Study On High Temperature Creep Behavior Of Magnesium Matrix Composites Reinforced By Magnesium Borate Whisker

Magnesium alloys are widely used in aerospace, automotive, electronics andother industrial fields because of their high specific strength and stiffness. However,the low heat resistant properties of magnesium alloys limit their applications in hightemperature environment. The effective way to improve the high temperature creepresistance of magnesium alloys is magnesium matrix composites reinforced by highstrength, high melting point, and high elastic modulus of fiber, particle or whisker.At present, research on creep behavior of magnesium matrix composites havecarried out extensively, but few study on high temperature creep behavior ofMg₂B₂BO₅whisker reinforced AZ31B matrix composites.In this dissertation, creep tests of magnesium matrix composites prepared byliquid press forming were conducted under constant tensile stresses of50MPa and60MPa, and constant temperatures of200and573K. Through analysis of tensilecreep curve and creep fracture topography, creep properties and mechanism ofmagnesium matrix composites have been obtained. Main research results are asfollows:（1） The steady-state creep stage accounts for the whole curve for70%-80%,while the accelerated creep stage is only around1%. By analysis of the creep curve,when temperature is473K or573K, the initial variable and the steady state creeprate of Mg₂B₂BO₅w/AZ31B composites increase with the increase of stress; whenstress is50MPa or60MPa, the initial variable and the steady state creep rate ofMg2BB2O5w/AZ31B composites increase with the rise of temperature.（2） By calculation of the steady-state creep rate under different temperature andstress, the apparent stress exponent and apparent creep activation of Mg2B2O5w/AZ31B are obtained. When temperature is473K, the apparent stress exponent isl.72; when temperature is573K, the apparent stress exponent is3.43. When stress is50MPa, the apparent creep activation is72.98kJ/mol; when stress is60MPa, the apparent creep activation is76.85kJ/mol.（3） The true stress exponent and threshold stress value of the composite material are calculated using the standard linear extrapolation method. The true stress exponent is7. When temperature is473K, the threshold stress value is45MPa; when temperature is573K, the threshold stress value is36MPa. The threshold stress value decreases with the rise of temperature. The true creep activation of Mg2B2O5w/AZ31B magnesium matrix composite is127.4kJ/mol. It’s quite as magnesium self diffusion activation energy （135kJ/mol）. The true stress exponent and true creep activation indicate the high-temperature creep mechanism of Mg2B2O5w/AZ31B is climb of dislocations due to core diffusion.（4） When temperature is473K, the creep constitutive equation is ε=7.96×1022（σ-45/G） exp（-127400/RT）; when temperature is573K, the creep constitutive equation is ε=6.73×1022（σ-36/G）7exp（-127400/T）.