Nucleation: stray grains and threading dislocations

April 28, 2011

[1] Formation of Stray Grains during Directional Solidification of a Nickel-Based Superalloy

Y Zhou

Superalloy CMSX-4 is directionally solidified and initiated by bi-crystal seeds. It has been found that diverging boundaries are the most favorable location for stray grain formation. The phenomenon cannot be attributed to nucleation of crystals. Reasonable mechanism is bending or detachment of side arms during extension of secondary arms and development of tertiary branches at the diverging boundaries. Solute interaction of the neighboring dendrites promotes likelihood of bending or detachment and then leads to an enhanced frequency of stray grains.
Highlights

► Stray grains are formed particularly around re-entrant features such as the platforms or shroud regions of the turbine blade airfoils and lead to rejection of single crystal superalloy components since the boundSuperalloy CMSX-4 is directionally solidified and initiated by bi-crystal seeds. It has been found that diverging boundaries are the most favorable location for stray grain formation. The phenomenon cannot be attributed to nucleation of crystals. Reasonable mechanism is bending or detachment of side arms during extension of secondary arms and development of tertiary branches at the diverging boundaries. Solute interaction of the neighboring dendrites promotes likelihood of bending or detachment and then leads to an enhanced frequency of stray grains.

[2] Sapphire surface pits as sources of threading dislocations in hetero-epitaxial GaN layers

F Y Meng et al

Sapphire substrates showed nanosized surface pits after the growth of GaN layers using a two-step process by hydride vapor phase epitaxy. Threading dislocations with Burgers vectors of c and c+a were found to originate from the pits. Mechanism of their generation is developed from cross-sectional transmission electron microscopy observations.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: