Laws And Mechanisms For The Effects Of Metallic Glass Inoculant On The Microstructures, Strength And Ductility Of The Al-Cu Alloys

With the progress in scientific technology and the demands of environmentalprotection and saving natural resources, it becomes more and more urgent to developlightweight material. Hence, developing aluminum alloys with high strength andductility becomes a research focus. Grain refinement through addition of inoculant isa common, simple and effective way to obtain an equiaxed grain structure, gaining asimultaneous improvement in mechanical properties of Al alloys. Currently, thecommon-used inoculant is mostly crystal morphology, such as Al-Ti-B and Al-Ti-Cmaster alloys. However, metallic glass used for inoculant of Al alloys has beenscarcely reported. The laws and mechanisms for the effects of metallic glass inoculanton microstructures, strength and ductility of Al-Cu alloys is also unknown. Hence,investigation of the laws and mechanisms for effects of metallic glass inoculant on themicrostructures, strength and ductility of Al-Cu alloys can provide a theoretical basisand the application foundation for developing new kinds of inoculant, enriching theinoculation theory as well as broadening the application region of metallic glass.This thesis mainly investigates the laws and mechanisms for effects of metallicglass inoculant on microstructures, strength and ductility of Al-Cu alloys. The effectof metallic glass inoculant on-Al grains in Al-Cu alloys has been revealed;simultaneously the fade mechanism of metallic glass inoculant as well as the effect ofmetallic glass inoculant on strength and ductility of the Al-Cu alloys at both room andelevated temperatures has been revealed.The main results are as following:1) It is found that the refinement ability of-Al grains in Al-Cu alloys inoculatedis as following: ZrCuAlNi metallc glass> ZrCuAlNiLa metallc glass> NiNbTimetallc glass> FeBSi metallc glass, with the smallest-Al grain size of about35m,38m,39m,41m, respectively; the effect inoculation time of the four metallic glassinoculants follows that: NiNbTi metallc glass=FeBSi metallc glass> ZrCuAlNiLametallc glass> ZrCuAlNi metallc glass,with the effect inoculation time of30min,30min,10min and3min, respectively. In particular, as compared with ZrCuAlNi metallic glass, the effective inoculation time of the inoculated alloys by ZrCuAlNiLaincreases from3min to10min.2) Compared with the uninoculated alloys, the θ′precipitates in the inoculatedAl-Cu alloys by metallic glasses have smaller size, increasing number density andmore homogeneous distribution. It is because that-Al grains of the inoculated Al-Cualloys are refined obviously, associated with increased grain boundary length, leadingto a more homogeneous distribution of the second phase during solidification.Moreover, because of the smaller grain size, the diffuse distance of Cu atoms fromgrain boundaries to grain interiors becomes short, which is benefit of Cu atoms todistribute homogeneously. Hence, the activity energy of θ′precipitates decreases, andit is easier for more θ′precipitates to nucleate, precipitate and growth simultaneously,which in turn causes a lack of Cu atoms and inhibit the growth of θ′precipitates.Therefore, the θ′precipitates have smaller diameter and thickness in the inoculatedAl-Cu alloys.3) The strength and ductility of the inoculated Al-Cu alloys at room temperatureare higher than those of the uninoculated alloys, and the law of the enhanced strengthand ductility is accord with the law of the refined grain size of the inoculated Al-Cualloy. The simultaneous improvement in strength and ductility of the inoculated Al-Cualloys is as following: the inoculated Al-Cu alloys by ZrCuAlNiLa metallc glass> theinoculated Al-Cu alloys by ZrCuAlNiLa metallc glass> the inoculated Al-Cu alloysby NiNbTi metallc glass> the inoculated Al-Cu alloys by FeBSi metallc glass. Theoptimal strength and ductility of the inoculated Al-Cu alloys are536MPa and16.3%,569MPa and11.1%,531MPa and14.0%,520MPa and14.2%, which is improved by14.0%and123%,19.7%and58.5%,12.9%and100%,10.6%and102%, respectively,as compared to uninoculated alloys.These enhanced properties are suggested to be mainly induced by both thesmaller-Al grains and nano-sized θ′precipitates.4) The refinement mechanism of-Al grains in the inoculated Al-Cu alloys bythese four metallic glasses has been revealed, i.e., when metallic glasses are addedinto the Al-Cu melt, Zr2Cu, ZrCu(B2), Fe2B and NiTi phases crystallized and they canact as effective heterogeneous nucleation sites of-Al grains during solidification,leading to significant grain refinement. The criteria for metallic glass acting as theinoculant of Al-Cu alloys has been proposed, i.e., the formed crystallization phasesshould act as effective heterogeneous nucleation sites for-Al grains duringsolidification. 5) The effect inoculation time of FeBSi and NiNbTi metallic glasses is longerthan ZrCuAlNi metallic glass. Due to the melt point of the crystallization Fe2B andNiTi phases, acting as the effective heterogeneous nucleation sites of-Al grains, ismuch higher than Zr2Cu and ZrCu(B2) phases. The effective inoculation time of theinoculated alloys by ZrCuAlNiLa increases from3min to10min, as La can facilitatethe formation and stabilization of ZrCu compound phases that can act asheterogeneous nucleation sites for-Al phase.6) The law of developing metallic glass inoculant having long-term effect forAl-Cu alloys is to improve the stability of the crystallization phase that can act asheterogeneous nucleation sites for-Al phase in the Al-Cu melt. Two methods can beadopted:(1) improving the melt point of crystallization phase which can act asheterogeneous nucleation sites for-Al phase;(2) adding some elements to metallicglass, in order to increase the stability of the crystallization phase which can act asheterogeneous nucleation sites of-Al phase.7) It has been revealed that the deformation mechanisms at elevated temperaturesof the uninoculated and inoculated Al-Cu alloys by ZrCuAlNi and ZrCuAlNiLa arecontrolled by dislocation climbing. Under elevated temperatures from433K to493Kwith strain rates increasing from10-4s-1to10-1s-1, θ′precipitates in the inoculatedAl-Cu alloys by metallic glasses have smaller size, higher number density and morehomogeneous distribution than the uninoculated alloys, which is more helpful forimpeding the dislocation movement. Hence, thermal activation energies of theinoculated Al-Cu alloys by ZrCuAlNi and ZrCuAlNiLa are much higher thanuninoculated alloys, resulted in much higher strength and ductility at elevatedtemperatures of the inoculated Al-Cu alloy, as compared to uninoculated alloys.8) It is proposed that FeBSi metallic glass is an effective inoculant for Al-Cualloys with long inoculation time, excellent refining effect and lower cost.