Decoding key materials’ stress loads in new ways

Paul Mayne//Western News

Mechanical and Materials Engineering professor Hamidreza Abdolvand has discovered never-before-seen deformation and stress levels in two technologically important materials – titanium and zirconium – used in the aerospace and nuclear industries.

Mechanical and Materials Engineering professor Hamidreza Abdolvand has discovered never-before-seen deformation and stress levels in two materials – titanium and zirconium – both technologically important to the aerospace and nuclear industries. The discovery may lead to safer and longer lifespans for these materials.

“There are always problems with these materials,” Abdolvand said.

Because of their unique mechanical properties, hexagonal closed-packed (HCP) polycrystals like titanium and zirconium are used extensively in many manufacturing and engineering sectors. But this industry-wide reliability is not without its concerns.

For instance, the interaction and load-sharing between the crystals – known as grains – of titanium alloys can lead to ‘cold dwell fatigue,’ which limits the life span of commercial aerospace components. Likewise, such interactions in zirconium alloys control the process of delayed hydride cracking in the key core components of nuclear reactors.

“If you have a strong grain, located in the middle of a lot of other stronger grains, the load in that middle grain relaxes because the other grains around it are now carrying the load. This simple concept can be used for tailoring and manufacturing stronger, and better engineering materials,” Abdolvand said.

Despite previous comprehensive analysis of HCP polycrystals, it has never been reported – until now – that grain-resolved stresses, along the loading direction, can drop while applied stress increases.

While a postdoctoral student ­at the University of Manchester’s Materials Performance Centre and the University of Oxford’s Department of Materials, Abdolvand spent a week at the European Synchrotron Radiation Facility in France. Since returning to Ontario a year ago (he completed his PhD at Queen’s University) and joining Western’s Faculty of Engineering, the analysis of his research found some surprising results when it came to tension loads.

According to Abdolvand, the current assumption is that the load of each grain increases as tension is applied. But what Abdolvand found was, in 30 per cent of grains in zirconium and 20 per cent in titanium, that the effect is the opposite.

“It’s always been hard to determine how these grains in materials interact and we wanted to explain it. They share the load in a very specific way,” he explained. “But they have not been behaving in a way that we have been perceiving and thinking about for so long. This has become possible by developing new and advanced modeling and experimental toolboxes.

“By creating a new code, and using this code to help predict lifespan when it comes to manufacturing, would be a plus to industry. The fact (is) we thought we knew how these things behave – and then we realized we didn’t know. The final aim is to ensure, in the manufacturing route, we get better material and safer and longer lifespans. We’ve discovered something new and that’s the joy of doing it (research), because you never know what could happen. Our objective was totally different. It wasn’t this plan to capture this phenomenon, but wow.”

His study, Strong grain neighbour effects in polycrystals, recently appeared in the open-access journal Nature Communications.