As per my knowledge, the ductile to brittle transition is characterized by a sudden and dramatic drop in the energy absorbed by a bcc metal subjected to impact loading. The starting temperature at which the transition (50% ductile and 50% brittle) occurs is generally known as DBTT.  However, ductility of bcc material may suddenly decrease to almost zero at a particular temperature. This transition is often more abrupt than the transition determined by the energy absorbed. This temperature is called the critical temperature of brittleness or nil-ductility transition temperature. The critical temperature is generally lower than the fracture energy transition temperature or DBTT and is merely investigated.

Thank you very much for your explanatory answer. As far as i know the Tc is explained as a temperature at which flow stress becomes insensitive to test temperature, screw and edge dislocations have equal mobility due to thermal activation of the screw dislocations. These are Tc values of some bcc metals: 340K for Fe, 350K for Nb, 450K for Nb, 480K for Nb and 800K for W. On the other hand, the DBTT temp of some bcc metals: Ta< -195C, V

Sorry for my misinterpretation about Tc. I confused it with the critical temp. of brittleness. Well, in that I can not accurately say what temp. you should take as a reference, because it will depend upon your experimental procedure and material. For example, DBTT is generally used to understand the deformation characteristics of a material under impact loading and researchers often use it as a reference. In case of tensile deformation at room temperature, it may not give the desired out come for most of the materials. However if the test temp. is cryogenic or so, then I think DBTT can be taken as a reference. If your test temperature is room temp. or higher, then, in my personal opinion, you may take Tc as a reference. 

With regards, 

Thanks. Yes. for compression and tension experiments, researches always used Tc rather than DBTT in literature.  So I will test my Fe nanopillars in-situ compression at ambient temp. or may be at higher temp. So i still need to understand the effect of DBTT on the deformation characteristics of single crystal bcc metals if it exists. 

Regards

Dear Halil,

I did not understand the meaning of "flow stress becoming insensitive of test temperature", does it mean that flow stress does not change with temperature? if yes then I politely dont agree with the values you have given, at least for Fe. As for iron and steel the flow stress keeps decreasing as you go high. The temp suggested by you is 67 deg C for Fe. where flow stress is not saturated to be constant.

Moreover, screw dislocation has always more mobility than edge, so why screw dislocation enhancement with thermal activation will equalize the mobility with edge one.

Even if both type of dislocations have equal mobility how the flow stress will equalize?

I searched the keyword critical temperature but got equivocal results. Could you give us a link or a reference where the term is defined in the way you mentioned.

I guess the term is specific to nano-structured materials, which I am not aware of.

May be when I know about Tc, I may suggest a correlation but about the effect of DBTT on bcc, you may refer this document,( I can send you the full doc privately if you want)

https://www.researchgate.net/publication/278049128_Determination_of_reference_transition_temperature_of_In-RAFMS_in_ductile_brittle_transition_regime_using_numerically_corrected_Master_Curve_approach

The transition temperature, as Manidipto said can be characterized by impact testing but that is a dynamic condition. The quasi-static transition temperature is also measured according to ASTM E1921. More details are in the reference.

With regards,

Abhishek

Article Determination of reference transition temperature of In-RAFM...

Thanks very much for your answer. I am sending the paper that i mentioned about Tc. 

I attached another paper which is mentioning the Tc of iron. 

Thanks 

Halil 

Dear Halil, 

Thanks a lot for sending the papers on this frontier subject of study.

Although I have read only first page of the first paper so far, but I understand that the terminologies used (Tc and d) as the parameters on which flow stress is dependent, is strictly restricted to the nano pillers, presumably ( I am not sure) due to very small grains where the grain boundary perhaps acts as another phase; I just think this may be the case.

These relations however do not hold good for bulk materials. As the size effect d power also changes and I am not sure about Tc in bulk materials as  more than mobility pinning of dislocations, climb precipitates and more wider grains perhaps are the flow controlling factors.

As I suspected in the earlier reply, this is the case specific to small scale materials [200 nm to 6 micron in your paper].

Thanks once again for updating me with these and one suggestion if you dont mind:

Please add this term of bcc pillers instead of bcc metals in your questions title, may be then it will attract attention of people working in this field. 

I wish you all the best.

With regards,

Abhishek

I think you may contact Prof. Zhi-Wei e Shan on ResearchGate who works in this area. (Prof. Zhi-Wei e Shan, Xi'an Jiaotong University, Xi’an, Shaanxi, China)

Hi Halil,

I am curious to know the experimentally obtained DBTT values of small size ( d= 300 - 5000 nm) BCC pillars, particularly, Fe.

Here you can see our recent paper on DBTT in Fe nanowires with size < 50 nm. 

Regards

Sainath

Article Atomistic simulations on ductile-brittle transition in ...