论金属间化合物的脆性及其克服途径

多晶材料的脆性主要表现为:晶内解理和晶界断裂。这和其本身的原子结合特性及晶界结构有关。本文从晶体结构,原子的结合特性,晶界结构讨论了金属间化合物的脆性。同时,从调整原子键合成分的“合金化”这一最简单易行的冶金手段和细化晶粒方面,认为改善金属间化合物的延性是可能的。     

时效处理对Cu-3Si-2Ni合金组织及性能的影响

通过室温拉伸性能测试、X衍射和断口分析等方法研究了时效工艺对Cu-3Si-2Ni合金组织性能的影响.结果表明:Cu-3Si-2Ni合金时效后,强化相粒子可在晶界处及晶粒内部析出,有效提高室温屈服强度和抗拉强度,但导致合金的伸长率降低,室温拉伸断口呈现韧性和沿晶脆性混合断裂特征,而固溶态合金的室温拉伸断口均呈现典型的韧性断裂特征.根据实验结果,得出该合金较佳时效工艺为450℃×2 h.     

锌铝合金中稀土及铁的晶界偏聚电子理论

根据分子动力学理论建立液态ZA27合金的原子集团,结合计算机编程构造出ZA27合金α相与液相共存时的原子构形及α相大角度重位点阵晶界模型.利用递归法计算铁、稀土元素固溶于晶粒内、游离于固液相界面及其在α相晶界处的环境敏感镶嵌能.结果表明:铁、稀土处于固液相界区比在晶内更稳定,这解释了铁、稀土在α相内溶解度很小,结晶时富集于固液相界前沿液体中,从而导致凝固结束后铁、稀土元素偏聚于晶界,并形成成分复杂的稀土化合物的事实.     

Fe中刃型位错扭折处掺S的电子结构

用基于密度泛函理论的自洽离散变分方法研究了bcc Fe中[100](010)刃型位错扭折处掺S的电子结构,计算了杂质偏聚能、原子间相互作用能、电荷密度及态密度.计算结果表明:S引入体系后,电荷发生了重新分布,S原子得到电子,其周围Fe原子失去电子,且S原子的3p轨道与近邻Fe原子的3d4s4p轨道之间杂化,使得它们之间的成键有较强的方向性,可影响材料的韧性;而近邻Fe原子因失去电子使自身之间相互作用比未掺杂时减弱,有利于扭折的迁移,可能影响材料的强度.     

纳米面心立方金属的变形机制及影响其力学性能的因素

综述了纳米面心立方金属的变形机制随晶粒尺寸的减小而发生的变化,即变形机制由晶界处发射不全位错、形成孪晶转变为晶界滑移、晶粒转动.当变形机制为晶界处发射不全位错、形成孪晶时,存在最佳孪晶形成晶粒尺寸范围,此时的孪晶形核应力最小.另一方面,随着晶粒尺寸的减小,在变形机制发生转变的临界晶粒尺寸附近存在韧-脆断裂方式的转变.提高孪晶密度、在纳米晶材料中加入微米晶相形成双峰晶粒材料可以提高纳米晶材料的塑性,得到更好的综合机械性能.     

稀土Ce在α-Fe中占位倾向与作用机理的密度泛函理论研究

采用基于密度泛函理论(DFT)的平面波超软赝势方法,结合广义梯度近似(GGA),计算了Ce原子在α-Fe晶内、晶界和自由表面的杂质形成能,在此基础上,研究了稀土元素Ce在α-Fe中的占位倾向,并根据Rice-Wang热力学模型讨论了Ce元素在晶界的韧脆化作用。结果表明:Ce倾向于偏聚在α-Fe晶界区,此外,Ce在晶界和自由表面的杂质形成能之差小于零。α-Fe晶界掺杂Ce前后的电子结构显示,Ce占位于晶界后,为掺杂区提供了更多的电子用于成键,与晶界上相邻Fe原子的作用加强,使掺杂区产生由金属键向离子键变化的趋势,也使晶界上Fe原子的成键态能量比重增加,从而增强了掺杂区原子之间的结合能力。同时,Ce加入后与晶界上Fe原子外层轨道发生较为强烈的相互作用,并使晶界上的原子成键区态密度左移,降低了体系总能量。     

晶界氧化对GH4738高温合金疲劳裂纹扩展的作用

测定了GH4738合金在650、700、750及800℃空气环境下的疲劳裂纹扩展速率da/d N-ΔK曲线及疲劳裂纹扩展寿命a-N曲线,得出了温度对合金疲劳裂纹扩展的影响规律,并结合组织性能、疲劳特征、高温及室温下晶界氧化情况等分析了温度对合金疲劳裂纹扩展的影响。结果表明,随着温度升高,GH4738合金的疲劳裂纹扩展速率(FCGR)增加,合金的断裂方式由沿晶和穿晶混合型断裂向完全沿晶断裂转变;在初始应力强度因子幅度DK为40 MPa·m1/2、晶粒尺寸为30~40 mm时,合金的疲劳裂纹扩展寿命在650~700℃内显著下降,存在一个温度敏感区间,其原因并不是材料的组织和力学性能的变化,主要是高温下的氧化作用所致;O通过裂纹尖端、滑移带间接进入晶界或O直接渗入晶界的方式,与晶界处的活性元素Co、Ti、Al反应生成脆性氧化物,从而降低了晶界强度,使合金的抗疲劳性能显著下降。     

静液挤压及退火处理对AZ80镁合金组织及力学性能的影响

通过不同状态AZ80镁合金的金相组织观察、力学性能试验和断口SEM分析,研究了静液挤压及退火热处理对AZ80镁合金组织及力学性能的影响。结果表明:经静液挤压后,镁合金组织发生动态再结晶,生成细小再结晶晶粒,再结晶晶粒尺寸10μm左右,镁合金抗拉及屈服强度明显提高,分别达到325 MPa和241 MPa。退火热处理后,在高温热能的驱动下镁合金组织发生了静态再结晶,同时位错能得到释放,使镁合金伸长率大幅度提高,达到8.1%。均匀化态的镁合金断裂属于韧性和脆性混合型断裂;挤压态镁合金断裂是脆性解理断裂;退火热处理态镁合金表现为沿晶脆性断裂,但与挤压态相比,断口有向韧性断裂过渡的倾向。     

二维超声磨削微纳米复相陶瓷表面变质层特性研究

利用二维超声振动磨削,对微纳米复相陶瓷磨削表面变质层的结构、晶粒度和晶格畸变等进行研究.在一定的磨削条件下,微纳米复相陶瓷二维超声振动磨削表层是以晶粒碎化的材料粉末化以及少量的材料压碎和晶粒脱落等方式为主的塑性变质层,亚表层是以晶格畸变、晶界滑移为主的塑性变形层,材料脆性碎裂去除方式极少,以塑性变形去除机理为主.为此提出了微纳米复相陶瓷二维超声振动磨削表面变质层结构模型.通过TEM,SEM观察发现:纳米材料微观变形机理为内晶型结构增强相的晶粒内位错,基体晶粒的晶界滑移、晶间第二相的变形为其变形协调机制.由于纳米粒子分散在晶界间,阻碍了裂纹的扩展,使得材料表现为穿晶断裂行为,从而获得良好的加工表面.     

合金钢稀土渗氮层的硬度增强机理

为研究合金钢稀土渗氮层硬度增强机理,采用扫描电镜、透射电镜和显微硬度计等测试方法,对3种常用合金钢的常规渗氮和稀土渗氮的显微组织和渗层硬度进行分析。结果分析认为,稀土原子的渗入导致钢表层晶粒缺陷密度增加,在缺陷处生成气团,使晶粒内氮原子偏聚区增多,晶粒亚结构细化,导致硬度提高;同时稀土原子沿晶界渗入和扩散,会诱导晶界迁移,使晶界表面突壁和扭折密度发生变化,可以使常规渗氮中容易析出的网状ε氮化物减少,降低了沿晶界断裂的可能性,在晶界强化。这些作用的叠加,提高了渗氮层的硬度,增强了材料的力学性能。     

LOCAL CHEMISTRY AND THE COHESIVE STRENGTH OF GRAIN BOUNDARIES IN Ni_3Al

ＬＯＣＡＬＣＨＥＭＩＳＴＲＹＡＮＤＴＨＥＣＯＨＥＳＩＶＥＳＴＲＥＮＧＴＨＯＦＧＲＡＩＮＢＯＵＮＤＡＲＩＥＳＩＮＮｉ_３ＡｌＳｈａｎｔｈｉＳｕｂｒａｍａｎｉａｎ；ＤａｖｉｄＭｕｌｌｅｒ；ＪｏｈｎＳｉｌｃｏｘａｎｄＳｔｅｐｈｅｎＬ．Ｓａｓｓ（Ｄｅｐａｒｔ?..     

Mo对高强度钢延迟断裂行为的影响

在含V和Nb的40Cr钢中添加不同质量分数(0-1．54％)的Mo元素,采用缺口拉伸试样和改进的M—WOL型试样研究了Mo对高强度钢延迟断裂行为的影响．结果表明,随着Mo含量的增加,实验钢的延迟断裂抗力逐渐提高;当Mo含量超过1．15％时,延迟断裂抗力不再提高．EDS分析结果表明,钢中Mo元素在晶界发生偏聚,偏聚范围在几个纳米尺度内．通过电子能量损失谱(EELS)证明,Mo元素在原奥氏体晶界的偏聚能够提高钢的晶界结合强度．在钢中添加Mo能够显著提高钢的回火抗力和晶界结合强度,这是其具有高的延迟断裂抗力的主要原因．碳化物Mo2C对氢的捕集作用亦能够提高钢的延迟断裂抗力．Mo和V元素的二次硬化碳化物在半共格和非共格状态时,实验钢的延迟断裂抗力显著提高．     

Solute segregation at grain boundaries in superplastic SiO2-doped TZP

Grain boundary structure, chemical composition, and bonding state in superplastic SiO₂-doped TZP and undoped TZP were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a field emission type transmission electron microscope (FE-TEM). No amorphous phase was observed at any grain boundaries in either SiO₂-doped TZP or undoped TZP. Yttrium ions segregated over a width of 4–6 nm at grain boundaries in both materials, and silicon ions segregated over a width of 5–8 nm at grain boundaries in SiO₂-doped TZP. The average dihedral angle between grain boundaries in SiO₂-doped TZP was as high as 80°, which agreed well with the fact that no grain boundary had glass phase. The strain energy is accumulated by the dissolution of silicon ions into the tetragonal zirconia lattice. However, the grain boundary energy of SiO₂-doped TZP is likely to be low enough to compensate the increase of strain energy near grain boundaries. OK-edge EELS spectra taken from grain boundaries in SiO₂-doped TZP were shifted 3–4 eV to the higher energy side in comparison with those from the grain interior. This may suggest that the chemical bonding is strengthened at grain boundaries by the presence of solute silicon. The strengthening may be responsible for the enhanced superplasticity in SiO₂-doped TZP.     

Grain boundary segregation and fracture behavior of boron containing Fe-Mn-Ni-(Mo,w) age hardening alloys

The effects of boron addition on the grain boundary segregation and fracture behavior of tempered Fe-Mn-Ni-Mo and Fe-Ni-Mn-W steels were investigated. High segregation of Mn to prior austenitie grain boundaries resulted in severe grain boundary embrittlement in W-bearing alloys. Boron addition did not significantly affect the grain boundary segregation of other alloying elements. Nevertheless, improvement of tensile properties is observed in 16 ppm boron doped W-bearing steel. Segregation of boron itself to grain boundaries is believed to affect the grain boundary strength of this alloy. Lower Mn segregation in Mo containing steels resulted in the ductile fracture when tempered at 480°C.     

Grain boundary segregation and intergranular fracture in Ferrite containing Mo,Si or Al

Roles of molybdenum, silicon or aluminum in ferrite on grain boundary segregation and hence on intergranular fracture have been investigated by using Auger electron spectroscopy (AES) and tensile test. Competitive segregation between sulfur and carbon or nitrogen, which caused the decrease below 700°C of sulfur content at the grain boundaries, was observed in the pure iron. The intergranular brittleness of the pure iron was caused by sulfur at the grain boundaries. When molybdenum was added to the pure iron, the sulfur contents at the grain boundaries were lowered in comparison to those in the pure iron. The molybdenum-bearing alloy showed higher fracture strength than that of the pure iron, and fractured mostly in the transgranular mode. This arises from the intrinsic effect of molybdenum on the grain boundaries as well as the decrease in sulfur content. Tn the 3.37 wt.%Si alloy, silicon and carbon or nitrogen competitively segregated to the grain boundaries, and such a competitive segregation was also observed between sulfur and carbon or nitrogen. The sulfur content at the grain boundaries decreased with increasing silicon content. The fracture modes in the 3.37- and 4.26 wt.%Si alloys were transgranular in the rolling direction, but were mostly intergranular in the transverse direction and in the as-rolled condition. The intergranular characteristic in the fracture behavior may be attributed to the detrimental effect of silicon as well as sulfur on the intergranular cohesion. Carbon and aluminum only were found at the grain boundaries of the aluminum-bearing alloy. This suggests that aluminum is a strong repulser of sulfur or nitrogen at the grain boundaries. Additionally, it was found that aluminum has a detrimental effect on grain boundary strength of ferrite.     

用正电子湮没技术研究Zr和Nb在TiAl合金中的行为

测量了ＴｉＡｌ,Ｔｉ５０Ａｌ４８Ｚｒ２和Ｔｉ５０Ａｌ４８Ｎｂ２的正电子寿命谱,并利用正电子寿命参数分别计算了合金基体和缺陷态的自由电子密度。ＴｉＡｌ合金基体的自由电子密度比金属Ｔｉ和金属Ａｌ基体的低,当Ｔｉ和Ａｌ组成ＴｉＡｌ合金时,Ｔｉ原子和Ａｌ原子的部分价电子被局域化,ＴｉＡｌ合金中金属健和共键共存。ＴｉＡｌ合金晶界缺陷的开空间较大,晶界缺陷处的自由电子密度较低,金属键结合力较弱,材料易发生沿晶     

Intergranular cracking susceptibility of 2.25Cr1.3W and 9Cr1MoVNb weld metals at elevated temperatures

The difference in elevated temperature fracture behaviors of 2.25Cr1.3W (P23) and 9Cr1MoVNb (P91) steels has been investigated through simple rupture tests. P91 showed a ductile fracture behavior in the test conditions. This is due to the strong grain boundary strengthening effect of segregated molybdenum. The intergranular fracture behavior observed only in P23 is due to the strong segregation of impurities (especially, phosphorus) and the decrease in bulk content of the grain boundary strengthener, tungsten, which arises from the formation of precipitates containing tungsten.     

EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides were observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample with planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.     

Phase-Field Modeling of Hydrogen Diffusion and Trapping in Steels

Hydrogen embrittlement of steels is directly linked to hydrogen diffusion and trapping in the microstructure,which can hardly be precisely measured by modern experimental techniques.A phase-field model,in which a chemical potential well of hydrogen in the grain boundaries is introduced,is proposed to simulate hydrogen diffusion and trapping in the polycrystal-line iron.It was interestingly found that grain boundaries,as connected trap sites,have a complex influence on the effective diffusivity of hydrogen,which are strongly linked to grain boundary diffusivity and binding energy.     

Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.