ALLOYING STOPS DISLOCATION MOVERNMENT JUSTIDIED

INTRODUCTION

Steels are mainly composed of iron and carbon and special properties are reached by introducing additional alloying elements. Stainless steels are among the most important engineering materials. They are alloy steels containing more than 12 percent Cr. Chromium forms a passive oxide film on the surface, which makes these alloys resistant against corrosion in various chemical environments (Wranglén, 1985). The main building block of ferrite stainless steels is the Fe-Cr alloy having the ferromagnetic a-Fe structure. Austenitic stainless steels form the largest sub-category of stainless steels and comprise a significant amount of Ni as well. At low temperature, these alloys exhibit a rich variety of magnetic structures as a function of chemical composition, ranging from ferromagnetic phase to spin-glass and antiferromagnetic alignments (Majumdar & Blanckenhagen, 1984). At ambient conditions, Ni changes the ferromagnetic a-Fe structure to the paramagnetic g-Fe structure.

Today austenitic stainless steels dominate the steels applications, where high corrosion resistance and excellent mechanical properties are required. The austenitic grades represent the primary choice also when nonmagnetic properties are concerned.

THE ALLOYING STOPS DISLOCATION MOVEMENT

The stress required to cause dislocation motion is orders of magnitude lower than the theoretical stress required to shift an entire plane of atoms, so this mode of stress relief is energetically favorable. Hence, the hardness and strength (both yield and tensile) critically depend on the ease with which dislocations move. Pinning points, or locations in the crystal that oppose the motion of dislocations, can be introduced into the lattice to reduce dislocation mobility, thereby increasing mechanical strength.

Dislocations may be pinned due to stress field interactions with other dislocations and solute particles, or physical barriers from grain boundaries and second phase precipitates. There are several strengthening mechanisms for metals, however the key concept to remember about strengthening of metallic materials is that it is all about preventing dislocation motion and propagation; you are making it energetically unfavorable for the dislocation to move or propagate. For a material that has been strengthened, by some processing method, the amount of force required to start irreversible (plastic) deformation is greater than it was for the original material.

In amorphous materials such as polymers, amorphous ceramics (glass), and amorphous metals, the lack of long range order leads to yielding via mechanisms such as brittle fracture, crazing, and shear band formation. In these systems, strengthening mechanisms do not involve dislocations, but rather consist of modifications to the chemical structure and processing of the constituent material.

Unfortunately, strength of materials cannot infinitely increase. Each of the mechanisms elaborated below involves some trade off by which other material properties are compromised in the process of strengthening.

An alloy:

An alloy is a metal that’s combined with other substances to create a new metal with superior properties. For example, the alloy may be stronger, harder, tougher, or more malleable than the original metal. Alloys are often thought to be a mixture of two or more metals. However, this is a misconception, as alloys can be composed of one metal and other non-metallic elements.

Using Alloying to Prevent Corrosion

We’ve seen that alloys can be created to increase a metal’s resistance to corrosion. The traditional method used to prevent corrosion was to cover the metal with a surface coating, such as polymer. This creates a barrier between the surface of the metal and the elements.

EonCoat isn’t a barrier coating. Fundamentally different from a polymer you paint on the surface of a metal, it’s a surface treatment that actually alloys the steel that it comes into contact with. Since rust starts at the surface of a metal, if the surface is alloyed, there’s nothing exposed, and therefore, no place for rust to form.

Steel can be strengthened by several basic mechanisms, the most important of which are:

1. Work hardening or strain hardening.
2. Solid solution strengthening by interstitial atoms.
3. Solid solution strengthening by substitutional atoms.
4. Refinement of grain size.
5. Effects of heat treatment on microstructures.

6. Precipitation strengthening.

7. Grain boundary strengthening.

8. Dispersion strengthening, including lamellar and random dispersed structures.

The most distinctive aspect of strengthening of iron and steel is the role of the interstitial solutes carbon and nitrogen. These elements also play a vital part in interacting with dislocations, and in combining preferentially with some of the metallic alloying elements used in steels.

The Nature of Dislocations

Plastid deformation is a measure of material strength, plastic deformation is irreversible. Therefore, the
configuration of the atoms must be changed during plastic deformation, for otherwise they would return to their
original position on unloading. If we consider shearing a single crystal as an example, it can be deformed
plastically by sliding whole layers of atoms against each other as shown in figure above for this sliding to
happen, the bonds between the atoms have to be stretched elastically until they can switch to the next atom.
The stress required for this process can be estimated and is of the order of one fifth of the shear modulus of
the crystal. The yield strength predicted this way for metallic single crystals is thus between 1GPa and 25GPa.
If we measure the strength of single crystals of pure metals, the values found are several orders of magnitudes
below this theoretical value and even lie below that of engineering alloys. Typical values are in the range of a
few mega Pascal. As single crystals always contain lattice defects, one possible explanation could be that
these are responsible for the reduced strength. If, however, the number of defects is reduced further, for
instance by a heat treatment, the yield strength becomes even smaller. Only an absolutely perfect single
crystal without any defects would possess a yield strength agreeing with the theoretical prediction. This can
only be nearly realized in so-called whiskers, which, however, are extremely small. The reason for this
spectacular failure of the theoretical prediction is that plastic deformation does not occur by sliding of complete
layers of atoms. Instead, it proceeds by a mechanism that is based on a special type of lattice defect, the
dislocations. To understand plastic deformation of metals thus requires an understanding of dislocations.

Dislocation densities

Dislocation is a lattice imperfection in a crystal structure which exerts a profound effect on a structure sensitive properties such as strength, hardness, ductility and toughness. There are two types, edge and screw or combination of both, all of which are characterized by a Burgers vector which represents the amount and direction of slip when the dislocation moves. Click on the web links provided to read further.

Transmission Electron Micrograph of                         Transmission Electron Micrograph of

Dislocations                                                                Dislocations

Grain boundaries act as an impediment to dislocation motion for the following two reasons:

•

Dislocation must change its direction of motion due to the differing orientation of grains.

•

Discontinuity of slip planes from grain 1 to grain

References

Callister, W. D. "Materials Science and Engineering: An Introduction" 2007, 7th edition, John Wiley and Sons, Inc. New York, Section 4.3 and Chapter 9.

Bauccio, Michael (1003) ASM metals reference book. ASM International. ISBN 0-87170-478-1.

Verhoeven, John D. (2007). Steel Metallurgy for the Non-metallurgist. ASM International. p. 56. ISBN 978-1-61503-056-9. Archived from the original on 2016-05-05.

Davis, Joseph R. (1993) ASM Specialty Handbook: Aluminum and Aluminum Alloys. ASM International. p. 211. ISBN 978-0-87170-496-2.

Mills, Adelbert Phillo (1922) Materials of Construction: Their Manufacture and Properties, John Wiley & sons, inc, originally published by the University of Wisconsin, Madison

Hogan, C. (1969). "Density of States of an Insulating Ferromagnetic Alloy". Physical Review. 188 (2): 870. Bibcode:1969PhRv..188..870H. doi:10.1103/PhysRev.188.870.