固溶温度对挤压态Mg-13Al-6Zn-4Cu合金组织与性能的影响

采用不同的固溶温度对挤压态Mg-13Al-6Zn-4Cu(质量分数,%)合金进行热处理,然后在(150℃/10 h)条件下进行时效处理,通过金相显微镜、扫描电镜及能谱分析、维氏硬度与极化曲线测试,研究固溶温度对挤压态合金显微组织、硬度与腐蚀性能的影响。结果表明:固溶处理促进晶界处的β-Mg_(17)Al_(12)相充分溶入α-Mg基体中。提高固溶温度使基体晶粒再结晶长大,逐渐缩小T-MgAlCuZn相心部的Cu元素富集区,改变β析出相的形态和分布,促进层片状β相在α-Mg晶界析出,从而提高时效态合金的硬度。但固溶温度超过420℃时,合金晶粒粗化并发生过烧。固溶温度升高导致合金腐蚀电位负移,腐蚀电流增大,腐蚀速率加快。     

表面纳米化工业纯钛组织性能研究

采用表面机械研磨处理(SMAT)实现工业纯钛的表面纳米化,利用金相显微镜(OM)、显微硬度计对其微观组织及显微硬度进行表征、测定;利用动电位极化曲线和电化学阻抗谱(EIS),结合X射线光电子能谱分析(XPS)技术,研究了表面纳米化工业纯钛在Hank's人工模拟体液中的腐蚀行为。结果表明:SMAT处理工业纯钛表层晶粒尺寸达到纳米级,从表层到基体晶粒尺寸由小到大呈梯度分布,其表层显微硬度明显提高,并由表层逐步降低至基体硬度。SMAT处理工业纯钛的自腐蚀电位较原始工业纯钛正移,腐蚀电流密度较原始工业纯钛低,其EIS高频容抗弧半径较原始试样大,表现出更稳定的钝化现象。表面纳米化过程中产生的高密度晶界和位错为Ti~(4+)提供了更多的扩散通道,有助于表面产生稳定的保护钝化膜,提高其耐蚀性。XPS分析结果表明,SMAT处理工业纯钛钝化膜的主要成分为Ti O_2。     

Ni-30Cr-1Cu合金晶粒度及晶界沉淀与脆性的关系

用晶界萃取复型技术研究了晶界一次相极少、二次相析出倾向很大的Ni-30Cr-1Cu合金的晶界沉淀规律.采用不同温度固溶后盐水快速冷却以抑制晶界二次相的析出,获得了晶界沉淀状态相近的不同尺寸晶粒.结果表明,随单纯晶粒长大,合金的室温冲击韧性、拉伸塑性均有不同程度的升高.于1180℃固溶后以不同速度冷却,结果显示随冷却速度的减慢,晶界M23C6显著增多、分布密度增大;其沉淀形态分别由空冷的精细薄片状向缓冷的颗粒状、棒状、薄膜状乃至枝晶状转变,使晶界在室温脆化,冲击韧性和拉伸塑性显著下降;断裂方式由以穿晶为主的混合型断裂向沿晶脆性断裂转变.     

Ni-30Cr-1Cu合金晶粒尺寸及晶界沉淀与脆性的关系

用晶界萃取复型技术研究了晶界一次相极少、二次相析出倾向很大的Ni-30Cr-1Cu合金的晶界沉淀规律。采用不同温度固溶后盐水快速冷却以抑制晶界二次相的析出获得了晶界沉淀状态相近的不同尺寸晶粒，结果表明，随单纯晶粒长大，合金的室温冲击韧性、拉伸塑性均有不同程度的升高。于1180℃固溶后以不同速度冷却，结果显示了随冷却速度减慢，晶界M23C6显著增多、分布密度增大；其沉淀特征分别由空冷的精细薄片状向缓冷的颗粒状、棒状、薄膜状乃至枝晶状转变，使晶界在室温脆化，冲击韧性和拉伸塑性显著下降；断裂特征由以穿晶为主的混合型断裂向沿晶脆性断裂转化。     

退火时间对FeCuSiBAl纳米晶/非晶复合材料的纳米压痕力学行为的影响

采用充氩气的电弧熔炼加单辊甩带法制备一种FeCuSiBAl非晶合金,然后在831K下恒温退火,研究退火时间对非晶合金的显微组织与纳米压痕力学行为的影响。结果表明,FeCuSiBAl非晶合金退火时在非晶基体中产生纳米晶,纳米晶化程度随退火时间延长而增加。在60min退火时间内,合金的硬度随退火时间延长而增大,退火时间超过60min后,硬度开始下降。退火以及降低载荷都导致合金的弹性模量增大。     

细晶W-15Cu材料的制备研究

利用机械合金化工艺制备了颗粒尺寸为0．5μm、W晶粒尺寸为10am的纳米晶W-Cu粉末，研究了其烧结致密化行为、烧结合金显微组织及其性能，并考察了其晶粒长大特性。结果表明：烧结温度和时间的增加有利于致密化，1300℃烧结30min，合金取得了99％的相对密度；硬度随烧结温度、时间的增加而增加，在1375℃烧结30min，硬度为HB321。W晶粒随着烧结温度的升高而增大，1200℃烧结30min获得了相对密度为98％以上、硬度为HB264、晶粒尺寸约为350am的细晶W-Cu合金。     

高能球磨法对TiAl基合金表面的纳米化改性

采用常规高能球磨机对TiAl基合金进行表面纳米化改性。利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)和X射线衍射分析仪(XRD)对试样表层的微观结构和相组成进行观察分析,研究表面纳米化过程中表层晶粒细化的机制;并采用纳米压痕仪测定试样表层的显微硬度,研究表面纳米化改性对合金表面性能产生的影响。结果表明:高能球磨技术能够实现TiAl基合金表面的纳米化改性。改性后试样表层晶粒尺寸约为10 nm。晶粒主要通过孪晶交割和位错缠结重组进行细化;表面显微硬度提高至920 HV,约为未处理试样的2.8倍。     

DC铸造7075铝合金微观偏析的量化分析

采用直接水冷半连续铸造(DC铸造)工艺制备7075铝合金铸锭.利用电子探针(EPMA)观察合金元素在样品中的分布,并通过绘制元素分布图判断合金元素在基体中的分布趋势.由于合金相的形成,Zn,Mg和Cu元素易在晶界偏聚；Cr和Ti作为形核质点,易在晶粒中心偏聚.利用5种排序方法处理收集到的电子探针数据,分别绘制成分曲线....     

对Ni3Al晶界脆性的再思考

对Ｎｉ３Ａｌ晶界脆性的近期研究进展表明，环境因素是造成脆化的主要原因。本文在综合分析文献资料的基础上指出，Ｎｉ３Ａｌ对环境因素的敏感性与其晶界结构密切相关。在Ａｌ含量略低于定比时，晶界存在纳米级的富Ｎｉ区，形成夹在Ｎｉ３Ａｌ有序点阵中的晶界无序区，它是此类合金对环境脆化高度敏感的内在原因。     

单一纳米及纳/微米SiC混合颗粒增强铝基复合材料研究

通过真空热压工艺制备了单一纳米及纳/微米SiC混合颗粒增强的Al-Si复合材料,研究了SiC颗粒的加入对复合材料的组织、致密度及硬度的影响。结果表明:纳米SiCp/Al-Si复合材料与基体合金相比晶粒细化,随着纳米SiC含量的增加,纳米SiCp/Al-Si复合材料的硬度、致密度都是先增大后减小,当纳米SiC含量为3%时硬度取得最大值64.4HV,较基体材料提高了28.8%;用扫描电镜对纳/微米SiCp/Al-Si复合材料的组织、形貌进行观察,发现微米SiC颗粒与基体合金结合紧密,界面无明显反应物生成。纳米SiC含量为3%时,随着微米SiC含量的增加,纳/微米SiCp/Al-Si复合材料的硬度、致密度都是先增大后减小,当增强颗粒含量为3%SiCnm+15%SiCμm时硬度取得最大值76.7HV,较基体材料提高了53.4%。     

Tempered martensite embrittlement and intergranular fracture in an ultra-high strength sulfur doped steel

This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However, some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement.     

Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-1Mo Steel

 The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 225Cr-1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 225Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a de-embrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. As a result, some stages of step-cooling test would change into a de-embrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or de-embrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve.     

High temperature ductility loss in carbon-manganese and niobium-treated steels

The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation. Formerly Graduate Student, Lehigh University     

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.     

Hydrogen induced grain boundary fracture in high purity nickel and its alloys—Enhanced hydrogen diffusion along grain boundaries

Embrittlement of nickel by solute hydrogen is usually accompanied by a change in fracture mode from ductile rupture to an intergranular mode. When hydrogen is supplied at the external surface, the kinetics of this embrittlement is shown to be more rapid than can be accounted for by lattice diffusion of hydrogen. The embrittlement kinetics are shown to be consistent with hydrogen diffusion along grain boundaries and the grain boundary diffusivity is derived over the temperature range 274–314 K. The effects of carbon and sulfur grain boundary segregation on the kinetics of grain boundary embrittlement were also investigated. Segregation of carbon decreases the extent of embrittlement while sulfur segregation increases the amount of embrittlement relative to that observed in pure nickel. These effects are interpreted in terms of the effects of segregated solutes on hydrogen grain boundary diffusivity and on the critical hydrogen concentration for intergranular fracture.     

The Influence of Alloying Elements on Impurity Induced Grain Boundary Embrittlement

A nonequilibrium thermodynamic model which describes the effect of solute grain boundary segregation on grain boundary cohesion was extended to Fe ternary systems. The extended model directly and simply predicts the effect of alloying elements on impurity-induced grain boundary embrittlement. According to the extended model, Mo, W, and Zr strongly reduce, Ni, Ti, and V slightly reduce, and Cr and Mn enhance impurity-induced grain boundary embrittlement in an Fe ternary system. For the evaluation of the extended model, Fe-P, Fe-P-Mn, Fe-P-Mo, and Fe-P-W alloys were studied by Auger electron spectroscopy, scanning electron microscopy, 4-point slow bend tests, and tension tests. The experimental results show that for a given amount of P grain boundary segregation the grain boundary strength increases with increasing Mo or W grain boundary segregation and decreases with increasing Mn grain boundary segregation. These experimental results showing the remedial effect of Mo or W and the embrittling effect of Mn on P-induced grain boundary embrittlement are consistent with the predicted results from the extended model. The nonequilibrium model is also used to evaluate impurity-induced interfacial embrittlement in continuous fiber metal matrix composite materials.     

Effects of impurities in steels on mechanical properties and corrosion behaviour

In steels produced and utilized in the Fed. Rep. of Germany the elements P and Sn may occur as impurities. Both these elements tend to enrich (segregate) at grain boundaries. The equilibria of grain boundary segregation in iron and the effects of alloying elements have been studied for P and Sn by Auger-electron-spectroscopy and were thermodynamically described. For a 3.5% NiCrMoV-turbine steel the grain boundary segregation of P and its effect on ductility have been studied in detail, with the results that the long-term embrittlement of this steel during application at temperatures around 400°C can be predicted and the maximum bulk concentration of P can be given. The effect of Sn on the creep of a 1% CrMoNiV steel at 550°C has been investigated, Sn favours cavity nucleation and growth, therefore tertiary creep starts earlier and premature failure occurs with increasing Sn content. Therefore, the Sn content should be kept as low as possible in heat resistant steels. Since carbon also segregates to grain boundaries and can displace P and Sn if there is enough free C in a steel, plain carbon steels are not subjected to embrittlement by P and Sn. The susceptibility to intergranular stress corrosion cracking in nitrates and other electrolytes is somewhat enhanced by P, however, only in a restricted range of potentials. In the range of maximum susceptibility the impurities have no effect, all carbon steels are susceptible to IGSCC, independently of their purity. So stress corrosion cracking cannot be suppressed by diminishing the content of phosphorus – only by avoiding the critical corrosion conditions concerning electrolyte and potential.     

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony-induced embrittlement kinetics in the nickel-chrominm steel. Based on these, the activation energy and frequency factor of diffusion of antimony-vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.     

The effect of fe additions on the embrittlement of Cu-Bi alloys

The effect of iron additions on the embrittlement of Cu-Bi alloys was studied by monitoring the ductility and grain boundary chemistry of embrittled specimens as a function of iron content. Mechanical properties improved for the same embrittling heat treatment as the bulk iron level increased, and this was correlated with a decrease in bismuth segregation to the grain boundaries. No iron was detected segregated to the boundaries, and several possible mechanisms were proposed to explain the beneficial effect of the iron additions. It was also found that approximately 70 min at 530°C is required to attain equilibrium for segregation of bismuth to the grain boundaries, and a diffusion coefficient derived from this data was found to be reasonable for bulk diffusion of bismuth in copper.     

The effect of fe additions on the embrittlement of Cu-Bi alloys

The effect of iron additions on the embrittlement of Cu-Bi alloys was studied by monitoring the ductility and grain boundary chemistry of embrittled specimens as a function of iron content. Mechanical properties improved for the same embrittling heat treatment as the bulk iron level increased, and this was correlated with a decrease in bismuth segregation to the grain boundaries. No iron was detected segregated to the boundaries, and several possible mechanisms were proposed to explain the beneficial effect of the iron additions. It was also found that approximately 70 min at 530°C is required to attain equilibrium for segregation of bismuth to the grain boundaries, and a diffusion coefficient derived from this data was found to be reasonable for bulk diffusion of bismuth in copper.