i work on Cobalt base alloy (Haynes 188 Co-Cr-Ni-W), we reduced initial bulk thickness into 60% and 85% (or strain). then we conducted tension test in environment temperature, it was interesting, the yield stress of 85% reduction sample increased 2 times than 60% sample. but elongation fall down half. how might we compensate this drop?
hello mehdi samie, naturally the grain size also decreased from fine to ultra fine size and this makes to increase the tension properties, for further more details you can refer diter text book and you can refer our papers on Mg alloys
Yeah I agree with Ahmad Zafari
Bimodal grain structure consisting of ultra-fine grains in coarse grains matrix may be one of the reason to increase the ductility of SPD sample without scarifying the strength of the materials
I too believe that Stress relaxation is also one of the process by which we can improve the ductility of the UFG materials in which the dislocation density is reducing significantly.
I tend to see all limitations to ductility as the result of defects in the matrix. I do not have any experience whether such considerations would apply to a super fine grained material, but I would expect this to be true. The only defects I am aware of which can give rise to cracks, which will initiate failure by either brittle or ductile macroscopic processes, are bifilms. These are double oxide films which are introduced, accidentally, during the pouring of the liquid metal. They usually survive working processes, and tend to nucleate second phases. For this reason cracks associated with second phase particles seen at the base of ductile dimples are there because of the presence of bifilms. It is the bifilms which initiate the local failures, which link to create the familiar dimple array.
It is not necessary to entrain the oxide films during the metal casting process. Without the presence of bifilms I would expect only ductile failures, with 100 per cent reduction in area, no matter how strong the alloy.
I agree that these suggestions are not mainstream metallurgy. I recommend my book (apologies for such apparently brazen self promotion) Complete Casting Handbook 2nd Edition 2015. It is not reading for the faint hearted. In addition, I attach a couple of papers which I hope will be helpful.
Thanks dear sir Kun-Kun Deng.
but annealing could cause decreasing in strength. is it noticeable? i don't want strength decreasing be observe.
Its quite natural. Reduction in grain size after a certain size will not increase your strength and ductility. There s a critical grain size below which Hall - Petch relationship is not valid. Below that your strength increases at the cost of ductility.
Hi Mehdi
Are you using ECAP?
Are you able to increase the hydrostatic pressure during your process, e.g. by means of back pressure etc.? In ECAP and many similar processes, back pressure has proved to produce stronger materials. In samples we produced by AFSE, the back pressure improved ductility as evidenced by micro shear punch tests.
It is quite common that increasing strength results in reduced ductility because of a reduced strain hardening capability. In your case, a high dislocation density and small grain sizes could be the key factors responsible for the mechanical behavior that you mentioned. There have been many publications on how to appropriately design ultrafine-grained structures for high strength and good ductility. Some of these papers are listed below:
1. Y. H. Zhao, et al., Simultaneously Elevating the Ductility and Strength of Ultrafine-Grained Pure Cu, Adv. Mater. 18 (2006) 2949-2953
2. Y. H. Zhao, et al., Tougher ultrafine-grain Cu via high-angle grain boundaries and low dislocation density, Appl. Phys. Lett. 92 (2008) 081903
3. Y. H. Zhao, et al., Determining the optimal stacking fault energy for achieving high ductility in ultrafine-grained Cu-Zn alloys, Mater. Sci. Eng. A 493 (2008) 123 – 129
4. Y. H. Zhao, et al., Simultaneously elevating the ductility and strength of nanostructured alloys, Adv. Mater. 18 (2006) 2280 -2283
5. Y. M. Wang and E. Ma, Three strategies to achieve uniform tensile deformation in a nanostructured metal, Acta Mater. 52 (2004) 1699-1709
Dislocation density increased in SPD processes.It caused increasing the strength,but as always increasing the strength doesn't mean increasing the elongation.These phenomenons are always against each other.Also, Dieter book in mechanical metallurgy has a very good explanation. Read my article in Persian,my friend.If you like to improve the ductility you can heat treat your alloy,refer to TTT diagram for your alloy and be careful about changing phase and grain size.Normally annealing or normalizing can help you to improve ductility.
1. You have NECKING in tension and hence the UNIFORM elongation is in question as further disintegration becomes fast. According to famous Considera's criterion (dS/dE=S), at a fixed maximum stress S the highest strain hardening rate (dS/dE) shortens (accelerates) a strain magnitude before necking. Meanwhile SPD strength is close to ultimate (saturation) level i.e. a hardening interval, if any, where the hardening rate gradually vanishes is very short= elongation is small. So, in tension the problem is INEVITABLE. On the other hand, under other conditions of detail operation where the localization does not matter, the material may safely accumulate more strain magnitude.
2. The TRUE strain ( Ln(Ho/H) in reduction/compression ), that is relevant physical measure, is about 1 and 2 for 65% and 85%, respectively. Hence your observation is quite expected.
Thank you sir Xiaozhou Liao, thank you sir seyed, and thank u sir A. A. Zisman
- Badges
- Science topic
- Similar topics
- Correction