hi everyone, we conducted hot tension test on Haynes 188 (Co base superalloy) at 1100 C, 1150 C, 1175 C and 1200 C. but magnitude of ductility decrease after 1150 C. we evaluate microstructure (OM) for cause of this issue, but we didn't observe incipient melting in vicinity of grain boundaries too. what other mechanism can cause this drop?
Hello Mr Xu Wang. yes of course it had grain growth, but that wasn't considerable. i think it had an other mechanism for this drop of ductility.
it was as cast and homogenized. we conducted hot tension test for prediction of hot working behavior. but after 1150 C hot ductility was drop. we expected hot ductility increase with increasing temperature.
@Samie
When designing structures to work at temperatures above room temperature you have to know the creep curve at given stress and Temperature. Unfortunately not enough enough info available for many alloys. Normally they show how the rate of loading plays different roles at different stresses and different temperatures.
A creep change appears between 50-70% of Tm in the Stain-Time curve.
The only thing you can get from that is the fact that above 70%Tm the increase in temperature starts to contribute negatively on ductility. A few explanation that have been provided so far are based on the fact that the diffusion of impurities become critical. During the migration the structure suffer a braking/reconstruction of bonds which can easily be a starting point for structure failure.
I guess it might be also associated with loading rate you can try different loading rate(slower) and see it may help you to understand more.
For many superalloys this is the problem faced in welding and alternative joining techniques must be used. There is liquation occuring and this causes high drop in ductility and strength. In fact, welding technique becomes useless. If you want to check for liquation, you must use SEM. OM is not adequate here.
Liquation causing microcracks is responsible for decreasing strength as temps are being increased in the range of temps you're talking about. You will do best to observe this phenomenon using Scanning electron Micrography.
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