Fabrication and Characterisation of Microstructure and Properties of Ultrafine-grained AZ61 Magnesium Alloy Strengthened with Ti Dispersions

Author:Yu Huan

Supervisor:hu lian xi


Degree Year:2018





Magnesium alloys are the lightest of all metals used as the basis for constructional alloys.Magnesium alloys have been widely applied in aerospace,a utomotive and railway industries,and electronics.However,the low absolute strength of magnesium and magnesium alloy is the fatal and urgent limit for further wide application.With the development of national defense and military industry,aerospace and transportation,the demands for lighter and higher strength materials are pressing,which accelerates the developing ultrahigh strength magnesium alloys.Most notably,magnesium alloys possess the high Hall-Petch coefficient being several times of metals with body-centered cubic and face-centered cubic structure,presenting quite attractive grain boundary strengthening.However,when the deformation temperature is above 393K,the stress of magnesium alloys would decrease evidently with the temperature increasing.More seriously,the microstructure of ultrafine grained would aggravate the decreasing of strength at elevated temperature.Therefore,the primary problem of ultrafine grained magnesium alloys is poor thermal stability,which limits the further application.Accordingly,ultrafine grained AZ61 magnesium alloys dispersed with Ti particulates were prepared by mechanical milling and plastic deformation.The AZ61Mg-Ti is strengthened by fine-grained strengthening,precipitation strengthening and solid solution strengthening.During hot deformation,the dispersions would hinder grain boundary slide and migration.Therefore,the strength of ultrafine grain AZ61Mg-Ti at elevated temperature was improved.In this paper,the nanocrystalline Ti/AZ61 magnesium matrix composite powders were prepared by mechanical milling.The effect of mechanical milling parameters on microstructure evolution including grain refining,cracking and dispersing of Ti particulates was studied.The appropriate mechanical milling processing was established.After mechanical milling,the average grain size of magnesium is approximately 46nm,the particulate size is about 274nm,and all the Mg17Al122 phase is resolved.The hardness of milled composite powders is 147HV.Comparing the microstructure evolution,the effect of Ti addition on the microstructure,including Mg grain and microstrain,solid solution status,morphology evolution,second-phase transformation and distribution,was analyzed.Nanocrystalline AZ61-10at.%Ti composite and AZ61 alloy powders,prepared by mechanical milling,were annealed in the temperature range of 573-723K for various durations.Based on microstructural evolution,the mechanism for the precipitating of Ti and Al from magnesium matrix was analyzed.The precipitated phase is Ti3Al phase with particulate size of10nm.In addition,the kinetics of magnesium grain growth,at temperature between 573 K and 723 K,is well interpreted by generalized parabolic grain growth model,Dn-D0n kt=.The grain growth exponent(n)and the activation energy(Eg)for nanocrystalline AZ61-10at.%Ti composite were calculated to be 8 and 134 kJ/mol,while the values of nanocrystalline AZ61 are 6 and 118kJ/mol.In addition,influence of temperature and time on hardness was investigated.After annealing treatment at 723 K for 600min,the hardness of annealed Ti/AZ61 composite decreases from 147HV to 130 HV.Based on the grain growth pattern of nanocrystalline AZ61-10at.%Ti composite powders,AZ61-10at.%Ti composite bulks were prepared by vacuum hot pressing.The effect of processing parameters on microstructure and properties is investigated.After hot pressing at 573K,the relative density is 98.5%and the average grain size is about 82nm.The hardness and yield strength are 136HV and 440MPa respectively.Annealing treatment is studied to improve strength and plasticity.After annealing treatment at 623K for 40h,the yield strength and ultimate strength are 493MPa and556MPa respectively.And the compressive strain to failure is 5.6%increasing by65%.The deformation behavior of nanocrystalline AZ61-10at.%Ti composite at elevated temperature is studied.The effect of temperature and strain rate on stress-strain curves is analyzed.It is certified that dynamic recrystallization and grain growth coexist during hot deformation processing.Ultrafine grained AZ61-10at.%Ti composite was prepared by hot extrusion.The extrusion ratio and die temperature were confirmed by numerical simulation.The effect of material temperature on microstructure and properties is investigated.When the temperature is 523K,the grain of extruded material is uniform and the average grain size is about 180nm.After annealing treatment at 423K for 40h,the hardness,yield strength,ultimate strength and fracture to failure are 152HV,606MPa,698MPa and 12.0%.The mainly strengthening mechanisms are fine-grained strengthening and precipitation strengthening which accout for 49%and 48%respectively.The yield strengths at 423K,473K and 523K are 520MPa,380MPa and 162MPa.During hot deformation,the submicron sized titanium particulates,together with nanometer sized ones,would hinder grain rotation,grain boundary slide and migration.Based on annealing treatment,it is certified that ultrafine grained AZ61-10at.%Ti composite is of high thermal stability.