This isn't for school so please don't bash me for "not doing my homework."

Why does the temperature independent factor Do get smaller for C in Fe and H in Fe.

I'm talking about the equation:

D =  Do * e ^ (Q/KT)

Also, is Do dependent on weither the host metal is BCC or FCC?

I'm interested in the following systems

Fe in FCC Fe
Fe in BCC Fe

C in FCC Fe
C in BCC Fe

H in FCC Fe
H in BCC Fe

I have an old textbook but it doesn't say much about this.

I know Fe in Fe is a vacancy diffusion and C/H in Fe are interstitial diffusion.  I know it takes less activation energy for intersitial diffusion.

Scott

It seems like Do for Fe in FCC Fe is smaller than for the same system in BCC.

However, Do for C in FCC Fe is larger than for the same system in BCC.

Why is this?  Is there a pattern?

@ "low" temperatures, the equilibrium phase of Fe is ferrite, or alpha-Fe, a bcc structure.
@ "high" temperatures, it's austenite (gamma-Fe), fcc

fcc is a "close pack" structure, whereas bcc is not. Simply put, there's more displacement of Fe atoms to allow C and H to diffuse interstitially.


On the other hand, the system I work with most often is Si, with either B or P diffusion....

Correct but I already know that.

btt

btt