A Key to Optimizing Materials for Energy, Transportation, and Structural Applications

Mechanical metallurgy can be defined as the interface between an alloy's mechanical behavior, the processing used to produce the alloy, and the underlying structure ranging from the atomic to macroscopic level. Because mechanical metallurgy is most strongly related to how metals break and deform, subfields have developed largely because of disastrous failures such as the Versaille train crash (fatigue), the breakup of the Titanic (toughness), and the sunken Liberty Ships (fracture mechanics). More recently, mechanical metallurgy has evolved in response to challenges in the areas such as energy and transportation. This short article is focused on examples of future opportunities within mechanical metallurgy, but the historical context of the field is addressed in many sources^1-5.

An excellent example of microstructural engineering to achieve specific mechanical properties is the new generation of steels called advanced high strength steels (AHSS). These alloys, which include dual phase (DP) and transformation induced plasticity (TRIP) steels have combinations of strength and formability that are superior to prior generation steels, so they are being implemented more widely into the automotive industry as car body panels, door beams, and body pillars among many other potential applications. Since these materials are stronger than prior generation steels, they can be used with smaller section thicknesses to allow for vehicle weight reduction and greater fuel economy.

DP steels consist of ferrite (relatively soft) and martensite (relatively hard), and the morphology, volume fraction, and composition of each phase can be tailored to achieve different combinations of strength and ductility. TRIP steels contain pro-eutectoid ferrite with varying amounts of retained austenite, martensite, and bainite (Figure 1). The mechanical behavior of TRIP steels is largely dependent on the stability and transformation of retained austenite to martensite during loading. There are many variations of composition, processing, and microstructure of both DP and TRIP steels that a mechanical metallurgist can use to optimize mechanical properties (Figure 2). However, due to the complexity of these steels, there is still much to be understood in areas such as damage evolution, fatigue behavior, and weld mechanical behavior⁶.

Figure 1. Micrograph, produced by heat tint etching, of TRIP 780 steel. The beige and orange areas are ferrite, the blue areas are martensite, and the dark blue/purple regions are retained austenite6.

Figure 2. Engineering stress versus engineering strain curves for a low carbon steel compared to several different types of AHSS. TRIP and DP steels are discussed in the text. TWIP steels rely on mechanical twinning to achieve large amounts of deformation, and M220 is a low ductility, martensitic sheet steel7.

Mechanical metallurgy has benefited from the advent of new tools to characterize mechanical response and microstructure. The effects of local grain orientations or overall texture on mechanical behavior can be studied by electron backscatter diffraction (EBSD) imaging. Figure 3 shows an EBSD image of the inverse problem where a mechanical load imposed during hot deformation influences grain recrystallization behavior in a nickel-base superalloy⁸. Other tools such as thermomechanical processing simulators with advanced control and measuring capabilities⁹, nanoindenters, atomic force microscopes¹⁰, in-situ mechanical testing devices for electron microscopes, neutron diffractometers¹¹, and high strain rate testing devices¹² have enabled many advanced mechanical metallurgy studies that have shed light on physical behavior on a microscopic or macroscopic level that could not be achieved until recently.

Figure 3. EBSD inverse pole figure map of partially recrystallized Inconel 945, a nickel base superalloy intended for pipelines exposed to corrosive environments. The small grains are recrystallized grains, and the large grains are deformed but not yet recrystallized. Grain size, texture, and boundary misorientation data can be extracted from EBSD data to help elucidate recrystallization mechanisms during hot deformation that simulates forging practices (Image Courtesy of S.P. Coryell).

In summary, mechanical metallurgy is a field with expanding opportunities due to the new tools that are available to explore mechanical behavior and also opportunities for advanced materials development as both transportation and energy industries seek to become more efficient.

References

1. G. E. Dieter, Mechanical metallurgy, 3rd ed. ed. New York: McGraw-Hill, 1986.
2. M. A. Meyers, et al., Mechanical behavior of materials, 2nd ed. / Marc André Meyers, Krishan Kumar Chawla. ed. Cambridge: Cambridge University Press, 2009.
3. T. H. Courtney, Mechanical behavior of materials, 2nd ed., International ed. ed. Boston, Mass. ; London: McGraw Hill, 2000.
4. N. E. Dowling, Mechanical behavior of materials : engineering methods for deformation, fracture, and fatigue, 3rd ed. ed. Upper Saddle River, NJ: Pearson Prentice Hall ; London : Pearson Education, 2007.
5. H. Petroski, To engineer is human : the role of failure in successful design. New York, NY: St. Martin's Press, 1985.
6. K. O. Findley, S. Liu, K. Clymer, M. Liu, J. Davis., "Weldability, Processing, Microstructure, and Mechanical Behavior Relationships in Advanced High Strength Steel," AIST Transactions, vol. August 2010, 2010.
7. J. Ronevich, "Hydrogen Embrittlement in Advanced High Strength Steels," M.S., Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, 2009.
8. S. P. Coryell, et al., "Flow Behavior of Superalloy 945 During High Temperature Deformation," in TMS 2010 Supplemental Proceedings Vol. 1, Seattle, WA, 2010, pp. 291-298.
9. "Dynamic Systems, Inc.," www.gleeble.com 2010.
10. L. Cretegny and A. Saxena, "AFM characterization of the evolution of surface deformation during fatigue in polycrystalline copper," Acta Materialia, vol. 49, pp. 3755-3765, Oct 2001.
11. D. W. Brown, et al., "Uniaxial tensile deformation of uranium 6 wt pct niobium: A neutron diffraction study of deformation twinning," Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, vol. 32, pp. 2219-2228, Sep 2001.
12. I. D. Choi, et al., "The effect of retained austenite stability on high speed deformation behavior of TRIP steels," Metals and Materials International, vol. 12, pp. 13-19, Feb 2006.