Effect of grain flow on mechanical properties 2

[Blank man]

I've been doing some research on grain sizes, grain boundaries, and crack propagation in metals. Every article I've read confirms what I've always known at a high level, that grain flow has an impact on fatigue strength... But I'm not sure how grain flow direction (specifically in a forged component) actually affects the mechanical properties of the part at the grain structure level. Does crack propagation in metal components (aluminum, steel) begin along the grain boundary or in the actual grain itself due to dislocations in the grain? And what is the preferred way to align grains/boundaries? Along the principal stress direction or perpendicular to the principal stress, or some other way?

My intuition tells me that grain flow should be aligned with the principal stress, but I'm just having trouble understanding how all of these things work together.

So I guess there are multiple questions:

1. How does crack propagation usually occur in metals (in general)? Along a grain boundary or within the grain?

2. How is grain flow direction related to crack propagation, or how does grain flow help control crack propagation?

Thank you!

 

[Metalero]

1. How does crack propagation usually occur in metals (in general)? Along a grain boundary or within the grain?
The two paths are possible and occurs: transganular or intergranular propagation.

2. How is grain flow direction related to crack propagation, or how does grain flow help control crack propagation?
More important than grain flow direction is the grain orientation (cristallographic texture or cristallographic microtexture). Studies have shown that some orientations (more or less compact) are the crack preferred paths.

 

[EdStainless]

In materials with non-cubic structures (Al and Ti) this is a very serious issue. You not only have distorted grains but you also get a preferred grain texture. This can result in both physical (modulus) and mechanical (strength, toughness) properties being direction dependent.
This is rarely an issue in Q&T steels or annealed stainless.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

Bm...

WRDC-TR-89-4060 [ADA219747] Failure Analysis Handbook

Quite extensive coverage of metallic materials failure... 'grain' size, orientation, etc... is critical to understanding the failures. Includes a extensive list of references.

Also... ASM

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]