BCC-FCC Phase Transformation Mechanism Of Single Crystal Iron

Because iron is the king structural material in the world,its physical and mechanical properties are the subject of extensive theoretical and experimental investigations.The phase transformation mechanism of iron had also been the focus of researchers.By applying different loading stresses,body-centered-cubic iron will undergo stress-induced phase transition,which will be transformed into face-centered-cubic or hexagonal close-packed phase.Obviously,the stress-induced martensitic phase transformation of iron can be divided into two categories: the first one is stress-induced phase transformation from body-centeredcubic to face-centered-cubic,which can be realized by uniaxial stretching a body-centeredcubic phase along one of its cubic axes to let the ratio of its three cubic axes reach√2^1/2:1:1.The other stress-induced martensitic transformation of iron is from body-centered-cubic to hexagonal-close-packed phase.The researchers had proved that when the loading process does not meet the stability conditions,the alkali metal would lose its stability due to bifurcation,that is,it would sudden expansion along one lateral direction and contraction along the other lateral direction,completing the transformation from body-centered-cubic to face-centered-cubic phase.We cannot help asking that whether there exists a similar phase transformation pathway when the body-centered-cubic iron was compressed along [100] direction.Phase transformation process of iron nanoplate and bulk material under [100] direction compressive loading was investigated by using molecular dynamic simulation method.The simulation results showed that the iron nanoplate would undergo body-centered-cubic to face-centered-cubic phase transformation;while iron bulk material would undergo body-centered-cubic to hexagonalclose-packed phase transformation.With applying sufficient large lateral stress,the iron bulk material would transform into face-centered-cubic phase.The critical transverse stress corresponding to the transformation of iron bulk material into face-centered-cubic phase and hexagonal-close-packed phase was obtained by further analysis.Moreover,these phase transformation processes were further analyzed by using elastic stability theory,the theoretical result was consistent with the simulation results very well.Body-centered-cubic to hexagonal-close-packed phase transition occurred when iron was impacted along [100] direction,and the mechanism of phase transition was very clear.However,during the impact process along the [101] direction,a large number of facecentered-cubic structures were found in the phase transition products except hexagonalclose-packed phase.Although it had been proved that the phase transformation mechanism of body-centered-cubic to hexagonal-close-packed was the same as that of impact along the[100] direction,the formation of face-centered-cubic structure had not been solved so far.In the present paper,molecular dynamics method wad used to study phase transformation mechanism of body-centered-cubic single crystal iron impacted along [101] crystal direction.The simulation results showed that body-centered-cubic phase would transform into close packed structure（hexagonal-close-packed and face-centered-cubic phase）.The formation mechanism of face-centered-cubic phase was analyzed: during the impact process,the single crystal iron contracted suddenly along [101] and [-101] crystal directions,and expanded along [010] crystal direction,leading to the transformation from body-centered-cubic to face-centered-cubic phase.In addition,phase transformation mechanisms of single crystal iron under different stress states were further studied.It was found that the transformation from body-centered-cubic to face-centered-cubic phase occurred in the case of uniaxial compression along [101] crystal direction and biaxial compression along [101] and [-101]crystal directions;while the transformation from body-centered-cubic to hexagonal-closepacked phase occurred in the case of biaxial compression along [101] and [010] crystal directions and triaxial compression.