Hot Deformation Behaviors Of SiCp/Al Composites

The poor workability of discontinuously reinforced aluminum(DRA)composites has always been a key issue that restricts their cost reduction and wide application.In the hot deformation process,due to the addition of the stiff reinforcements,thus leading to an increasing of deformation resistance.When the hot working parameters were not appropriate,the nonuniform distribution of reinforcement particles occurred readily,and that would bring the risk of damage such as interface debonding,cavity,crack,etc.However,the complicated structures of DRA,as well as the difficulty of sample preparation for microstructural observation,impede a further investigation on the hot deformation behavior of DRA.The previous study on deformation mechanisms of the DRA from different fabrication routes were still inadequate to some extent.Therefore,considering the microstructural distinction from different routes,a comprehensive investigation on the structural evolution of DRA during hot deformation is highly desirable.In this work,the SiCp/Al composites were prepared by stir casting and powder metallurgy which were the two most common routes.The deformation behavior of the composites was studied in detail by hot compression experiments.A more accurate stress-strain rate fitting method was developed,which improved the solution accuracy of the modified dynamic material model(MDMM).The hot deformation mechanism were studied by combining strain rate sensitivity map,temperature sensitivity map and processing map with microstructural observations.In addition,the phenomenological constitutive model and the physical constitutive model based on the dislocation kinetics were established to describe the deformation behavior of the composites during hot working more accurately.The kinetic parameters of the Arrhenius constitutive model corresponding to the deformation mechanism were solved.The strain rate sensitivity map indicated that temperature had a more significant effect on the deformation mechanism for the fine-grained 14 vol.%SiCp/2014Al composites prepared by stir casting and following extrusion process.From the s map,the critical transition point(TAGG)of abnormal grain growth(AGG)was ascertained to be 440 ?.The deformation mechanisms corresponding to different temperature ranges were determined by the ? curves and the microstructural characteristics:(?)dynamic recovery(DRV)was the dominant mode at 350-400 ?;(?)dynamic recrystallization(DRX)occurred in manner of dynamic grain growth(DGG)at 400-440 °C;(iii)450 ?corresponded to the equicohesive point(Teq)of the matrix alloy.Above the Teq,grain boundary sliding would contributed to the deformation.The instability map based on the MDMM exhibited a well coincident correlation with the external crack of overall specimens.In the range of 355-495?and 0.001-1 s-1,the internal damage gradually increased with the increasing temperature and strain rate.At 460 ? and 0.001 s-1,although there was no macroscopic cracks on the lateral surface,a number of small cracks,cavities and other damages generated near the edges where was subjected to tensile stress.For the fine-grained 14 vol%SiCp/2014Al composites,AGG operated above 440?.The yield strength at 460? and 495 °C was higher than that at 425 ? under the low strain rate of 0.001 s-1.This anomalous behavior was caused by the multiple impediments to grain boundary gliding(GBS)and intragranular slip by AGG and intragranular SiC particles respectively.It could be evaluated by the Zener limit dimension that the fine-grained matrix is not a stable system.The grain growth would occur readily above the recrystallization temperature.The large strain and multiaxial deformation contributed to eliminate the particle free bands(PFBs)and improve the particle uniformity,while the strain and temperature made a less effect on it.For the 17 vol.%SiCp/2009Al composite fabricated by powder metallurgy route,according to the ? map,the processing parameters can be divided into four domains with different deformation mechanisms:(i)350-450 °C and 0.001-0.056 s-1,DRV was dominant mechanism for both the matrix alloy and the composite;(ii)350-450 ?and 0.056-1 s-1,DRV and partial DRX were predominant at different positions of specimen interior respectively for the both materials;(iii)450-500? and 0.056-1 s-1,the dominant mechanism was GBS at shear deformation region,and partial DRX at center region for the matrix alloy,while DRX was predominant at various regions for the composite;(iv)450-500 °C and 0.001-0.056 s-1,the dominant mechanism was GBS for the matrix alloy,while it was partial DRX for the composite.The recrystallization mechanism was geometric dynamic recrystallization(GDRX).The analysis of DRX kinetics indicated that the SiC particles accelerated dislocation multiplication and impeded the restoration in the composite.At the temperatures below 450?,dislocations restored gradually by DRX in the subsequent deformation after yield peak,while above 450 °C most of the dislocations could be restored by DRX in the stage of yield peak due to the increased mobility.The two types of instability criteria based on Ziegler's continuum principle and Lyapunov function could not present well correlation with the composite and the unreinforced alloy fabricated by PM route.The microstructural observation indicated that the damage more readily occurred at the high temperatures(?450 0C)and high strain rates(>0.1 s-1).Nevertheless,due to the impediment of SiC particles,the damage level in the composite was less than that in the matrix alloy.The hot flow behavior of the powder metallurgy processed 17 vol.%SiCp/2009Al composite and the unreinforced alloy were described by the different types of constitutive models.The calculation errors of Johnson-Cook(JC)model and the modified Khan-Huang-Liang(M-KHL)model were compared.According to the temperature partition of 350-450?(DRV)and 450-500 ?(grain boundary softening and DRX),the apparent activation energy was solved by the power law function of Arrhenius model.The deformation activation energy QHW of the unreinforced alloy and the composite were 64.69 and 173.65 kJ/mol in the temperature range of 350-450?,whereas those were 276.52 and 449.02 kJ/mol in the range of 450-500?.The QHW of the composite was higher than that of the unreinforced alloy.The physical constitutive model based on MTS(mechanical threshold tress)was established to describe the softening behavior during hot working.The results showed that both the short-range and long-range barriers increased simultaneously because of the addition of SiC particles.