超硬铝LC9合金的超塑性研究

微细晶粒超硬铝LC9合金在470～530℃温度范围内,速率为8.33×10~(-4)～1.66×10~(-2)s~(-1)条件下拉伸呈显出良好的超塑性。在最佳超塑性条件下(T=515℃,ε=1.66×10~(-3)s~(-1))获得延伸率δ=1300％、流变应力δ=1.7MN/M~2应变速率敏感性指数m=0.66 金相和电镜观察表明,在超塑流变过程中,除发生晶界滑动,扩散蠕变外,还发生明显的动态再结晶以及晶内结晶学滑移,晶内位错密度随变形量增大而增加。扩散蠕变导致在横向晶界上形成新的条带区,出现晶界迁移和无沉淀区,同时存在晶内和晶界扩散。空洞在三角晶界处萌生,沿横向晶界方向的扩展连结,导致突然断裂。     

Al-6Cu-0.5Zr合金超塑变形后微观结构的研究

相分析表明该合金由Al基固溶体和CuAl_2、ZrAl_3两种粒子所组成。合金超塑性变形后,利用标记线法测定了晶界滑移量对总变形量的贡献为60％左右。在δ≈30％的试样上观察到晶界变宽,在晶界上呈现折皱区,并在遇到第二相时改变方向。透射电镜分析表明,晶界滑移时出现晶界位错,在三晶交界处或晶界坎处向晶内激发位错,晶界是位错源与壑,激活的晶内位错通过滑移和攀移会形成位错亚晶界,晶内位错的激活与运动是晶界滑移的重要协调机制,晶界滑移与晶界位错运动有关。合金超塑性变形时,在晶界和CuAl_2相界处有空洞形成,研究了空洞面积分数与面缩率的关系。靠近断口处,空洞数和面积分数急剧增加,说明空洞的增殖、扩展和连接导致断裂。     

MB15在半固态等温处理中的组织和成分演变

为考查等温处理技术制备半固态成形用MB15镁合金非枝晶锭料的可能性,借助于液淬技术、光学和电子显微分析,研究了MB15镁合金的微观组织和成分演变.结果表明:MB15镁合金在半固态等温热处理过程中能形成晶粒细小、晶粒粗化速度缓慢的球状组织,但其圆整程度低于AZ91D合金.随着温度的升高和保温时间的延长,Zn向晶界扩散速度加快,晶界处的Zn含量逐渐增加,晶内Zr偏析逐渐扩散.冷却时,α-Mg与Zr在晶内富Zn处发生包晶反应,晶界处发生共晶反应和共析反应.     

ZM5镁合金TIG焊接接头组织与力学性能

采用TIG焊对ZM5镁合金进行焊接,利用光学显微镜、显微硬度仪和拉伸试验机对ZM5镁合金接头的组织特征和力学性能进行研究。结果表明:ZM5合金TIG焊接接头是由热影响区、部分重熔区和焊缝组成。热影响区组织是由初生α-Mg相基体和主要分布在晶界上的α-Mg+β-Mg17Al12共晶相组成;部分重熔区中共晶相不仅大量析出在晶界上,在晶内也呈现出较均匀的弥散析出,而且其β-Mg17Al12相出现显著长大;焊缝组织则是典型的树枝晶形貌,枝晶为初生α-Mg相,枝晶间是α+β共晶相。组织形貌的差异导致接头各区域有着不同的显微硬度,也使得接头的抗拉强度和塑性都低于母材。     

物理金相三级人员试题参考答案

二　问答题2 4 .沿晶断裂是裂纹沿晶界扩展而造成金属材料的脆断。在失效分析中常见的有以下几种类型的沿晶断裂形式 :(1)晶界沉淀相造成的沿晶断裂。晶界上不连续的沉淀相在外力作用下 ,在其周围首先形成微孔 ,后经长大连接成为晶界裂纹 ,最后造成沿晶断裂 ,此时为沿晶韧窝断裂。若晶界上沉淀相粒子形成脆性网状薄膜 ,此时断裂为脆性薄膜分裂型。(2 )杂质元素偏聚造成的沿晶脆断。杂质元素Ｐ ,Ｓ ,Ｓｂ ,Ａｓ和Ｓｎ等在晶界上存在并达到一定浓度时 ,它们将降低晶界内聚能 ,从而引起脆断。如高强度低合金钢的回火脆性就是典型例子。(3)由环…     

用α粒子径迹法研究硼微合金化后的Fe_3(SiAl)中的硼分布

用α粒子径迹法考察经硼微合金化后的 Fe_3(SiAl)合金在部分有序状态下的硼分布,用透射电镜观察反相畴,结果表明硼含量在 Fe(SiAl)合金中存在一临界浓度约150ppm。硼含量低于此临界浓度时,硼在晶内的大量反相畴界上呈均匀分布,降低了长程有序度和反相畴界能,从而改善了该化合物的脆性;高于此临界浓度时,硼开始在晶界上偏聚,晶内出现贫硼区,削弱了硼在晶内畴界上均匀分布带来的降低反相畴界能和改善化合物脆性的有益作用。硼与反相畴界之间可能存在化学交互作用。     

DISTRIBUTION OF BORON IN BORON-DOPED Fe_3(SiAI)BY MEANS OF α-PARTICLE AUTORADIOGRAPH

用α粒子径迹法考察经硼微合金化后的Fe_3(SiAl)合金在部分有序状态下的硼分布,用透射电镜观察反相畴,结果表明硼含量在Fe(SiAl)合金中存在一临界浓度约150ppm。硼含量低于此临界浓度时,硼在晶内的大量反相畴界上呈均匀分布,降低了长程有序度和反相畴界能,从而改善了该化合物的脆性;高于此临界浓度时,硼开始在晶界上偏聚,晶内出现贫硼区,削弱了硼在晶内畴界上均匀分布带来的降低反相畴界能和改善化合物脆性的有益作用。硼与反相畴界之间可能存在化学交互作用。     

双相不锈钢中奥氏体沉淀相的晶粒取向及界面特征分布

利用电子背散射衍射(EBSD)技术和基于Rodrigues-Frank(R-F)空间的取向差分析方法研究双相不锈钢中过饱和铁素体(α)经1323K沉淀析出的奥氏体(γ)相的晶粒取向和界面(晶界和相界)特征分布。结果表明:α晶粒经ε=2预先轧制变形后,γ析出相具有较强的织构,晶粒内取向差呈现以小角度晶界为主,孪晶界次之的晶界特征分布,这些晶粒与α基体的取向关系满足K-S,N-W和Bain关系的数量基本相当。具有相同取向而未经轧制变形的α晶粒沉淀析出的γ晶粒取向基本随机分布,与基体的取向关系主要以K-S为主;但内部发生孪晶的γ晶粒与基体不再满足K-S关系,其周围的相界约有1/4满足35°/〈110〉的新型取向关系。     

5Cr21Mn9Ni4N钢锻造裂纹的研究

过高的锻造加热温度(1220℃)使冷却时形成大量晶界枝晶片状M_7C_3,造成晶界严重弱化;在1200℃以下加热后的冷却过程中,晶界和惯析面上均形成一定数量的片状M_(23)C_6,且随着温度的降低钢的最薄弱环节由晶界向晶内转移.晶界和晶内惯析面的弱化,易导致锻坯、尤其是锻坯表面形成裂纹.确定了合适的锻造温度参数.     

ECAP形变对高纯铝微结构及冲击层裂损伤的影响

在高应变率冲击载荷下,金属材料的主要失效方式之一是层裂损伤。为探讨微结构对层裂损伤的影响,本文利用等径角挤压(Equal Channel Angular Pressing,ECAP)技术制备了不同微结构的高纯铝,并对改性后的高纯铝进行平板撞击实验,通过电子背散射衍射表征加载后样品的孔洞分布和晶界取向差。研究发现:高纯铝在ECAP挤压变形过程中发生动态再结晶和动态回复,导致采用ECAP技术无法获得超细晶高纯铝;降低挤压温度可抑制动态再结晶,并得到尺寸相对较小的晶粒(约50 μm)。平板撞击实验结果表明,晶粒尺寸对弹性极限、层裂强度等宏观力学性能影响较小。回收样品分析表明,大晶粒样品中孔洞尺寸大而数量少,小晶粒样品中孔洞尺寸小,但数量多。孔洞在晶界(沿晶孔洞)及晶界附近(晶内孔洞)均可成核。晶内孔洞比沿晶孔洞数量多但尺寸小。大角度晶界比小角度晶界更容易导致沿晶孔洞成核,而晶内孔洞倾向于在45°～55°晶界附近成核。     

含微量稀土元素的Al-Zn-Mg合金的超塑变形机制

用透射电镜详细观察了含微量稀土的 Al-Zn-Mg 合金在超塑变形过程中晶界位错的形态、运动及其与晶界第二相粒子的相互作用情况。发现晶界中的第二相粒子对位错沿晶界运动有明显的阻碍作用。实验还发现在含第二相粒子较多的晶界的超塑性合金中,晶界滑动是通过位错沿晶界运动和晶界的粘滞性流动两种机制同时发生作用进行的,它们相互促进并共同推动了晶界滑动的发展。首次在透射电镜下观察到超塑变形使晶界加宽的现象,在面缩率ψ=72.2%的样品中测得某晶界的宽度约为100nm,从而证实了超塑变形的晶界滑动和晶粒转动机制。     

Accumulation of coincidence site lattice boundaries during grain growth

Abstract

Monte Carlo simulations were used to investigate the effect of grain growth on the coincidence site lattice (CSL) boundary content of randomly textured polycrystals. Each grain was assigned an orientation, and grain boundary properties were dependent on both the boundary misorientation and the CSL character. While low misorientation angle boundaries (LABs) increase during growth, the fraction of CSL boundaries does not change with time. Decreasing CSL boundary energy and mobility did not alter these results. In contrast with LABs, which are characterised by a scalar misorientation angle, a particular combination of three independent rotation variables is required to create a low energy CSL boundary; thus, these boundaries are unlikely to form or to persist in a random polycrystal. While texture influences boundary formation, a texture that can enhance CSL boundaries is not apparent. Boundary plane effects should not increase CSL fraction during grain growth.     

晶界设计在多晶体金属材料中的应用

对多晶体金属材料的晶界结构进行设计,能够显著提高这类材料的力学、化学及磁学性能。因此,晶界设计已成为金属材料改性的一项重要技术。对晶界设计的基本理论进行了介绍,在此基础上综述了近年来这项技术的应用研究概况,并提出了未来的进一步研究方向。     

Development of non-rare earth grain boundary modification techniques for Nd-Fe-B permanent magnets

The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure.Intergranular addition and grain boundary diffusion(GBD)process are effect...     

Relation between grain boundary segregation and grain boundary character in FCC alloys

An analytical model of segregation at grain boundaries, which takes into account all five macroscopic parameters of grain boundary character, has been developed. The model is based on a combination of previous bond energy treatments of grain boundary energy and of segregation to free surfaces. It is tested by comparing its predictions against previous computations of segregation to symmetrical twist grain boundaries in simple fcc alloys obtained by Monte Carlo simulations in conjunction with embedded atom method potentials. The comparison shows good agreement with the previous computer simulations. Examples of model predictions in the case of asymmetric grain boundaries are also provided.     

FEM modelling of the combined effect of grain boundaries and second phase particles on the flow stress of nanocrystalline metals

In this study, numerical FEM simulations to provide an insight into the possible combination of grain boundary and second phase particle strengthening in nanocrystalline metals were made. Because grain boundary sliding is one of the major deformation mechanisms in these materials special attention was paid to the role of nanoparticles located at the grain boundaries. It was shown that second phase particles located at the grain boundaries may contribute to the strengthening process, thereby compensating for the loss of strength brought about by grain boundary sliding. However, the effect of the particles strongly depends on the relative contribution of grain boundary sliding to the overall deformation process. In addition, particles located at grain boundaries under conditions of grain boundary sliding significantly influence the distribution of plastic deformation making it more homogenous at the nano-meter scale.     

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.     

Structure of special grain boundaries in SiAlON ceramics

The microstructure of hot-pressed samples of the 15 R polytype phase in the Si-Al-O-N system was studied by means of TEM. Emphasis was put on studies of high angle grain boundaries. In this material high angle boundaries of arbitrary orientation usually possess a vitreous grain boundary phase. However, special grain boundaries were found, which were free of any vitreous grain boundary phase. From the orientation relation of the adjacent grains Σ=1 (coherent reflection twin boundary), Σ=7 and Σ=13 boundaries were found. For their explanation a coincidence site lattice (CSL) model was developed for (0001) twist boundaries. By assuming an exponential form for the potential of atomic interaction, the calculation of minimum grain boundary energies for special twist angles was in accordance with the experimental observations.     

Energy of Grain Boundaries of Different Type in fcc Solid Solutions,Ordered Alloys and Intermetallics with L12 Superstructure

The study of the triple junctions of the grain boundaries in some fcc solid solutions, ordered alloys and intermetallics with L12 has been carried out using the optical metallography and TEM methods. Two-types of the triple junctions were found in the alloys investigated (1), consisting of the boundaries of the random (RT), and (2), consisting of the RT boundary and the special boundaries (ST). The relative values of the RT and ST boundary energy were determined on the basis of the measurements of the angles between the grain boundary planes. It has been shown that the energy of ST boundaries increases with the increase of the stacking fault (SF) energy. The energy of the RT boundaries does not depend on the SF energy.     

A practical approach to the determination of the crystallography of grain boundaries

Methods available for determining the crystallography of grain boundaries are surveyed. It is suggested that the reason why relatively few studies of the crystallography of large numbers of boundaries have been made is the intensity of labour involved in the data analysis. A scheme is described which allows a large sample population of grain boundaries to be handled with maximum efficiency, whilst maintaining high precision in the parameters which describe the boundary. It also permits a comparison of these parameters with those which describe special cases (e.g. high-density coincidence site lattice, etc). The method involves a combination of stereographic manipulation and matrix algebra.