Title: Summary Part Heat Treatments and Processes

Choose the correct statement.

- Some strategies for reducing dislocation mobility and hence increasing the strength of metals are:

Decrease of the grain size, dispersion of precipitation, alloying to form solid solutions, forming multi-phase materials. (correct)
- Some strategies for reducing dislocation mobility and hence increasing the strength of metals are:

Increase of the grain size, elimintation of percipitates and different or secondary phases, and quenching.

- Sintering is a heating process which primarily serves to densify and reduce porosity in materials without metling the material.  It is enabled by diffusion of atoms along grain boundaries. (correct)
- Sintering is a heating process where a material is held at a constant temperature which relaxes the material microstructure after mechanical deformation and is often associated with grain growth.
- Sintering is a rapid cooling process which traps a materials' microstructure at a high temperature state.

- Time-dependent phase transformations begin with the nucleation of the new phase followed by its susequent growth – it is an inhomogeneous process. (correct)
- Time-dependent phase transformations are characterized by homogeneous transformation of the pervious phase into the new phase.

- The competition between diffusion and phase transformation is the reason for the knee-shaped curves observed in TTT diagrams. A TTT diagram plots percentage of phase transformation in a temperature-versus-time plot. (correct)
- The competition between diffusion and phase transformation is the reason for the knee-shaped curves observed in TTT diagrams. A TTT diagram plots the equilibrium stability of different phases in a temperature-versus-time plot.

- Alloys with rich phase diagrams filled with many different equilibrium phases are likely to form eutectic and eutectoid microstructures with heat treatment rather than percipitates at grain boundaries. (correct)
- Alloys with limited solubility are likely to form eutectic and eutectoid microstructures with heat treatment rather than percipitates at grain boundaries.

- Precipitates harden an alloy by acting as barriers to dislocations. (correct)
- Precipitates harden an alloy because they are themselves harder than the matrix. The strengthening is hence similar to that of an aggregate composite.

- Overaging describes a heat treatment in which the precipitation process in continued so long that the precipitate have an opportunity to grow and coalesce. The originally fine dispersion of precipitates turns into a coarse dispersion, which is less effective in strengthening. (correct)
- Overaging describes the fact that a dispersion of fine precipitates is not thermodynamically stable at room temperature. Hence, every dispersion-strengthened alloy degradates in time, as it is "growing older": the originally fine dispersion of precipitates turns into a coarse dispersion, which is less effective in strengthening.

Feedback (0 to 80%): Try reading over the TTT Diagram, Heat Treatment and Processes, and Precipitates chapters again to strengthen your understanding
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