| The Al-Sr master alloy modifies the eutectic Si in the hypoeutectic Al-Si cast alloy（Si content 10-12 wt.%）to gain fine fibrous morphology.This improves the thermal conductivity and mechanical properties of the alloy,which can be used as the raw material of bolt-type radiators.However,residual B makes the Al-Si alloys modification level below Class 5.Because the mechanism of this B-Sr poisoning effect has not yet been clarified,many"trials and errors"are inefficient.Therefore,there is an urgent need to elucidate the B-Sr poisoning mechanism,which is helpful for the development of an effective modification process.This article takes eutectic Si in Al-10Si-xSr-yB（x+y≤0.15）system as the research object,focusing on the B-Sr poisoning effect and La-B-Sr non-poisoning phenomenon.From the experiment,according to changes of the length and aspect ratio of the eutectic Si with the content of trace elements B and Sr to explore the range of B-Sr poisoning components and the influence of La on this range.Electron Backscatter Diffraction is used to detect the phases of the B-Sr poisoning effect before and after adding La.X-ray Powder Diffraction measures the twin density of eutectic Si.Theoretically,CALPHAD（CALculation of PHAse Diagram）method is used to explain the formation conditions of B-Sr poisoning products.Get the following conclusions:（1）When the Sr/B mass ratio is equal or less than 1.5,B-Sr poisoning effect occurs,and the eutectic Si returns to the lamellar one with a length of 8.2-14.8μm and an aspect ratio of 6.1-8.8.The formation of SrB6,accompanying Sr-containing fluoride salt inclusions,causes Sr to lose its effectiveness.Meanwhile,growth steps appear on the surface of the eutectic Si and the eutectic Si achieves growth by twins.（2）The area of modification components is raised to 3 times by 1000 ppm La addition.When Sr is no less than 0.015 wt.%,the critical Sr/B ratio of B-Sr poisoning effect decreases from 1.5 to 0.4 by La addition.When the effective La/B mass ratio is2.5-5,the eutectic Si has a length of 1.3μm with an aspect ratio of 2.4-2.5,and the average twin spacing in Si fibers is 26.9 nm.Meantime,these high-density twins limit the growth of eutectic Si and cause the Si to grow into fine fibers.The production of LaB6 and the disappearance of SrB6 indicate that La reacts with B and releases Sr.The appearance of La and the disappearance of Sr in the fluoride salt inclusions reflect the protective effect of La on Sr.（3）After La prevents B-Sr poisoning effect,the comprehensive performances of Al-Si cast alloys including thermal conductivity and mechanical properties are improved.The thermal conductivity of the Al-10Si-B-Sr alloy at room temperature is increased from 170.2 to 193.3 W/（m·K）by La addition.The tensile strength is increased from 172.8 to 201.4 MPa with an enhancement of the elongation from 2.28 to 6.0%by La addition.The reason for the improvement is the morphology of eutectic Si changes from lamella to fiber and the rare earth La reduces the porosity of the cast alloy. |
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