| Complex brassses possess good mechanical and wear properties and can be working under aggressive service conditions such as high load,high impact and low lubrication.Therefore,they are commonly used to manufacture the friction parts of machines,particularly the ring synchronizer in the automobile.At present,the performance of domestic complex brass materials still falls much behind foreign advanced level.The fundamental problem is that it is short of understanding about the formation and growth mechanisms of hard phases and the influence on wear mechanisms.Thus,it is lack of effective means to controll the hard phases.Mn5Si3 and(Mn,Fe)5Si3 are the significant hard phases in the complex brasses,which are the core of determining the excellent wear resistance.In this paper,the growth mechanism and morphology evolution of the hard phases Mn5Si3 and(Mn,Fe)5Si3 were studied by investigating different complex brasses.The effects of volume fraction,size and morphology of hard phases on the mechanical and wear properties were systematically analysed.The effect of trace Sr addition on the microstructure and properties of HMn64-8-5-1.5 brass was also investigated.In addition,the formation mechanism of primary and eutectic hard phase during solidification,and the precipitation mechanism of Mn5Si3 phase from the 3 matrix during isothermal heat treatment were researched by inveatigating the brasses HMn62-3-3-1 and HMn64-8-5-1.5.The effect of fine and dispersive Mn5Si3 precipitates on the mechanical and wear properties of complex brasses was also analysed.The main results can be described as follows:(1)By means of phase extraction and deep etching technologies for different complex brasses,3D morphologies of the hard phases were systematically observed and the growth mechanism was revealed combined with the analysis of crystal structure.Both the(Mn,Fe)5Si3 and Mn5Si3 phases are formed in the complex brasses with small Fe addition,which can be collectively called[Mn(Fe)]5Si3 in this paper due to the same crystal structure.The preferred growth directions of the D88-type[Mn(Fe)]5Si3 phase are<0001>and<1120>and the growth velocity in the<0001>direction is higher than that of<1120>.Thus,the crystal morphology of[Mn(Fe)]5Si3 phase will be long hexagonal prism through faceted crystal growth pattern.Two kinds of hollows,i.e.internal hollow and prism facet hollow,are formed on the coarse primary[Mn(Fe)]5Si3 prisms,which can be ascribed to the restrained solute diffusion within the prism and attachment limited kinetics on the prism facet,respectively.(2)During the solidification process of complex brasses,the primary[Mn(Fe)]sSi3 phase is nucleated from the melt and grows up with decreasing the temperature.The nano-sized MnsSi3 particles are observed to precipitate from the ? matrix after isothermal heat treatment at higher temperatures.A crystallographic orientation relationship is found between the precipitated MnsSi3 phase and ? matrix:(110)p//(1100)Mn5Si3 and[111]p//[1122]Mn5Si3.Fine and dispersive Mn5Si3 precipitates in the matrix results in a great improvement of wear resistance of the complex brasses.(3)The strengthening mechanisms of complex brasses can be mainly attributed to the load transferring effect from matrix to[Mn(Fe)]5Si3 particles,followed by grain refinement strengthening.With increasing the[Mn(Fe)]5Si3 content,the hardness and strength increase effectively,and the fracture mode transforms from ductile failure to quasi-cleavage and cleavage failure.Severe cracking tendency of coarse,high aspect ratio primary[Mn(Fe)]5Si3 particles containing large-sized hollows leads to slight increment in tensile strength and great ductility decrease at high[Mn(Fe)]5Si3 content.(4)The wear resistance of complex brasses is significantly influenced by the size and morphology of[Mn(Fe)]5Si3 particles under higher friction loads.The formation of coarse primary[Mn(Fe)]5Si3 particles with high aspect ratio and large hollows greatly reduces the wear resistance due to the severe crack formation at the subsurface.The proper combination of thinner primary and eutectic[Mn(Fe)]5Si3 particles are found to efficiently restrain the crack creation and result in a good wear resistance.(5)With the trace Sr addition into the HMn64-8-5-1.5 brass,the grains are refined and the amount of primary[Mn(Fe)]5Si3 particles increases.The prism diameter and aspect ratio of particles decrease and also the dimension of both the internal hollow and prism facet hollow.For the brass with proper Sr addition,the hardness and tensile strength increase.The wear resistance greatly improves due to the reduced cracking tendency of primary[Mn(Fe)]5Si3 particles.The HMn64-8-5-1.5 brass achieves the highest mehcnical and wear properties when the Sr content is 0.15wt.%. |
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