Research On The {10-12} Twinning Deformation Mechanism Of Pure Magnesium Induced By Focused Ion Beam(FIB)

In this work,the experimental techniques including Electron Backscatter Diffraction(EBSD)and Focused Ion Beam(FIB)were employed to study the nucleation and growth of twinning mechanical properties of pure Magnesium alloy in different test conditions.With the help of the particle bombardment-induced local microscopic stress,the mechanism of twinning nucleation and growth,is systematically studied via twinning pattern,twinning configuration and twinning variants with different process parameters(i.e.,bombardment beam,time and the orientation of c-axis: 0o and 90o)of pure Magnesium.From the visual conditions such as {10-12} twinning nucleation,the plastic deformation mechanism of Magnesium alloys is improved to explore the nature about {10-12} twinning nucleation in different process conditions by establishing the twin nucleation and growth model,thus revealing the essence of plastic deformation,which is beneficial to enrich microstructure and texture control theory in the process of twinning.Based on FIB technology,the nucleation mechanism of {10-12} twinning produced by a particle beam bombardment on the surface of the sample via different process parameters were studied,including ion beam machining,the collision between particles and energy exchanging.Furthermore,a subsequent study of the trend of ion sputtering yield change under the different incident ionic energy and angles,and the number of ions were also paid attention to as well.In addition,the surface damage of ion bombardment material is also theoretically analyzed.The main results are as follows:(1)With the aid of FIB,the accelerated and then focused ion beam was used as the incident beam.During the process of atomic collisions,high-energy ions can work with solid surface and then the solid atom would be sputtering out(called bombardment),resulting in defects.In that case,it would create an ideal environment for stress concentration during plastic deformation,which was beneficial for the {10-12} twin nucleation and growth and later made it into reality successfully.(2)The scope of the current and time to motivate twinning are about 1nA~16nA,10s~30s,respectively.Additionally,the morphology of the twins is related to bombardment currents and time as well as the crystal orientation.(3)In the micro-scale plastic deformation process,the {10-12} twin is produced by focused ion beam irradiation both in vertical load and parallel load to the c axis,differing from that of the macroscopic deformation,which has a unidirectional loading dependency.That is,the occurrence of {10-12} twin depends on compression perpendicular to c axis or tension parallel to the c axis.(4)When FIB bombardment direction is perpendicular to the c axis of the sample,the {10-12} twin trend to occur in the hole tip under different bombardment current and bombardment time.A hole-tip nucleation(HTN)mold was put forward to explain the essence of the nucleation and growth of {10-12} twinning.HTN: when bombarded perpendicular to c axis,it creates a severe stress concentration at the tip of the hole,partial dislocations are strongly prevalent expended rapidly from hole tip to the nearby,which is beneficial to the occurrence of the {10-12} twinning.After the characterization analysis by channel 5 software,the {10-12} twinning is the most common to see near the hole,thus confirming the {10-12} twinning is one of the most basic twinning model in magnesium alloy.(5)On the contrary,when bombarded parallel to c axis,the {10-12} twin is luck to emerge near the hole at the bottom of the tip after the EBSD data analysis.According to the different morphological characteristics of twinning occurs in the experiment,another mold called Hole-wall nucleation(HWN)may account for this phenomenon.HWN: when bombarded parallel to c axis,the stress state at the bottom of hole tip is more suitable for the nucleation of twin,thus making twinning into reality.Then,the atoms return to steady state,namely the twin growth process.(6)The essence for accounting for the nucleation of {10-12} by HTN and HWN is related to bombardment direction and crystal orientation.