Christian Doppler Laboratory

          Early Stages of Precipitation

Christian Doppler Laboratory

          Early Stages of Precipitation

 

 

Project manager: Mohammad Reza Ahmadi

 

Precipitation hardening is a one of four mechanisms, which improve the mechanical strength in materials. In this mechanism, precipitates act as small barriers, which inhibit easy dislocation movement to increase yield strength. Precipitation hardening is commonly performed in three steps:

  1. Solution heat treatment: Heating into single phase solubility region to establish a homogeneous solid solution.
  2. Quenching: Quenching form single phase region to low temperature to form a supersaturated solid solution (SSSS).
  3. Aging: Heating up to increased temperature to facilitate diffusive process. Precipitates nucleate and grow.

At the early stage of aging, precipitates are small and coherent to the lattice (coherent particles) but after a while they will grow more and more until they lose their coherency and become incoherent particles. Dislocations react in two different ways with coherent and non-coherent precipitates:

 

When dislocations interact with a coherent precipitate, they can pass through the precipitate, because atomic lattice continuity remains intact at the precipitate – matrix interface. For a quantitative description of the dislocation – particle interaction, we have to consider the energy changes in the dislocation as well as the properties of the precipitate. The type of precipitates where dislocations can pass through the precipitate volume are called soft or “weak precipitates”. The corresponding strengthening mechanism are categorized into

  • Chemical strengthening
  • Coherency strengthening
  • Modulus hardening
  • Order strengthening
  • Stacking-Fault strengthening

If dislocations interact with non-coherent precipitates, they cannot traverse through them. Because atomic continuity is missed at precipitate interface. In this case precipitates will be bypassed by dislocations under Orowan equation and make a loop around precipitates. This is the reason that these precipitates are called hard or “strong precipitates”.

 

 

Goal of the project is to calculate and predict strengthening due to precipitation hardening considering all dislocation interaction mechanisms at different stages of hardening.