I find most of the properties being different for the martensite formed in steel when compared to that in shape memory alloys. Do they represent the same? If not, then what are the significant differences between the former and the latter?
Only the name is same. Martensite is common name for low tmeperature phase. and austenite for high temperature phase. By SMA is transformation between this 2 phases reversible, but it is ireversible in steels. There are few reasons why by steels is the transformation is not reversible. The main one is that change in volume is not aproximetly 0 as it is by SMA!
Only the name is same. Martensite is common name for low tmeperature phase. and austenite for high temperature phase. By SMA is transformation between this 2 phases reversible, but it is ireversible in steels. There are few reasons why by steels is the transformation is not reversible. The main one is that change in volume is not aproximetly 0 as it is by SMA!
In a sense these martensites are similar because of the same character (pronounced shear and extremely low time lapse) of lattice transformation. What is different is the highest dislocation density which arise in the steel martensite during the fast plastic accommodation of the pronounced phase strain i.e. misfit of two lattices (not only volumetric but also deviatoric!). In heating, the reverse transformation of SMA is much easier since there is no need to remove (annihilate) excessive defects. At the same time, to call the transformation in steels irreversible is inacceptable oversimplification. Indeed, any martensite in steel still eventually (later, at higher overheating) transforms into austenite.
Well, it depends on chem composition. Martensitic steels vary in tensile strength from say 700 MPA to 2500 MPa. At the lower limit it is comparable to the strength of some
SMA (Ni-Ti for instance).
Both are different.
In steel, due to quenching process, austenite gets transformed to martensite, which is a supersaturated solid solution of carbon in gamma iron, having BCT, BODY CENTERED TETRAGONAL structure..leads to very hardness.
But, in shape memory alloys, the concept is Psuedo plasticity, which is by thermo mechanical deformation, helps to generate energy.
Hi Santosh,
the significant difference between them is crystallographic reversibility which is due to one important factor - type of austenite/martensite interface. The "sessile" interface of steel martensite has higher overall energy caused by huge dislocations density, higher chemical free energy, higher values of lattice elastic constants, and so on. On the other side, these items are opposite in the SMA martensite.
The higher energy of steel martensite cause that heated martensite does not transform to austenite directly. Its interface is fixed by higher energy and transformation to austenite is realized throught decomposition of martensite to more stabile phases and after further elevation of temperature these phases diffusionaly goes to austenite phase.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Metals
- Alloys
- Steel