by Professor Kip Findley
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 sources1-5.
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.
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.
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
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
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.
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,
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.
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