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Chapter 10: Problem 31

Cite two reasons why martensite is so hard and brittle.

Short Answer

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Answer: The two main reasons for martensite's hardness and brittleness are its body-centered tetragonal (BCT) crystal structure and rapid transformation rate from austenite. The BCT structure introduces a high level of stress within the crystal lattice, and the rapid transformation rate leaves a high number of dislocations locked in the martensite.

Step by step solution

01

1. Crystal Structure of Martensite

One of the reasons why martensite is so hard and brittle is due to its crystal structure. Martensite has a body-centered tetragonal (BCT) crystal structure. The BCT structure has elongated unit cells along one crystallographic direction, which makes it less symmetric and introduces a high level of stress within the crystal lattice. This increased stress and distorted lattice with strong atomic bonds result in increased hardness. The high stress within the structure also makes it difficult for dislocation movement, preventing it from undergoing plastic deformation and resulting in brittleness.
02

2. Rapid Transformation Rate

The second reason for martensite's hardness and brittleness is the rapid transformation rate of martensite from the parent phase, austenite. The transformation process, known as martensitic transformation, involves a sudden shear motion of the crystal lattice that happens at very fast cooling rates. This rapid movement doesn't allow enough time for dislocations to rearrange and for vacancies to be eliminated. Consequently, a high number of dislocations remain locked in the martensite, leading to the material being hard and brittle. In summary, the hardness and brittleness of martensite are primarily caused by its body-centered tetragonal crystal structure and rapid transformation rate from austenite.

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Most popular questions from this chapter

For a eutectoid steel, describe isothermal heat treatments that would be required to yield specimens having the following Rockwell hardnesses: (a) \(93 \mathrm{HRB}\) (b) \(40 \mathrm{HRC}\) (c) \(27 \mathrm{HRC}\)

(a) From the curves shown in Figure \(10.11\) and using Equation 10.18, determine the rate of recrystallization for pure copper at the several temperatures. (b) Make a plot of \(\ln (\) rate) versus the reciprocal of temperature (in \(\mathrm{K}^{-1}\) ), and determine the activation energy for this recrystallization process. (See Section 5.5.) (c) By extrapolation, estimate the length of time required for \(50 \%\) recrystallization at room temperature, \(20^{\circ} \mathrm{C}(293 \mathrm{~K})\).

(a) Briefly describe the phenomena of superheating and supercooling. (b) Why do these phenomena occur?

Briefly describe the simplest heat treatment procedure that would be used in converting a \(0.76 \mathrm{wt} \% \mathrm{C}\) steel from one microstructure to the other, as follows: (a) Spheroidite to tempered martensite (b) Tempered martensite to pearlite (c) Bainite to martensite (d) Martensite to pearlite (e) Pearlite to tempered martensite (f) Tempered martensite to pearlite (g) Bainite to tempered martensite (h) Tempered martensite to spheroidite

Name the microstructural products of eutectoid iron-carbon alloy \((0.76 \mathrm{wt} \% \mathrm{C})\) specimens that are first completely transformed to austenite, then cooled to room temperature at the following rates: (a) \(200^{\circ} \mathrm{C} / \mathrm{s}\), (b) \(100^{\circ} \mathrm{C} / \mathrm{s}\), and (c) \(20^{\circ} \mathrm{C} / \mathrm{s}\).
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