May 2011

Searching for the optimal balance

Leading the way in the world of superalloys

By Alexandra Lopez-Pacheco

Mahesh Chaturvedi is recognized as a world leader in the area of superalloys and grain-boundary engineering. His pioneering research and work has led to breakthroughs in superalloy welding. Chaturvedi’s discoveries have had a profound effect on the aerospace industry, and won him more than a few awards along the way.

Currently a professor emeritus at the University of Manitoba, Chaturvedi was a Tier 1 Canada Research Chair in aerospace materials (2002-2009) and an NSERC Industrial Research Chair from 1995 to 2005. He was also associate dean of engineering for over a decade and associate vice-president of research. As a professor of materials engineering, he has contributed extensively to materials education through his lectures and was an ASM–IIM Lecturer in 2006.

He also received funds from the Canadian International Development Agency to develop a three-year institutional linkage program with Khon Kaen University in Thailand, a five-year linkage program with Beijing University of Aeronautics and Astronautics, and another five-year program with China’s Lanzhou University to improve the teaching of materials science and engineering.

cHe is a speaker in this season’s CIM Distinguished Lecturers Series, sharing his findings from some of his recent research on the role of boron in the design of superalloys.

CIM: Why did you decide it was important to research the role of boron in the design of superalloys?

Chaturvedi: I was doing research on the joining of aerospace materials and one of the factors I found was that boron is very detrimental to welding. Boron has been used in high-temperature materials to provide high-temperature strength. So the question is, what does boron actually do in these alloys and why do we need it?

With that in mind, we started to look at the various aspects that are affected by boron, both the good and the bad, to see if we could come up with the best possible solution – an optimal concentration of boron that could be prescribed for aerospace material. What we found was that, yes, boron is necessary, but not as much as people had been proposing and were starting to use. The goal was to minimize the adverse effects and maximize the beneficial effects.

CIM: What are these key adverse and beneficial effects?

Chaturvedi: In any manufacturing process, joining is very important. When you weld materials, there is a fusion zone where the actual components melt and are joined together as they solidify. In addition to that, there’s the heat-affected zone. Very often, materials can crack in fusion zones, as well as in the heat-affected zones, after the welding has been completed during the cooling stage. The adverse effect is that boron influences the cracking in the heat-affected zone – the larger the amount of boron, the greater the cracking. The flip side of the coin is that the boron improves the high-temperature properties of the material. For example, nickel superalloys are used for turbine blades in aircraft engines. If you want to raise the engine to improve its efficiency, you need to operate it at a very high temperature and you need materials that can better withstand that temperature – so you increase the amount of boron. In doing so, you improve the mechanical properties of the material at high temperatures, but you can’t weld it.

Welding is also used to repair materials. When cracks form you can weld them, but it becomes more difficult to repair if the materials have a high boron content. We have to find the compromise, the optimal balance.

CIM: Were you able to identify the optimal concentration for boron?

Chaturvedi: Yes, there is an optimal concentration. The interesting thing is that some of these superalloys were developed 50 to 60 years ago when there were not sophisticated methods of conducting research or improving purity. Also, the materials were not as carefully produced. Discovered by “trial and error,” researchers determined that the optimal range was 50 to 84 ppm – and they were right. Now, we also know why they were right.

CIM: What does this mean for the metallurgical industry?

Chaturvedi: This research will help engine manufacturers and repairers understand why this cracking occurs. Now, the aircraft industry knows and understands this and they’ll be able to make sure that boron doesn’t exceed the optimal amount; if they want to improve the properties of the material, they’ll have to follow another process rather than increasing the concentration of boron. In addition to the aerospace industry, my research can also be applied to gas-fired and coal-fired power-generation turbines.

CIM: What other research are you working on?

Chaturvedi: All of my research is related to high-temperature materials for aircraft engines. I’m now working on single crystal alloys and their weldability and studying “brazing” – another technique used for joining materials. As well, I’m looking at different methods of welding, such as fusion and laser.

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