MSE 220 EXAM 2 QUESTIONS WITH VERIFIED
ANSWERS
eutectoid reaction - Answers - upon cooling, a solid phase transforms isothermally and
reversibly into two new solid phases
peritectic reaction - Answers - upon cooling, a solid and liquid phase transforms
isothermally and reversibly into a solid phase having a different composition
What are the two fundamental steps in the production of a new phase? - Answers -
Nucleation and growth.
What is the difference between homogeneous and heterogeneous nucleation? -
Answers - Homogeneous nucleation occurs uniformly throughout the parent phase,
while heterogeneous nucleation occurs preferentially at structural inhomogeneities like
surfaces or impurities.
What does activation free energy (ΔG*) represent in nucleation theory? - Answers - An
energy barrier that must be overcome for nucleation to occur.
What is the critical radius (r*) in the context of nucleus growth? - Answers - The size a
cluster of atoms must achieve for growth to continue spontaneously.
How do superheating and supercooling occur? - Answers - They occur when
temperature changes are too rapid to maintain equilibrium conditions, raising or
lowering the transformation temperature respectively.
Which type of nucleation requires a smaller degree of supercooling or superheating? -
Answers - Heterogeneous nucleation.
What shape does a plot of fraction transformation versus the logarithm of time typically
take? - Answers - An S-shape curve.
How is the transformation rate defined mathematically? - Answers - As the reciprocal of
the time required for a transformation to proceed halfway to completion.
What information is presented in an isothermal transformation (TTT) diagram? -
Answers - The dependence of transformation progress on time while temperature is
held constant.
How do continuous-cooling transformation (CCT) diagrams differ from TTT diagrams? -
Answers - CCT diagrams account for continuous cooling by shifting transformation
curves to longer times and lower temperatures.
, What is the structural difference between coarse and fine pearlite? - Answers - Fine
pearlite has thinner alternating layers of alpha-ferrite and cementite than coarse
pearlite.
What is the composition and structure of bainite? - Answers - A very fine structure
composed of a ferrite matrix and elongated cementite particles.
How is spheroidite formed? - Answers - By heating fine or coarse pearlite or bainite at
approximately 700°C for several hours.
What is the crystal structure of martensite? - Answers - Body-centered tetragonal.
How is martensite produced? - Answers - By rapidly quenching austenite to prevent
carbon diffusion and the formation of pearlite or bainite.
What is the process to transform martensite into tempered martensite? - Answers -
Heating martensite at temperatures between 250°C and 650°C.
How do alloying elements affect the CCT diagram? - Answers - They shift the pearlite
and bainite noses to longer times, making the transformation to martensite more
favorable.
Which iron-carbon microstructure is the hardest and strongest? - Answers - Martensite.
Which iron-carbon microstructure is the softest and most ductile? - Answers -
Spheroidite.
How does the strength and ductility of tempered martensite compare to bainite? -
Answers - Tempered martensite is stronger, while bainite offers a desirable strength-
ductility combination but is less strong.
What causes embrittlement in some steel alloys? - Answers - The presence of specific
alloying/impurity elements combined with tempering within a certain temperature range.
What mechanism allows shape-memory alloys to return to their original shape upon
heating? - Answers - The migration of twin boundaries and a martensite-to-austenite
phase transformation.
What are the three primary factors to consider when designing laboratory tests for
material mechanical characteristics? - Answers - Nature of the applied load, load
duration, and environmental conditions.
How is engineering stress defined? - Answers - Instantaneous load divided by the
original specimen cross-sectional area.
ANSWERS
eutectoid reaction - Answers - upon cooling, a solid phase transforms isothermally and
reversibly into two new solid phases
peritectic reaction - Answers - upon cooling, a solid and liquid phase transforms
isothermally and reversibly into a solid phase having a different composition
What are the two fundamental steps in the production of a new phase? - Answers -
Nucleation and growth.
What is the difference between homogeneous and heterogeneous nucleation? -
Answers - Homogeneous nucleation occurs uniformly throughout the parent phase,
while heterogeneous nucleation occurs preferentially at structural inhomogeneities like
surfaces or impurities.
What does activation free energy (ΔG*) represent in nucleation theory? - Answers - An
energy barrier that must be overcome for nucleation to occur.
What is the critical radius (r*) in the context of nucleus growth? - Answers - The size a
cluster of atoms must achieve for growth to continue spontaneously.
How do superheating and supercooling occur? - Answers - They occur when
temperature changes are too rapid to maintain equilibrium conditions, raising or
lowering the transformation temperature respectively.
Which type of nucleation requires a smaller degree of supercooling or superheating? -
Answers - Heterogeneous nucleation.
What shape does a plot of fraction transformation versus the logarithm of time typically
take? - Answers - An S-shape curve.
How is the transformation rate defined mathematically? - Answers - As the reciprocal of
the time required for a transformation to proceed halfway to completion.
What information is presented in an isothermal transformation (TTT) diagram? -
Answers - The dependence of transformation progress on time while temperature is
held constant.
How do continuous-cooling transformation (CCT) diagrams differ from TTT diagrams? -
Answers - CCT diagrams account for continuous cooling by shifting transformation
curves to longer times and lower temperatures.
, What is the structural difference between coarse and fine pearlite? - Answers - Fine
pearlite has thinner alternating layers of alpha-ferrite and cementite than coarse
pearlite.
What is the composition and structure of bainite? - Answers - A very fine structure
composed of a ferrite matrix and elongated cementite particles.
How is spheroidite formed? - Answers - By heating fine or coarse pearlite or bainite at
approximately 700°C for several hours.
What is the crystal structure of martensite? - Answers - Body-centered tetragonal.
How is martensite produced? - Answers - By rapidly quenching austenite to prevent
carbon diffusion and the formation of pearlite or bainite.
What is the process to transform martensite into tempered martensite? - Answers -
Heating martensite at temperatures between 250°C and 650°C.
How do alloying elements affect the CCT diagram? - Answers - They shift the pearlite
and bainite noses to longer times, making the transformation to martensite more
favorable.
Which iron-carbon microstructure is the hardest and strongest? - Answers - Martensite.
Which iron-carbon microstructure is the softest and most ductile? - Answers -
Spheroidite.
How does the strength and ductility of tempered martensite compare to bainite? -
Answers - Tempered martensite is stronger, while bainite offers a desirable strength-
ductility combination but is less strong.
What causes embrittlement in some steel alloys? - Answers - The presence of specific
alloying/impurity elements combined with tempering within a certain temperature range.
What mechanism allows shape-memory alloys to return to their original shape upon
heating? - Answers - The migration of twin boundaries and a martensite-to-austenite
phase transformation.
What are the three primary factors to consider when designing laboratory tests for
material mechanical characteristics? - Answers - Nature of the applied load, load
duration, and environmental conditions.
How is engineering stress defined? - Answers - Instantaneous load divided by the
original specimen cross-sectional area.
