Page 45 - MetalForming Magazine May 2022

Metal Matters
By Daniel J. Schaeffler, Ph.D.
Crystallography 101
Atoms of metal alloys arrange them- selves in specific repeating patterns known as crystals. Picture two sim- ple patterns as a 3D arrangement of bil- liard balls with rows directly on top of one another, compared with a slight offset where every other row sits in the pocket of the prior one. In both scenarios, the balls all are the same size, but the gaps between them vary.
Reality typically is more complex. Crys- tal structures for common sheet metals are body-centered-cubic (BCC) for carbon and ferritic stainless steels; face-centered- cubic (FCC) for austenitic stainless steels, aluminum alloys and copper alloys; and hexagonal-close-packed (HCP) for titani- um alloys. The different atomic stackups influence the characteristics of material flow.
Upon heating of carbon steels, the BCC
structure known as ferrite transforms to
an FCC structure called austenite. If slowly
cooled, austenite transforms back to fer-
rite, along with an amount of pearlite
based on the carbon content in the steel.
When quenched, although steel wants to
convert back to BCC ferrite, there is insuf-
ficient time for the carbon in austenite to
diffuse out of the FCC structure. This extra carbon strains the BCC structure, elongating one axis into a body-cen- tered-tetragonal (BCT) crystal structure called martensite. Martensite’s high strength comes from the highly strained crystal structure (Fig. 1).
Metal flow is based on the movement of one layer of atoms relative to the next, so differences in crystal structure
Dr. Danny Schaeffler, with 30 years of materials and applications experience, is president of Engineering Quality Solutions (EQS) and chief content officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing compa- nies; 4M teaches fundamentals and practical details of material properties, forming technologies, process- es and troubleshooting needed to form high-quality components. Schaeffler is the metallurgy and forming technical editor of the AHSS Application Guidelines available from WorldAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Fig. 1—Face-centered cubic (FCC), body-centered cubic (BCC) and body-centered tetragonal (BCT) crystal structures: The arrangement of atoms and the gaps between them affect metal flow.
Fig. 2—In this 2D representation of grains in different orientations, all circles are the same element with different orientations represented by different colors.
change the ease by which atoms can move.
While the type of crystal structure is consistent throughout a sheet metal grade, the orientation of these crystals changes. A grouping of atoms in one orientation is called a grain; adjacent grains differ in their orientation, as seen in Fig. 2, showing a 2D represen- tation of grains in multiple orienta- tions. However, remember that grains grow and move in three dimensions. Smaller grains mean more orientation changes, leading to more resistance to atomic movement as sheet metals deform. This translates to higher strength. For perspective, the atomic diameter of common sheet metals typ- ically is 0.00025 to 0.00035 microns, and an ASTM grain size of 7 equates
to a diameter of 32 microns, or 100,000 times as large. Additions of alloying elements, each with a unique atomic size, distort and strain the regular pattern of atoms. This explains why aluminum alloys and steels are stronger than the elements aluminum and iron, which typically comprise
more than 95 percent of an alloy.
The distribution of different orientations is the crystal-
lographic texture of the metal alloy. Hot-rolled steels have a random grain orientation, so properties do not vary greatly relative to the rolling direction. This contrasts with cold- rolled grades, where grains elongate in the rolling direction. This leads to measurable differences in properties depending on the orientation. Isotropic materials are those where ori- entation does not affect properties, while orientation influ- ences anisotropic materials.
Taken together, the measured strength and ductility char- acteristics coming out of the sheet production mill reflect these differences in crystal structure, grain size, grain ori- entation and alloying. MF
42 MetalForming/May 2022
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