Effect Of La And TiB₂ Content On Microstructure And Corrosion Resistance Of TiB_2?p?/ZL205 Composites

Particle-reinforced aluminum matrix composites are gradually applied in various important fields due to their high specific strength,high specific stiffness,low thermal expansion coefficient and good thermal stability,however,the microstructure of composites will become heterogeneous due to the segregation of particles which might change the properties of composites even lead the cracking of them.Not only the mechanical properties but also other properties,such as corrosion resistance,are affected by the segregation of particles and their physical and chemical properties.Therefore,this study took TiB_2?p?/ZL205 composite as the research subject,and explored the influence of TiB₂ particle content and La content on the microstructure and properties of the composite by means of thermal analysis,SEM,XRD,electrochemical test and so on,no matter the composite was on in as-cast or after T6 heat treatment.According to the results of thermal analysis,the precipitation temperature of?-Al increased obviously,and the solidification rate of the composite slowed down,meanwhile both the precipitation temperature and the quantity of?phase also decreased with the increase of TiB₂ particles.However,when the content of TiB₂ particles was 10%,the solidification speed of the composite increased on the contrary.After adding different amounts of La to the 10%TiB_2?p?/ZL205 composite,the results of thermal analysis showed that the solidification rate of the composite and the precipitation of?phase reduced gradually with the increase of La content,and the Al_2.12La_0.88 phase appeared in the solidification process.Moreover,when the addition of La increased to 0.4%,a new phase has precipitated,which is speculated to be Al_xCu_yLa_z ternary rare earth phase.The SEM images of composite indicated that TiB₂ particles were mainly aggregated at the grain boundary and formed particle aggregates with?phase and other precipitated phase,and the aggregation of TiB₂ became more serious with the increase of TiB₂.After T6 heat treatment,the phenomenon of particle skewness did not change obviously.With the addition of La into 10%TiB_2?p?/ZL205 composite,the aggregation of TiB₂ was improved,and the size of particle aggregates also became smaller in the meantime.However,the dispersed TiB₂particles agglomerated again when La was added to 0.4%.In T6 heat treated10%TiB_2?p?/ZL205 composite with La addition,it was found that the particles were more diffused with the increase of La and did not agglomerate again even if the La content is added to 0.6%.The SEM images of composites immersed in 3.5%NaCl for 60 days showed that there are both pitting and intergranular corrosion occurring on the composites,which were mainly corrosion type.In the as-cast state,the shape of intergranular corrosion is crack-like,while the shape would become porous and both pitting and intergranular corrosion would decrease in the T6 state.Therefore,the corrosion resistance of composites were improved by T6 heat treatment.No matter in as-cast or T6 state,however,the corrosion status were more severe with the increase of TiB₂ particles.For 10%TiB_2?p?/ZL205 composites with La addition,La narrowed the range of poor Cu area so as to reduce the amount of intergranular corrosion indirectly.And the particles became more dispersed because of the effect of La,which hindered the spread of corrosion in the matrix,thereby improving the corrosion resistance of composites.Rockwell hardness test was carried out on all composite,and it was found that the hardness of the composites was improved with the increase of particles in both as-cast state and T6 state.And the more TiB₂ particles in the composite,the less effect of improving the hardness of composites in T6 state.The hardness of ZL205 alloy without particles is 50.92in T6 state,which was 118.92%more than the hardness of that in as-cast state in which the hardness was 23.25.When the particles content rose to 10%,the hardness of composite was66.25 in T6 state,which was only a 42.32%increase compared to the hardness?46.55?of that in as-cast state.However,for the composites with La addition,the hardness of the composites in the as-cast and T6 states was only improved slightly with the increase of La,and the hardness of composites was increased by approximately 43%generally after T6 heat treatment.The electrochemical teat results of composites showed that the i_corr value was raised with the increase of particles,and the i_corr value of ZL205 alloy without TiB₂ was1.504E^-6A/cm² while that of composite with 10%TiB₂ was 1.624 E^-5A/cm² and owned an order of magnitude larger.This meant that the corrosion rate of composites is high and the composites had poor corrosion resistance.For the 10%TiB_2?p?/ZL205 composites with La addition,it was found that the i_corr value became smaller due to the increase of La content.When the La content was raised to 0.6%,the i_corr value decreased from 1.624 E^-5 to 4.684E^-6,which meant that more La addition led to better corrosion resistance of composites.And the i_corr value of composites in T6 state was smaller than that in as-cast state with the same TiB₂content or La content,therefore T6 heat treatment could also improve the corrosion resistance of composites.