| As a kind of cast aluminum alloy,hypereutectic Al-Si alloys have been widely applied in aerospace and automotive fields,due to their better castability and weldability,high wear resistance,and better corrosion resistance.But a large number of coarse primary silicon phases exist in these alloys owing to lower cooling rate under conventional casting conditions,resulting in spliting of A1 matrix and deterioration of alloy properties.Thus,the application field for these alloys is restricted.Admittedly,high current pulsed electron beam(HCPEB)has been a new technology for surface modification in recent decades.Rapid heating and cooling processes induced by HCPEB trigger the nonequilibrium solidification on material surface,thus achieving a surface modification layer that possesses excellent properties.However,abundant microcracks and crater structures are prone to form on the material surface under the condition of pulsed electron beam treatment,severely deteriorating corrosion resistance of material surface.The existence of these structures hinders the development of HCPEB technology,becoming two main problems.Rare earth element,as a useful one,could refine material microstructures and eliminate microcracks or micropores.Additionally,the element diffusion effect induced by HCPEB results in uniform distribution of rare earth element,as a unattainable modification effect in other conventional treatment methods.Therefore,in the present work,surface modification of hypereutectic Al-Si alloys(Al-17.5Si alloys)by HCPEB and rare earth Nd is first reported with the main purpose of eliminating of microcracks and crater structures and refining of coarse primary silicon phase.And the evolution of surface microstructure for alloys after HCPEB treatment is investigated in detail,and the effect of pulsed number on the hardness and corrosion resistance for alloy surfaces is also carried out.Compound modification of primary silicon phases is obtained by rapid cooling effect of HCPEB and constitutional supercooling of rare earth at the crystallization frontier,and the optimum effect is achieved.Firstly,surface microstructure for alloys without Nd and with Nd after HCPEB treatment is systematically studied by means of SEM,XRD,EPMA and OM analyses.The main conclusions are as following:(1)Primary silicon phase becomes a halo microstructure resembling a bull's eyes on the Al-17.5Si alloy surface under the condition of pulsed electron beam.The size of this phase changes from 20?100?m of initial sample to 70nm of 25 pulse-treated sample,decreasing by more than three orders of magnitude.Nano Al cellular structures with 200?300nm in size and tiny eutectic structures are formed nearby the edge of halo microstructure;(2)Al,Si and Nd elements are uniformly distributed on the HCPEB-treated Al-17.5Si-0.3Nd alloy surface,and microcracks and crater structures decrease significantly and even disappear with increasing pulsed numbers.The crack density decreases sharply from 0.0669mm-1 of 5 pulse-sample to 0.00687mm-1 of 50 pulse-sample,and the crater density also decreases from 3.82mm-2 of 5 pulse-sample to 0.64mm-2 of 50 pulse-sample.The site-fixed analysis results reveal that microcrack increases first and then decreases in length with increasing pulsed numbers,and finally disappears at 50 pulses by adding Nd element.Microcrack elimination is attributed to a reduced stress concentration in the primary silicon phase induced by Nd element during the cooling process of HCPEB treatment.As a result,the microcracks decrease in length and quantity and even disappear.Crater elimination is also attributed to decreasing the surface roughness and casting flaws and eliminating the impurities in the original microstructure of Al-17.5Si alloy after adding Nd element.Large-sized crater structure finally sharply decreases and even disappears under the polishing effect of pulsed electron beam.Microcrack elimination contributes to elimination of crater structure;(3)The residual stress value of alloy surface basically increases with increasing pulsed numbers,indicating tensile stress state;(4)The diffraction peaks of Al and Si elements broaden and shift on the Al-17.5Si and Al-17.5Si-0.3Nd alloy surfaces after HCPEB treatment.The phenomena become more remarkable with increasing pulsed numbers.All the diffraction peaks firstly move to high angles and then to low angles.The largest shift angle appears in the 5 pulse-sample.The interplanar spacing of Al(220)and Si(311)and the crystal lattice of Al decrease firstly and then increase as pulsed numbers increase;(5)The compact and integrated remelted layer is formed on the alloy top surface after HCPEB treatment.For Al-17.5Si alloy,the average thickness of remelted layer increases from 5.4?m of 5 pulse-sample to 8.2?m of 50 pulse-sample;For Al-17.5Si-0.3Nd alloy,this value also increases from 4.1?m of 5 pulse-sample to 6.6?m of 50 pulse-sample.The thickness of remelted layer for Al-17.5Si alloy is decreased at the same pulsed number by Nd element.Secondly,the metastable structures formed on the HCPEB-treated alloy surfaces are investigated in detail by TEM analysis.The results are summarized as following:(1)High density dislocation tangle is generated in Al matrix on the Al-17.5Si alloy surface after 15 pulse treatment;(2)Amorphous alumina is formed on the surface layer of Al-17.5Si alloy after 25 pulse treatment,and amorphous crystallization occurs on the alloy surface.Nano A1 and Si phases with size range of 5?20nm are formed on the amorphous alumina layer;(3)Nano primary Si phase with size of 100-200nm is generated and uniformly distributed on the surface layer of Al-17.5Si-0.3Nd alloy after 25 pulse treatment;(4)Nano eutectic Si phase with size of 5-50nm is formed on the surface layer of Al-17.5Si-0.3Nd alloy after 25 pulse treatment,and is located in transitional zone between Al-rich and Si-rich phases;(5)Nano A1 cellular structure with size range of 100?200nm and tiny nano silicon phase with size of 5-20nm are generated on the surface layer of Al-17.5Si-0.3Nd alloy after 25 pulse treatment.Nano Al cellular structure is uniformly distributed in Al matrix,and tiny nano silicon phase is located in the grain boundary or subgrain boundary of Al.Finally,the hardness and corrosion resistance of alloy surfaces before and after HCPEB treatment are carried out by the vickers hardness tester and electrochemical workstation.The main conclusions are as following:(1)The microhardness of Al matrix on the alloy surfaces after HCPEB treatment is significantly increased.The microhardness of Al matrix for 50 pulse-treated Al-17.5Si and 50 pulse-treated Al-17.5Si-0.3Nd alloys is improved by factors of about 2.2 and 1.7,respectively,compared to their initial samples.The surface microhardness of Al matrix on the Al-17.5Si alloy is decreased at the same pulsed number after adding Nd element;(2)After HCPEB treatment,the microhardness of halo microstructure presents a gradient distribution from high hardness of centre to low hardness of edge.This gradient distribution gradually disappears with increasing pulsed numbers.Particularly,central hardness of halo microstructure after adding Nd element is higher than that of this microstructure for alloys without Nd at the same pulsed number treatment(not including 50-pulsed sample),but this value is conversely lower than the central hardness of halo for alloys without Nd at 50 pulses;(3)The microhardness of remelted layer for alloys without Nd and with Nd is obviously increased after 50 pulses,and this microhardness value is 1584.8MPa and 1106.8MPa,respectively.The microhardness of remelted layer is decreased at the same pulsed number treatment with Nd adding;(4)After adding Nd element,the corrosion resistance of Al-17.5Si alloy surface is significantly improved after HCPEB treatment.The corrosion current density decreases by two orders of magnitude from 3.954×1 0-6?A/cm2 of initial sample to 5.888×10-8?A/cm2 of 50 pulse-sample.The improvement of corrosion resistance is attributed to grain refining of alloy surface and eliminating of microcracks and large-sized crater structures.In short,microcracks and crater structures easy to form during HCPEB process are eliminated for hypereutectic Al-17.5Si alloy surfaces due to Nd element adding,and the size of primary silicon phases is refined from tens of micrometers to tens to hundreds of nanometers owing to rapid cooling effect of HCPEB,which induce improved corrosion resistance of HCPEB-treated sample.It is effective to solve two problems existing in HCPEB surface modification process and extend the application of this technology in industry. |
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