**MSE 104 Questions:**

The '**Question**' is as posed by a student in the cohort (with some minor editing by me). The '**Answer**' is my response. I've tried to include links to further (web) resources where I can. I will also respond to comments if you'd like!

**Question:** In the pile-up model, why does a larger grain size lead to a larger stress?

**Answer: **If we deform a metal to a fixed strain, then the total number of dislocations used to accommodate that strain is larger when we have a larger grain size, the number of dislocations is larger to accommodate the strain, as strain is "change in length over original length", change in length is related to the number of dislocations & the length of the Burgers vector, and the grain size is better. The strain ahead of a dislocation pile up is proportional to square root of the number of dislocations.

**Question:** Please can you explain the field plots around the dislocations (page 181-184 of the notes)?

**Answer:** These are plots of the magnitude of the stress field with respect to position around a dislocation. The 'take home' messages from these stress fields are given in the following slides.

**Question:** What is the difference between a coherent and incoherent interface/boundary?

**Answer: **A coherent interface is where the two grains/phases match at the interface perfectly. An incoherent interface is where they do not match. In the context of precipitate hardening, this interface is between the parent matrix and the precipitate.

**Question:** Can you please share more information on the dislocation model presented in the lecture?

Note that I made a mistake in the lecture and incorrectly said this was a dislocation dynamics simulation. It is in fact a molecular dynamics simulation, sorry!

**Question: **For the Single Arm Frank-Read Source, is the mobile segment the slip plane?

**Answer:** This cannot be the case, as the dislocation is a line defect. The slip plane is a planar defect.

The mobile dislocation segment is a dislocation segment that is contained on a glissile slip plane. This compares with the pinned, i.e. sessile, segment that is holding the single armed source in position.

**Question: **I have found several mistakes in the lecture notes and slides, please can you correct them?

**Answer:** Please can you direct me to where these mistakes are and I can assess where they are? There are a few minus sign errors that I have mentioned in the lectures themselves.

**Follow-up question:** There are many blank equations in the notes, such as the inter-atomic potential section.

**Answer:** These sections are intentionally blank in the lecture notes. The derivations were provided in the lectures themselves for you to include and annotate by hand. The blank spaces were provided for you to complete this activity.

**Question:** Why does the presence of a characteristic Burgers' vector for each dislocation mean that a dislocation must a dislocation terminate on another defect?

**Answer:** The Burgers’ vector is
the characteristic feature of a dislocation and it is the ‘translation’ defect that
is characteristic of the dislocation. At the ‘end’ of this dislocation, this
defect must be ‘put’ somewhere (to sort of ‘cancel’ out). This can only be done
on: (1) itself; (2) another dislocation; (3) or another defect like a grain
boundary, surface, or phase boundary.

**Follow-up question:** Could you explain this another way?

**Follow up answer #2 (following help from some Tweeps, answer courtesy of @egg_daddy):** "The dislocation is the boundary between slipped and unslipped crystal. Boundaries can't just stop, cf coastlines."

**Question:** please can you supply more information about grain boundary characterisation?

**Answer:** You can find out more on this within Hull &
Bacon. There are a few interesting resources on this: