipitation in more than one step! Curiosity: Al-brass (Cu alloys with Al) have same phenomenon as at steels → martensitic like transformations, after quenching high hardness and low inpact energy.

Increase of strength – precipitation hardening Process of precipitation in more than one step Precipitation hardening process of Al(Cu) alloys When an equilibrium process of precipitation has more than one step Spinodal decomposition Guinier- Preston zones: decomposition of solid solution starts with long range fluctuation in composition, there is no definite surface of phases

Precipitation hardening process of Al(Cu) alloys Increase of strength – precipitation hardening Precipitation hardening process of Al(Cu) alloys System reaches the free enthalpy minimum in more steps (the whole process in details is function of time and temperature)

Precipitation hardening process of Al(Cu) alloys Increase of strength – precipitation hardening Precipitation hardening process of Al(Cu) alloys The outcome of the process depends on the composition as well Description of precipitation process in alloys which capable for precipitation treatment

Precipitation hardening process of Al(Cu) alloys Increase of strength – precipitation hardening Precipitation hardening process of Al(Cu) alloys

Increase of strength – precipitation hardening Stabilization of the process in case of phase precipitation from Al(Cu) supersaturated solid solution

Increase of strength – precipitation hardening Stabilization with third component: Ti, Mg, B, etc. What should be improved? Decrease of changes in specific volume, mechanical properties during heat treatment (decrease property changes versus time), decrease of shrinkage during casting.

Effect of composition and ageing on strength Increase of strength – precipitation hardening Precipitation hardening process of Al(Cu) alloys Effect of composition and ageing on strength

Increase of strength – precipitation hardening With micro-alloying it is possible to adjust: - distribution of precipitated phase, - average size of prec. phase, - parameters of heat treatment.