Effects of Si alloying and T6 treatment on mechanical properties and wear resistance of ZA27 alloys

To improve the mechanical properties and wear resistance of ZA27 alloy, Si was introduced to the alloy, and the effect of Si alloying and T6 heat treatment on the microstructure, mechanical properties and wear resistance was investigated. The results show that with 0.55% Si, the microstructure of the alloy can be refined effectively, which leads to the increase of hardness. But the tensile strength and elongation decrease because Si undermines the integrity of the matrix. On the other hand, the dendrites are transformed into a desired α+η+(α+η)mixture with T6 heat treatment, which introduces a remarkable increase to the elongation and hardness of the alloy. The wear resistance of the ZA27 alloy with Si alloying is significantly better than that of the ZA27 alloy without Si. With the increase of Si addition, the wear resistance of the alloy firstly increases and then decreases.In the alloy without Si alloying, severe plastic deformation and large delamination were observed on the worn surface of the alloy. However, with the increase of Si, the main wear mechanism transformed to abrasive wear gradually. In addition, the T6 treatment can further improve the wear resistance of the alloy with Si alloying.

MICROSTRUCTURAL EVOLUTION OF AS-CAST ZA27 ALLOY DURING COMPRESSIVE CREEP

Phase transformation and micro structural change of an as-cast ZA27 alloy were investigated during compressive creep by X-ray diffraction technique, SEM and TEM. Compressive creep induced decomposition of α metastable η' phase and a four-phase transformation, α + ε → T' + η and T' → θ, occurred during the compressive creep testing. The occurrence of negative creep in the alloy resulted from the volume expansion caused by the four-phase transformation. A micro structural change was also observed from a lamellar structure into a spheroidized structure in higher creep strain of tested specimens. It provided evidence of compressive creep induced phase transformation which occurred in ageing process.

Effect of rare earths on phase transformation in as-cast ZA27 alloy dur-ing compressive creep

The effect of the mixed rare earths of Ce on the phase transformation in ascast ZA 27 alloy during compressive was investigated under 37 MPa and at 160 deg C by X-raydiffraction technique and SEM. The results showed that the as cast microstructure of ZA 27-RE alloyconsisted of a dendritic Al-rich alpha' surrounded by Zn-rich beta' phase, interdendritic epsilonphase and Zn-rich eta phase together with a complex Z phase which was a complex constitute compound,(RE,Cu)Al_5Zn_(16), dispersed in crystal interfaces or branch crystal interfaces and stable duringcompressive creep test at 160 deg C. The phase transformations of ZA 27-RE alloy, decomposition ofbeta' phase arid four transformation, were delayed by the addition of rare earths, also the lamellarstructure and the spheroidized structure in ZA 27-RE alloy were finer than in ZA 27 alloy duringcompressive creep test at 160 deg C at the same creep time, and the compressive creep resistance ofZA 27-RE alloy was higher than that of ZA 27 alloy.

Corrosion influence on surface appearance and microstructure of compo cast ZA27/SiC_p composites in sodium chloride solution

The influence of corrosion on the surface appearance and microstructure of particulate ZA27/SiCp composites was examined after 30 d immersion in a sodium chloride solution with the access of atmospheric oxygen. The composites with different contents of SiC micro-particles were synthesized via compo casting. Microstructural studies by means of optical microscopy （OM） and scanning electron microscopy （SEM） showed that corrosion occurred in the composite matrices, preferentially in regions of the η phase, rich in zinc. The corrosion processes did not affect the silicon carbide particles incorporated in the matrix alloy. According to the results of electrochemical polarization measurements, an increase in the content of SiC particles in the composite matrice has led to the lower corrosion resistance in the composites.

Aging microstructural characteristics of ZA-27 alloy and SiC_p/ZA-27 composite

The aging characteristics of as-quenched microstructures of ZA-27 alloy and SiCp/ZA-27 composite(ZMCp) were investigated using SEM, EDS and TEM. The structure, morphology and size of sub-grains in primary dendrite in ZMCp continuously change during aging process. Little tiny spherical Zn-rich η phase distributes in the dendrite. Amount of transitional α′phase, well coherent with equilibrium αf phase, in SiCp-neighboring dendrite edge zone is less than that in dendrite center zone. Both eutectic and peritectic β phase transform into lamellar α and η phases, obeying [ 2113]η∥[110]α, and (002)α∥ (1 101)η. In the like-eutecticum of ZMCp, less amount of β phase and decomposition products are found. The size of α phase decomposed from peritectic β phase in ZMCp is obviously larger than that in the monolithic alloy. The lamella decomposition of β phase beside SiCp is evidently more rapid than that in the alloy. SiC particulates strongly accelerate neighboring β phase decomposition in aging process.

An Investigation of Creep Resistance in Grade 91 Steel through Computational Thermodynamics

This study was conducted to understand the relationship between various critical temperatures and the stability of the secondary phases inside the heat-affected-zone(HAZ)of welded Grade 91(Gr.91)steel parts.Type IV cracking has been observed in the HAZ,and it is widely accepted that the stabilities of the secondary phases in Gr.91 steel are critical to the creep resistance,which is related to the crack failure of this steel.In this work,the stabilities of the secondary phases,including those of the M23C6,MX,and Z phases,were simulated by computational thermodynamics.Equilibrium cooling and Scheil simulations were carried out in order to understand the phase stability in welded Gr.91 steel.The effect of four critical temperatures—that is,Acl(the threshold temperature at which austenite begins to form),Ac3(the threshold temperature at which ferrite is fully transformed into austenite),and the M23C6 and Z phase threshold temperatures—on the thickness of the HAZ and phase stability in the HAZ is discussed.Overall,the simulations presented in this paper explain the mechanisms that can affect the creep resistance of Gr.91 steel,and can offer a possible solution to the problem of how to increase creep resistance at elevated temperatures by optimizing the steel composition,welding,and heat treatment process parameters.The simulation results from this work provide guidance for future alloy development to improve creep resistance in order to prevent type IV cracking.

Effect of Yttrium on Aging Resistance of Zinc Alloy ZA-27

By metalloscopy,scanning electron microscope,X-ray diffraction,electron probe,salt water erosion and hot steam erosion,the microstructures and the aging resistance of zinc alloy ZA-27 with different contents of yttrium were studied. The results show that with optimum addition of yttrium in the alloy ZA-27 the fine YAl_3 phase forms. The fine YAl_3 granules can act as the condensation nuclei of α phase and the number of the α phase′s nuclei increases greatly and the growth of the nuclei comes in for restrict during the crystallization and the alloy′s grains become fine and the segregation reduces. In addition,yttrium also combines with zinc and the fine dispersion Y-Zn phase forms which improves grain boundary conditions and the alloy′s aging resistance. But when the content of yttrium is overmuch some YAl_3 granules grow to be lumpy,so the fine and close texture of the alloy′s boundaries is damaged and the proportion of the interface among the phases increase which brings a decline in the alloy′s resistance to corrosion.

微量元素对ZA27合金腐蚀及老化性能的影响

ZA27合金是目前国内外发展迅速的新型材料。它具有优良的机械性能、耐磨性能和机械加工性能。该合金利用其价廉的优势,主要用于代替昂贵的青铜合金制作耐磨件。但ZA27合金与传统的锌合金一样,也有腐蚀和老化现象,这种现象历来是人们关注的问题,但迄今为止,其机理尚不清楚,也没能提出防止老化的根本措施。本文在研究ZA27于不同介质中的耐蚀性和人工时效时机械性能变化的基础上,探讨了微量Sn,Pb和RE元素对该合金腐蚀及老化性能的影响。

K_2O·6TiO_(2(w))/ZA-27复合材料显微组织与性能

采用挤压铸造法制备了Ｋ２Ｏ·６ＴｉＯ２晶须增强ＺＡ－２７合金复合材料，分析了复合材料的显微组织特性，利用ＴＥＭ对该复合材料界面的微观结构进行了观察与分析，测试了复合材料的抗拉强度和硬度。实验结果表明：该复合材料渗透完全，晶须分布均匀，观察有两种形态界面存在，复合材料的硬度随晶须含量的增加而显著提高。

Mg-Li合金及复合材料的抗蠕变性能

对Mg Li合金及液态反应合成MgO/Mg2 Si颗粒增强Mg Li基复合材料的室温和高温抗蠕变性能进行了研究。结果表明 ,复合材料的抗蠕变性能明显高于基体合金 ,室温时的蠕变速度仅为基体合金的 1/10 ,且随温度的升高效果更明显 ,160℃时的蠕变速度约为基体合金的 2 %。其机理为颗粒的存在提高了材料的屈服强度和弹性模量 ,以及颗粒限制晶界的相对滑动、阻碍位错的滑移及攀移。

ZnAl27合金基复合材料高温蠕变性能研究

利用HBE 75 0型高低温硬度仪和H 80 0型TEM研究了SiCp/ZA2 7,Al2 O3 p/ZA2 7复合材料和ZA2 7合金的高温蠕变性能 ,结果表明 ;随着温度升高 ,材料的抗蠕变性能下降 ,在不同温度下 ,SiCp/ZA2 7和Al2 O3 p/ZA2 7复合材料的抗蠕变性能均明显高于ZA2

钇对ZA-27锌合金老化性能的影响

采用金相、扫描电镜、X射线衍射、电子探针及盐水浸蚀、热蒸汽浸蚀等方法,对不同钇含量的ZA 27锌合金进行了显微组织及抗老化性能的研究。研究结果表明,在ZA 27合金中添加适量的稀土钇,钇可与铝生成细小的YAl3相,为α相提供凝结核,使α相形核量增加,在结晶过程中,α相枝晶不能充分发育,从而起到细化晶粒、减少偏析的作用;同时,还可与锌生成弥散、细小的钇锌相,从而改变晶界状况,提高合金的抗老化性能。当钇稀土含量较高时,由于在晶界与晶内均有大块钇锌相生成,破坏了合金晶界的致密度,并增加了相界面,从而使合金的抗腐蚀性能有所下降。

ZA-27合金摩擦磨损性能的研究

进行了ＺＡ－２７合金与牌号为ＱＳｎ６．５－０．１锡青铜合金摩擦磨损性能的对比实验。实验结果表明，在低速重载润滑条件下，ＺＡ－２７合金的减摩能力和抗磨损性能明显优于ＱＳｎ６．５－０．１合金，为ＺＡ－２７合金代替ＱＳｎ６．５－０．１合金制造纺机上纱锭轴承底座提供了实验依据。

K_2O·6TiO_(2(w))/ZA-27 合金复合材料高温磨损性能

通过ＳｑｕｅｅｚｅＣａｓｔｉｎｇ工艺制备了钛酸钾（Ｋ２Ｏ·６ＴｉＯ２）晶须增强锌铝合金（ＺＡ２７）复合材料，分析了Ｋ２Ｏ·６ＴｉＯ２（ｗ）／ＺＡ２７合金复合材料的微观组织特性，研究该复合材料及基体合金室温及高温（～１５０℃）摩擦磨损性能．实验结果表明：在边界润滑条件下，复合材料的高温耐磨性能比基体合金有显著提高；利用ＳＥＭ５０５扫描电镜对两种材料的磨损表面微观形貌进行了观察，并讨论了高温磨损失效机制．