Creep mechanism (thermal and irradiation induced/enhanced) and embrittlement in fcc and bcc

[eneacasucci]

TL;DR Summary

In comparing FCC and BCC metals, FCC has higher packing efficiency and more active slip planes, leading to more pronounced thermal creep due to easier dislocation glide. BCC, with lower packing efficiency, shows more induced/enhanced creep due to higher defect mobility. Helium embrittlement is more evident in FCC, likely due to nickel content, while irradiation embrittlement is more pronounced in BCC due to fewer slip planes, which hinder dislocation movement.

When comparing FCC (Face-Centered Cubic) and BCC (Body-Centered Cubic) metals, I typically consider their packing efficiency and the number of active slip planes. FCC structures exhibit higher packing efficiency and a greater number of active slip planes compared to BCC structures.

From various sources, I have learned that thermal creep is more pronounced in FCC metals. I attribute this to the higher number of active slip systems in FCC, which facilitates dislocation glide, leading to creep deformation.

Additionally, I’ve read that BCC metals exhibit a more significant presence of induced/enhanced creep. Induced creep in BCC is associated with the formation of dislocation loops, while enhanced creep is related to the climb motion of dislocations. I reasoned that this enhanced creep behavior in BCC could be due to its lower packing efficiency, which allows for higher diffusivity and, consequently, greater defect mobility.

Regarding embrittlement, some sources suggest that helium embrittlement is more pronounced in FCC metals, possibly due to the presence of nickel and the related (n,α) reaction. On the other hand, neutron embrittlement is reported to be more significant in BCC metals. I hypothesize that this is because the fewer slip planes in BCC structures make it easier for defects to obstruct dislocation movement, thereby contributing to embrittlement.

Are my explanations correct?
It is very difficult to find information about this online and in books. I studied on Olander book but still some things are not that clear to me. I usually try to use logic to justify things but I'm not sure my reasoning are correct. thank you deeply in advance for your help.

 

[eneacasucci]

@Astronuc I think I'll need your knowledge also to find an answer about this... of course everyone is more than welcome to join in!

 

[Astronuc]

Your understanding is essentially correct, but it is a bit more complicated. I will see if I can find some good references besides the classic text Mechanical Metallurgy by George E. Dieter, which has Chapter 13 Creep and Stress Rupture, see section 13-6 Mechanisms of Creep Deformation, in the 1986 Third Edition.

Some journal articles are quite good. For example, "Basic modelling of creep rupture in austenitic stainless steels" - https://www.sciencedirect.com/science/article/abs/pii/S0167844216303135

Ferritic (bcc) and martensitic (bct, bcc) stainless steels can achieve higher strength at room and low temperature than 300 series austenitic stainless steels depending on cold work levels (I will try to find some examples), but ferritic, martensitic, or ferritic-martensitic stainless steels lose strength rapidly at temperature > 650°C.

Some modern F-M steels use elements like W to obtain higher strengths. Adding amounts of larger atoms into fcc (austenitic) stainless steels also has a strengthening effect. One challenge in manufacturing such steels is obtaining chemical homogeneity (uniform distribution) of the larger atoms on the atomic levels. Ideally larger atoms are in solid solution, but some may segregate and form carbides, nitrides or oxides, or in some cases silicides, which can compromise performance at higher temperatures.

 

[eneacasucci]

Your understanding is essentially correct

Thank you!
But I think I made a mistake here

I have learned that thermal creep is more pronounced in FCC metals

because BCC metals, due to their more open lattice structure and lower packing efficiency, are generally more susceptible to thermal creep than FCC because thermal creep is mainly governed by dislocation climb (over dislocation glide) that is a diffusive mechanism.

 

[Astronuc]

because BCC metals, due to their more open lattice structure and lower packing efficiency, are generally more susceptible to thermal creep than FCC because thermal creep is mainly governed by dislocation climb (over dislocation glide) that is a diffusive mechanism.

Let me check my resources before responding. Meanwhile, one might review publications by John Emil Dorn (University of California), Oleg Sherby and Raymond Orr, and publications since, especially more recently following the advances in materials characterization.

I did a project recently regarding high strength (and creep resistant) stainless steels, and I did consider one martensitic, age-hardenable alloy with tensile strength up to 250 ksi (1722 MPa) after looking at several new alloys. I rejected that alloy in favor of a couple of advanced austenitic alloys. Creep resistance was one factor in addition to high strength, particularly at 'high' temperature.

 

[Astronuc]

There is a peper entitle, "Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel," by Kouichi Maruyama, Junichi Koike, 2001
See a figure here and a link to the paper.
https://www.researchgate.net/figure/Minimum-creep-rates-of-ferritic-steels-at-600C_fig3_200653249

The authors state in the introduction, "The present paperwill provide an overview of the strengthening mechanisms of the high Cr ferritic steel with a tempered martensitic lath structure under creep condition."

Note the change in creep rate at 600°C as a function of stress level and composition. Adding C, W, Nb, V and combinations of W, NB and V with C and N greatly increases the creep resistance. W, Nb and V are carbide/nitride formers. The paper discusses solution hardening: W and Mo in solution, as well as other mechanisms.

Note also with respect to the elements in the periodic table, Period 6 elements have greater atomic radius that corresponding elements (same group) in Period 5, which have greater atomic radius than those in Period 4. For example, W > Mo > Cr, or Os > Ru > Fe (but we wouldn't use Os because it's way too expensive and forms a toxic tetroxide).