What is the main distinction between the martensite and other transformations?
The Martensite transformation is a diffusionless mechanism.
It transforms via a change in crystal structure.
What is an allotropic transformation?
A change in crystal structure.
What is the Martensite start and finish temperature?
Martensite start: the temperature at which Martensite forming becomes possible
Martensite finish: Temperature below which no further Martensite forms
How does the Martensite formation compare energetically to the diffusive formation?
Martensite formation requires a higher driving force than the diffusive ones.
(thats why Ms is below the other starting Temps.)
Describe the microstructure of martensite.
Formation as plate inside parent austenite
additional plates form with the original plate as a starting point.
Plate formation is accompanied by shape change.
Describe the change in shape for the martensite transformation on the surface of an Austenite crystal.
What is the rough propagation speed of the martensite change?
Speed of sound
Describe the Bain model for martensite transformation.
resulting in minimum atom displacement
20% contraction in z
12% expanion in x and y
What is the full martensite transformation based on?
Bain distortion
shear strain which produces undistorted plain
rotation
Where does the Martensitic transformation nucleate?
The transformation nucleates at points of high deformation=strain embryos
What are the two key features of the Martensitic transformation demonstrated by this diagramm?
The Martensitic transformation is fully reversible and independant of time.
How is the shear induced into the material by martensite transformation taken up by the material?
Either twinning of dislocations
Why are the hysterisis temperatures far apart in the Fe-Ni system?
Because of the high required driving force.
What is special about the martensite formation in a Fe-Ni system?
slightly below the Ms temperature a burst like formation of martensite occurs, which catalyses the growth of further martensite
What limits the growth of martensite plates?
The transformation of the plates induces a large strain on the mother lattice and leads to a loss of coherence, which leads to the growth stop.
Is martensite formation strictly time independant?
No a smalll fraction forms after while holding at the temperature.
the lower the better
p 30
What is the influence of stress on the martensite transformation?
In general Ms can be increased or decreased by strain
<=> there is an influence
compression raises Ms
hydrostatic stress lowers Ms
What are the differences in consequence if the Martensite deforms the Material plasticly or elasticly?
elastic: the martensite can appear and disappear. the process is reversible
plastic: the dislocations lock the martensite in place and the temperature needed to reform Austenite are higher
Describe the relation for the needed applied stress for stress asissted martensite formation and the temperature
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