Microstructure Evolution And Deformation Mechanism Of Metals At High Strain Rates

As the main parameter to evaluate the application of metal steels in dynamic impact environment,strain rate is widely used to analysis the strengthening and failure mechanism of metals.In previous work,researchers have focused on improving mechanical properties of metals under different strain rates.However,the high strain rate dependence of dislocation,low angle grain boundary（LAGBs）and high angle grain boundary（HAGBs）has been studied less in previous work,and the conversion mechanism between the three microstructural factors remains unclear.In this paper,45#steel and single crystal aluminum were compressed by three loading methods:universal testing machine,Hopkins bar and light gas gun.By regulating the strain rate,the influence of 45#steel and single crystal aluminum on the microstructure evolution of the metal was revealed and the deformation mechanism was clarified.（1）A gas gun was used to generate shock wave to compress the AISI 1045 steels and investigated their structures under different stain rates of 4.3×10⁵s^-1（low）and 3.3×10⁶s^-1（high）,respectively.The gradient structure was found along the shock compression direction under different stain rates,which were strain-rate dependent.The microstructure at the low-strain-rate shows the banded ferrite/pearlite.High-strain-rate sample displayed a gradient structure（amorphous zone→streamlined martensite→composite martensite and troostite）.Compared with the low-strain-rate sample,the more fractions of the Geometrically Necessary Dislocation（GND）and LAGBs were obtained under the high strain rates.At high strain rates,the dislocations would form the LAGBs,while the LAGBs would difficultly transform into HAGBs.At low strain rates,the LAGBs preferred to transform into HAGBs.These results indicate that the rate effect of the structures under loading plays an important role in guiding the materials design and applications.（2）An obvious rate effect was found by studying the microstructure response of 45#steel at different strain rates.However,the structure of the AISI 1045 steels is complex,involving a variety of phase transition.The influence of strain rate on the microstructure of metal materials is not clearly studied.Hence,the deformation mechanism and microstructure evolution of single crystal Al at different strain rates were studied in detail.The structural evolution of（100）single crystal aluminum has been studied at a wide strain rate ranging from 10^-4 to 10⁵s^-1 under compression methods.It is found that dislocation motion has a strong strain rate response.At ultra-low strain rates(<10^-3 s^-1),the plastic deformation mediated by dislocation→LAGBs rearrangement process.At low strain rates(>10^-3 s^-1),the dislocation adapts to plastic deformation through an annihilation process by massive cross-slip.At high strain rates(>10^-1 s^-1),plastic deformation is also regulated by dislocation rearrangement,leading to the formation of a large amount of LAGBs.Under ultrahigh strain plastic(>10⁵ s^-1),the plastic deformation is dominated by the twinning process.The rearrangement process of dislocation→LAGBs are different in ultra-low strain rate and high strain rate.For ultra-low strain rate with low applied stress,a large amount of LAGBs is formed.Dislocation is difficult to cross the barrier and fixed by obstacles.For high strain rates,dislocation motion is hindered by the short loading time and the activation of large number slip systems.These results reveal the deformation mechanism of microstructure response behavior of single crystal aluminum at different strain rates.（3）Anisotropic plasticity is controlled by the activation of the slip system,which is affected by the crystal orientation.Therefore,the orientation of single crystal has a great influence on the plastic deformation of single crystal.The different orientation of single crystal aluminium in 3×10^-2 s^-1（low）and 1×10^-1 s^-1（high）structure evolution was studied by the universal testing machine.It is found that the structural response of single crystal aluminum has strong orientation dependence.Both the annihilation process at low strain rate and the slip process at high strain rate show that（111）single crystal aluminum exhibits slower dislocation motion than（100）orientation.With the increase of strain rate,the degree of dislocation movement of（111）single crystal aluminum increases.These results indicate that the strong difference between the microstructure of the dislocation under the two loading conditions is related to the orientation anisotropy.