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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

The kinetics of the austenite-to-pearlite transformation obey the Avrami relationship. Using the fraction transformed-time data given here, determine the total time required for \(95 \%\) of the austenite to transform to pearlite: $$ \begin{array}{lc} \hline \text { Fraction Transformed } & \text { Time (s) } \\ \hline 0.2 & 12.6 \\ 0.8 & 28.2 \\ \hline \end{array} $$

Cite two important differences between continuous cooling transformation diagrams for plain carbon and alloy steels.

(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, \(20^{\circ} \mathrm{C}(293 \mathrm{~K})\).

Name the two stages involved in the formation of particles of a new phase. Briefly describe each.

Briefly explain why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite.
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