Research On Damping And Microplastic Behavior Of Pure Mg And Mg Alloys

Hot extrusion, equal channel angular pressing （ECAP） and subsequentannealing treatment were performed on pure Mg to modify the microstructure. Theeffects of grain size and texture of pure Mg on microplastic deformation, dampingcapacity and the relationship between them were studied in detail. Two kinds ofalloy elements were added to Mg to study the microplasticity and damping capacityof Mg alloys. The microstructures characteristics of pure Mg and Mg alloys wereobserved by OM and TEM. The texture evolutions were analyzed by neutrondiffraction and EBSD. The strain dependence of damping capacities were studied byDMA. The developments of anelastic strain, friction stress and secant elasticmodulus, etc. in microstrain region were studied by cyclic loading-unloading tests.This paper revealed the damping and microplastic behaviors of pure Mg and Mgalloys, established a good foundation to develop high damping capacity, high fatigueand high dimensional stability Mg based materials.The average grain size of pure Mg after extrusion was about68μm. Withincreasing the ECAP passes, the degree of recrystallization was increased, whichresulted in the refinement of grain size. The grain size of pure Mg after ECAPprocessing for4passes at the temperature of250°C was refined to6μm. Theas-extruded Mg exhibits a texture with {0001}1010nearly parallel to theextrusion direction （ED）. With increasing the ECAP passes, the basal poles rotatedaround transverse direction （TD） and ED axis and formed a maximum componentlocating at about40ofrom normal direction （ND） and65ofrom TD. The texturecomponent with basal planes parallel to ED becomes much weaker. The textureevolution after ECAP led to a increase of the Schmid factor for basal slip towardsED. The grain size was gradually increased, but the texture almost kept stable afterannealing treatments.The microplasticity and damping related to plasticity of pure Mg have the samephysical mechanism and can be explain by the same dislocation model. The twostages of plastic damping and microplasticity correspond to each other. The firststage relates to the plastic strain below2×10^-4(the total strain of about8-9×10^-4),the basal dislocations in favorable oriented grains break away from strong pinningpoints and slide on the basal planes, which results in the larger volume activationsbut smaller hardening exponents; the microplastic deformation process enters intothe second region when the plastic strain is above2×10^-4, the tangle and piled-upof dislocations lead to the decrease of mobile dislocations, the materials are characterized by the larger hardening exponents but smaller volume activations.When the strain is smaller than the first critical strain amplitude （about1×10-4）,the materials almost deform in an elastic manner. At this time the dislocationsreversibly motion between the weak pinning points, the loading-unloading curvesalmost coincide with the elastic line, and the damping capacityQ10is amplitudeindependent; the dislocations unpin from weak pinning points with the increase ofstrain amplitude, as a result of the rapid increase of anelastic strain and decrease ofsecant elastic modulus. At this time the damping capacityQ1hincreases graduallywith the increase of amplitude. The first two anelastic stages can be explained bydislocation theory developed by Granato and Lücke （G-L）; however, a deviationfrom the straight line occurs beyond the second strain amplitude （about5×10-4）, thebasal dislocations break away from strong pinning points, which results in theoccurrence of microplastic deformation. In this case the damping capacityQ_p^-1rapidly increases; due to the tangle and piled-up of dislocations above the thirdstrain amplitude (about9×10^-4), the friction stress gradually increases but theincrease speed of modulus defect slows down. The last two microplasticdeformation processes should be explained in terms of the microplastic dislocationmodel. With increasing the grain size or the Schmid factor of basal slip, the dampingcapacity, anelastic strain and elastic modulus increase defect, but the secant elasticmodulus, friction and back stresses decrease.Two kinds of Mg alloys which containing elements that possess differentsolubilities in Mg were designed and fabricated. The effects of alloy elements ondamping and microplasyicity of Mg alloys were studied with the comparison of pureMg. The distance of weak pinning points on dislocations was reduced when Alelement was added to Mg, so Mg-1Al alloy exhibited the largest secant elasticmodulus and friction stress but the lowest damping capacity at low strain region.However, the addition of Si did not result in the derease of damping capacitybecause it was unable to dissolve in Mg. But the second phase Mg₂Si led to themarked decrease of grain size of as-cast Mg-1Si alloy, which blocked the motion ofdislocations and so resulted in the derease of damping capacity at high strain region.While the secant elastic modulus, friction stress and back stress were increased atthe same time.It is supposed that the area swept by a dislocation pinned by strong pinningpoints is a circle at the critical strain amplitude to generate new dislocation. Theaverage mobile dislocation densities and distances between pinning points wereestimated according to the quantitative relationship between the dissipated energy by dislocation motion against the friction stress on the slip plane and dampingcapacity. The average mobile dislocation densities of pure Mg and Mg alloys afterdeformation are in the order of10¹²m^-2, and reduced by one or two orders aftershort time annealing treatments.