New Methods On The Determination Of Martensitic Transformation Shear Angle And Lattice Parameters

Materials characterization is a substantial part of materials science, since the characterization of microstructure and its transformation is becoming more and more a key to reveal materials properties and nature.Atomic force microscope (AFM) with high space resolution is used to characterize martensitic transformation surface relief, by observing the morphology and measuring the surface relief angles of stress-induced martensite in Fe-25Mn-6Si-5Cr(wt%) shape memory alloy and thermal-induced martensite in Fe-23Ni-0.55C(wt%) alloy, respectively. The relief angles are measured by AFM as 3.058°, 2.424°, 2.835°in Fe-25Mn-6Si-5Cr alloy and 3.6°, 6.3°, 4.9°and 5.3°in Fe-23Ni-0.55C alloy. Shi et al's and Yang et al's method on calculating shear angle are commented respectively and the limitation induced by one inappropriate assumption on shear direction is analyzed. Calculating results derived from their methods on above two alloys also indicate a huge deviation from theoretical shear angle, up to several tens of degrees. On the basis of Bergeon's crystallography model of relief angle, a standard procedure on calculating sample surface index through both trial-and-error and tetrahedron model method is built to improve the calculation of martensitic transformation shear angle. Related tremendous calculations also solved by MATLAB programming. The calculating results on two above alloys using modified method are in good agreement with the theoretical value. The errors are only 1.33°, 0.20°, 0.26°in Fe-25Mn-6Si-5Cr and 0.13°, 1.42°, 0.12°, 1.22°in Fe-25Ni-0.55C, which indicates higher precise and universal validity of this improved method.Computer simulation of higher-order Laue zone (HOLZ) lines of convergent beam electron diffraction (CBED) is carried out by and standard HOLZ lines in [113] axis of fcc, bcc and hcp structure are plotted. Pure Al and transformation induced plasticity (TRIP) steel retained austenite are applied to study the effects of accelerating voltage and lattice parameters on HOLZ lines, respectively. A method on the characterization of carbon concentration in micro area of TRIP steel retained austenite through accurate measurement of lattice parameters is proposed and could be further applied to study the stability of retained austenite combining with transmission electronmicroscope (TEM) experiments.