Study On The Grain Refinement Of Si Phase And The Interfacial Characteristics In Si-high Al Alloys

Al-Si alloy,an important foundry alloy,has attracted extensive investigation for excellent properties,such as high strengh-to-weight ratio and good casting characteristic.The morphology of primary silicon is a key factor to the application of hypereutectic Al-Si alloys.There are many literatures on hypereutectic Al-Si alloys which silicon content is below 30 wt.%,but few papers focus on Si-high Al alloys,especially the alloys with more than 50 wt.%Si content in spite of their special performance.In addition,Si-high Al alloys have potential application for their good abrasion,corrosion resistance and hardness.Up to now,the refinement effect of AlP on primary Si is believed to be due to low mismatch between the two phases,but the knowledge of interfacial bonding between them,which is useful to understand the refinement mechanism,is absent.The experimental and computational methods were used in this dissertation. The refinement of Si-high Al alloys were systemtically studied using optical microscope(OM),electron probe micro-analyzer(EPMA),scanning electron microscope(SEM),differential scanning calorimeter(DSC) and high temperature diffractometer etc.The calculation software,Materials Studio,was used for the theoretical analysis.The refinement of Si-high Al alloys and the interfacial bonding in refined alloys were systemtically investigated.The major research efforts of the present study are as follows:Si-high Al alloys with Si content ranging from 30 wt.%to 70 wt.%can be effectively refined using Si-20P master alloy.The average size of primary silicon particles transforms from 2-4 mm to about 30-50μm.The minimal holding temperature and adding phosphorus amount for good refinement increase abruptly when Si content reaches 50%in the melt.The minimal overheating degreeΔT_min (ΔT_min is the difference between the minimal overheating temperature T_min and the liquidus temperature T_L) for good refinement is about 260 K when silicon content is above 50 wt.%in the melt.The X-ray diffraction data of the Al-50Si melt demonstrate that structural change occurs when the melting temperature varies from 1373 K to 1573 K.Furthermore,the reason of the abrupt temperature augment for the refinement is due to the variation of the melt structure.The short range order of the microstructure in the melt is strong at 1373 K,and weak at 1573 K.Silicon atoms have intense interaction force at the low overheating degree(60 K) in the Al-50Si melt.AlP particles and solvent phosphorus atoms have a limited refinement effect on the strong short range order of silicon,which is the reason for the formation of the plate-like primary silicon particle at a low overheating temperature.Two new refinement methods for Si-high Al alloys were presented,and Al-70Si alloy can be well refined easily now.The first method was using the newly developed Si-20P master alloy.Different silicon content Si-high Al alloys can be refined successfully using the master alloy at the critical superheating temperature. Experiential formulae about the critical superheating temperature and the adding phosphorus amount are.founded according to the experimental data.The other method was the combination of Si-20P master alloy and Al-TiO₂-C mixture powders.The primary silicon cannot be well refined at 1473 K using 1%Si-20P master alloy addition,but it can be refined at this temperature using 1%Si-20P master alloy and 2%Al-TiO₂-C mixture powders.Moreover,the refinement effect was also enhanced.Duplex nuclei of TiC and AlP particles were found in the center of the primary silicon in despite of their large mismatch.The reaction of Si-20P master alloy in the Al-Si melt was investigated.Si and P atoms diffuse into the melt when the Si-20P master alloy is added to the Al-50Si melt and AlP compound forms.The coarse AlP sticks becomes needle-like or granular in the high temperature melt.The morphologies of AlP compound play an important role in the refinement of primary silicon.Undoubtedly,the fine and dispersed AlP is favorable to the refinement efficiency.AlP compound can react with water in the air,so the actual AlP compound is difficult to be observed in the refined primary silicon.The difference between oxidized nuclei and actual AlP nuclei was investigated in this paper.The actual AlP nuclei have the same color as the primary silicon in backscattered electron images, which has not been observed until now.The oxidized AlP nuclei appear as a black Al-P-O compound,as have been observed in the past experiments.The differential scanning calorimeter results show that the precipitation temperature of primary silicon increases after adding sufficient phosphorus to the melt.The growth and the morphology of primary silicon were also investigated in this paper,and the primary silicon with a kind of coralloid morphology which is similar to modified eutectic silicon has been found.The growth mode of primary silicon changed at certain cooling rate with a little phosphorus in the melt.The surface energies of 2-6 layers of AlP(100),(110) and(111) surface were calculated.AlP(100),(110) and(111) surfaces show bulk properties when they reach 3,4,5-layer,respectively.The surface energy of(100) is the biggest and that of(110) is the least among them.The electron density that exposed to the vacuum decreased after the relaxation.The electron density at P atomic position decreased while that at Al atomic position increased.The height of DOS peak at the Fermi level decreased after the relaxation,which shows more steady.The geometry structure,the electron structure and bond characteristic are all varied during the relaxed process.Some electrons go to the vacuum,therefore,the surface DOS varies,and some new surface states occur.To further explore the electronic properties of AlP surface,I studied DOS of AlP(100),(110),(111) surface.DOS of AlP surface is different from the bulk AlP crystal because the DOS peaks become narrower and higher.Different AlP surfaces have different DOS peaks at Fermi level,which contributes to the variation of structure stability.The decrease of coordination number of atom on the surface contributes to metallic characteristics.The DOS peaks of Al atoms on the outmost layer are high at the Fermi level,and decrease sharply on the second and third layers.While the DOS curves of P atoms on different layers show the similar value at the Fermi level,which indicates that the superficial Al atoms are crucial to the surface structure.AlP(111) surface was choosed as an example to explore its structural stabilities.AlP(111) surface free energy is a function of the systematic potential.The Al-terminated surface is more stable than the P-terminated surface at low Al potential.The P-terminated AlP(111) surface is thermodynamically more favorable in most of the range of chemical potential ofΔμ_Al(Δμ_Al=μ_Al-μ_Al (bulk)).The atomic structure,bonding,and ideal work of adhesion(W_ad^ideal) of the interface formed between AlP(100)/Si(100),AlP(110)/Si(110) and AlP(111)/Si(111) are calculated.The favourable interfaces can be deduced for the reason that adhesive interface energies(W_ad) are different,which cannot be obtained from the traditional mismatch theory.The results show that the main bonding between AlP and Si is covalent P-Si or Al-Si bond,accompanying some ionic characteristic. Un-polar and polar AlP surfaces have different contribution to the AlP/Si interface. The values of DOS at Fermi level are very small when the bonding is between un-polar AlP surface and Si surface.The DOS curves often have sharp peak when the bonding is between polar AlP surface and Si surface.According to traditional disregistry theory,the favorable nucleation effect of AlP is attributed to the crystal similarity between AlP and Si crystals,but the position of Si and AlP is uncertain. The movement of Si atoms at lateral direction occurs at interface during relaxation process in this work.Afterward,Si atoms are not on the top of Al or P atoms,but on the bridge sites between Al and P atoms.The adsorption of Si atoms on the AlP surface was studied using first-principles.The bonding characteristic between Si and AlP surface,such as adsorption position,adsorption energy,bond type,bond length,DOS,charge density distribution and difference charge density distribution were investigated, which are very important to understand the initial stages of heterogenous nucleaion. The adsorption of Si atoms on the AlP surface is an exothermic reaction.Si-P bonds are sometimes double bond or triple bond while the Si-Al bonds are always single bond.The adsorption energy of Si-P bond is bigger than that of Si-Al bond. The Al-P bonds of exterior layer are weakened after the adsorption of Si on the AlP surface.In all,the superficial P atoms have strong effect on adsorptive Si atoms.Si/Al interface and TiC/AlP interface were also investigated in this dissertation.Two interface geometries,namely[112]_Al//[110]_Si and [110]_Al//[103]_Si were investigated.It is found that the main bonding between Al and Si is covalent Al-Si or P-Si bond,accompanying some ionic characteristic. Moreover,adhesion energies(W_ad) of several types of interfaces were calculated.It is a endothermal reaction to form TiC(100)/AlP(100) interface or TiC(110)/AlP(110) interface,which is deduced from the interfacial adhesion energy.Therefore,the duplex nucleation of TiC/AlP must be formed in the high temperatue melt.The exothermic reaction of in situ TiC provides the necessary formation energy of the duplex nuclei.