Thermal stability of electroless-nickel/solder interface: Part B. Interfacial fatigue resistance

The relationship between fatigue resistance and interfacial microstructure was studied along the interface between the Sn-Pb eutectic alloy and an electroless Ni-P coating. The fatigue resistance of the solder interface was measured from the flexural peel fracture mechanics specimens in the asreflowed, mild-aged, and overaged conditions. While the mild aging had only a marginal effect on the fatigue resistance of the interface, overaging was found to significantly degrade the interfacial resistance to fatigue-crack growth, resulting in a lower fatigue threshold and a much earlier onset of the fast fracture. The effects of the overaging were shown to result from the transformation of the interfacial microstructure, which weakened the crack-sliding resistance in the near-threshold regime and embrittled the interface in the high crack-growth rate regime.     

Thermal stability of the microstructure of silver films

The thermal stability of freely suspended silver films 100 nm thick is studied during isothermal annealing at temperatures of 350–600°C for different times. At temperatures of 350–450°C, only grain growth is observed. Above 450°C, along with grain growth, the formation and growth of hillocks and holes take place; in this case, grain boundaries are essential in the processes. A continuous film transforms into a cellular one. At 500°C, the growth processes of both grains and holes have the same incubation period, during which no grain growth, hole formation, and hole growth take place.     

等温时效对Cu/Sn-3.5Ag/Ni(P)UBM互连焊接件界面微观组织及剪切性能的影响

Sn-3.5Ag焊料与Cu基板及化学镀Ni(P)板通过回流焊接形成Cu/Sn-3.5Ag/Ni(P)UBM互连结构,在200℃下对焊接件进行等温时效,针对电子器件的可靠性评估,研究等温时效对其界面微观组织和剪切性能的影响。用扫描电镜(SEM)和能量色散谱仪(EDS)对接头双界面形成的金属间化合物层的组织结构进行观察和分析,采用力学试验机测试接头的剪切强度,并通过SEM观察分析断裂特征。结果表明:随时效时间延长,焊料基体中二次相Ag_3Sn明显粗化,由小颗粒状转变为细条状;双界面化合物层逐渐变厚,两侧(Cu,Ni)_6Sn_5层的形貌趋于相似;接头剪切强度随时效时间延长而下降,由时效24 h的33.04 MPa降至时效144 h后的24.78 MPa;时效24~120 h后接头的剪切失效均为焊料内部的韧性断裂模式,时效144 h后,断裂模式转变为韧脆混合断裂,部分断裂面在焊料基体内部,部分在焊料与界面形成的化合物层内。     

Al/Sn覆板机械振动复合界面的显微组织研究

借助电子显微技术研究了机械振动作用下两个异种金属复合界面的显微组织变化特征,透射电镜观察和EDX扫描分析表明,Al/Sn复合材料界面结合良好,其微观界面呈现具有过渡特征的结合状态,过渡层分为富A1和富Sn两层不同的组织形态,经历了两次形核结晶而成.     

生物油/生物柴油混合燃料的热稳定性

为了考察生物油/生物柴油混合燃料的热稳定性,分析了该混合燃料在不同的存储温度和存储时间下组分物化性质的变化,并测试了存储前后该混合燃料的黏度、水分含量、酸度、平均分子质量等理化特性.实验结果表明:当存储温度为80℃,存储时间为180 h时,样品的含水量小于0.7%;黏度随存储温度的升高和存储时间的延长变化不大.利用FTIR和GPC分析了样品在存储过程中的化学变化.GPC结果表明,样品的分子质量有所增大,原因可能是在存储过程中发生了聚合或缩合反应.FTIR结果表明,生物油/生物柴油混合燃料中存在O-H伸缩振动、C-H伸缩振动和C=O伸缩振动,这些官能团吸收强度的增加说明在存储过程中发生了酯化、醚化等化学反应.     

钯含量对Ag-Cu-Pd钎料合金组织和性能的影响

通过差热分析、扫描电镜显微组织分析、真空钎焊等手段,对不同Pd含量Ag-Cu-Pd钎料的熔化特性以及在无氧铜和镍上铺展性进行研究,并对铸态组织和钎焊界面组织进行分析.结果表明,随着Pd含量增加,Ag-Cu-Pd钎料的固相线温度、液相线温度有明显提高,固液相线温度间隔也随之增大;当Pd含量为10 %和20 %时钎料铺展性良好,且随着Pd含量增加,钎料合金铺展性降低,但当Pd增加到30 %时,钎料铺展性变差且有轻微侵蚀迹象;铸态Ag-Cu-Pd合金中Pd元素主要存在于富铜相中;Ag-28Cu-20Pd钎料焊接无氧铜板时的钎焊界面,形成连续的金属间化合物（IMC）层,钎料中Pd元素主要分布在IMC层上.      

Characteristics of lath martensite: Part II. The martensite-austenite interface

This investigation, using an Fe-20 pct Ni-5 pct Mn (wt pct) alloy, deals with the nature of the lath martensite-austenite interface. For the first time the misfit dislocation structure associated with a martensite interface has been observed experimentally. The interface consists of a single set of parallel dislocations having Burgers vector α/2[l?1l]_martensite = α/2[011]_austenite. Relative to the austenite, the observed dislocation line direction is [0?57], and the dislocation line deviates about 10 and 15 deg from the pure screw orientation in the austenite and martensite, respectively. However, the dislocations are in screw orientation on an atomic scale, although the interface step structure causes them to deviate from the exact screw orientation macroscopically. The spacing of the interface dislocations varies from 26 to 63Å. The observed interface dislocation array satisfies the requirements for a glissile interface, which suggests that the dislocations are misfit dislocations which accomplish the lattice invariant shear of the crystallographic theories.     

A Transient Thermal Model for Friction Stir Weld. Part I: The Model

Current analytical thermal models for friction stir welding (FSW) are mainly focused on the steady-state condition. To better understand the FSW process, we propose a transient thermal model for FSW, which considers all the periods of FSW. A temperature-dependent apparent friction coefficient solved by the inverse solution method (ISM) is used to estimate the heat generation rate. The physical reasonableness, self-consistency, and relative achievements of this model are discussed, and the relationships between the heat generation, friction coefficient, and temperature are established. The negative feedback mechanism and positive feedback mechanism are proposed for the first time and found to be the dominant factors in controlling the friction coefficient, heat generation, and in turn the temperature. Furthermore, the negative feedback mechanism is found to be the controller of the steady-state level of FSW. The validity of the proposed model is proved by applying it to FSW of the 6061-T651 and 6063-T5 aluminum alloys.     

Microstructural evolution during liquid phase sintering: Part I. Development of microstructure

During liquid phase sintering, numerous solid-solid particle contacts can be generated by particle motion within the fluid. It is shown that, somewhat surprisingly, Brownian motion can produce such contacts. If such contacts are accompanied by particle adherence, the particles can then subsequently fuse into one (i.e., coalesce) by the liquid state analog of the evaporation-condensation mechanism of sintering. An isolated microstructure will develop if the time for particle coalescence is much less than the time between contacts. A highly skeletal arrangement of particles will form under the converse condition. Using these principles, a “microstructure map” is calculated in which the expected morphology of microstructure (i.e., skeletal or isolated) is related to the solid particle volume fraction, the kinetic and thermodynamic parameters affecting particle coalescence, and the frequency of particle contacts by Brownian motion. Some discussion of the thermodynamic and morphological factors affecting the probability of particle adherence after contact is presented.     

Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

TA1/Q235钢累积叠压变形特性及界面组织

采用累积叠轧压方法,研究了TA1/Q235钢累积叠压变形特性及界面组织.研究结果表明:随着总应变的增加,TA1与Q235钢的变形程度的差别增大,当真应变大于1.0时二者的变形差基本上维持在1.0左右.TA1的应变在850~900℃范围内发生突变,并伴随着径向变形差值增大.当变形温度小于850℃时,在Q235钢侧,呈现出非常明显的垂直于压缩方向的变形组织;当复合变形温度为850~950℃时,在Q235钢侧出现了明显的铁素体柱状晶组织,晶粒排列整齐,几乎都垂直于界面,并且随着温度升高,铁素体晶粒粗化.当累积变形量小于1.5、变形温度为850℃时,金属间化合物的厚度为0.7~1μm,当累积变形量为2.0时厚度约1.7μm.当累积变形量为1.0时,在700~850℃条件下压制,界面化合物层厚度变化不大,为0.8~1μm,900℃时其厚度增加了1倍多,950℃时达到约2.3μm.     

The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part II. Recrystallization behavior

The recrystallization behavior of four PM2000 cold flow-formed tubes deformed to levels ranging from 72 to 92 pct has been investigated. It has been found that coarse, high grain aspect ratio (GAR) hoop-oriented grain structures can be produced in Fe-based oxide dispersion strengthened (ODS) alloy tubing by appropriate combinations of flow-forming and subsequent high-temperature annealing; a flow-forming deformation of 89.6 pct produced the coarsest hoop-oriented grain structures in the present study. In the absence of obvious particle alignment, it has been suggested that the formation of hoop-oriented grain structures may be explained by the anisotropic growth of recrystallizing grains into the aligned, anisotropic deformation cell structure observed in the flow-formed tubes.     

Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys

This article describes studies of phase transformations during aging in a variety of model maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was also used during the course of the work. The composition and morphology of precipitates were compared in several maraging systems aged at different temperatures for different times to investigate the aging sequence. The APFIM results are compared with studies by other workers using different experimental techniques. In Fe-Ni(-Co)-Mo model alloys, ω phase and Fe₇Mo₆ μ phase have been found to contribute to age hardening at different stages of aging; no evidence was found for the existence of Mo-rich clusters in the as-quenched Fe-Ni-Co-Mo alloy. In a high-Si Cr-containing steel, Ti₆Si₇Ni₁₆ G phase and Ni₃Ti have been found to contribute to age hardening; reverted austenite was found after aging for 5 hours at 520 °C. In a Mn-containing steel, Fe₂Mo Laves phase and a structurally uncertain phase with a composition of Fe₄₅Mn₃₂Co₅Mo₁₉ have been found to contribute to age hardening. Formerly Graduate Student with the Department of Materials, Oxford University.     

A process model for the microstructure evolution in ductile cast iron: Part I. the model

In the present investigation, the multiple phase changes occurring during solidification and subsequent cooling of near-eutectic ductile cast iron have been modeled using the internal state variable approach. According to this formalism, the microstructure evolution is captured mathematically in terms of differential variation of the primary state variables with time for each of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equivalent volume fractions of constituent phases utilizing the constraints provided by the phase diagram. The results may conveniently be represented in the form of C curves and process diagrams to illuminate how changes in alloy composition, graphite nucleation potential, and thermal program affect the microstructure evolution at various stages of the process. The model can readily be implemented in a dedicated numerical code for the thermal field in real castings and used as a guiding tool in design of new treatment alloys for ductile cast irons. An illustration of this is given in an accompanying article (Part II).     

The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part I. Deformation structure

Oxide-dispersion-strengthened (ODS) alloy tubes with a nominal composition of Fe-19.5 pct Cr-5.5 pct Al-0.5 pct Y₂O₃ have been cold flow formed to deformation levels between 72 and 92 pct. The deformation structure of the tubes has been studied using a variety of techniques including transmission electron microscopy (TEM). The deformation cells produced by flow forming are elongated in both the hoop and axial directions, especially at deformation levels above 80 pct. In this case, most deformation cells can be regarded as ribbons, lying roughly along the “rolling direction,” which for flow forming is a helix around the tube surface. No obvious particle alignment was found in the tubes. Although the initial grain size is submicron, transition bands composed of parallel-sided long deformation cells similar to those in deformed single-crystal specimens have been observed in the transverse section of the tubes and a macroscopic shear band has been observed in the longitudinal section of the 92 pct deformed tube.     

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni₃Ti, Fe₇Mo₆ μ phase, Fe₂Mo Laves phase, ω phase, Ti₆Si₇Ni₁₆ G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe₇Mo₆μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys. Formerly Graduate Student with the Department of Materials, Oxford University.     

A Transient Thermal Model for Friction Stir Weld. Part II: Effects of Weld Conditions

In Part II of this series of articles, the transient thermal model, which was introduced in Part I, is used to explore the effects of welding conditions on the heat generation and temperature. FSW of the 6061-T651 aluminum alloy is modeled to demonstrate the model. The following two steps are adopted to study the influence of welding conditions on the heat generation and temperature. First, the thermal model is used to compute the heat generation and temperature for different welding conditions, the calculated results are compared with the reported experimental temperature, and a good agreement is observed. Second, the analytical method is used to explore the approximate functions describing the effect of welding conditions on the heat generation and temperature. Based on the computed results, we discuss the relationship between the welding conditions, heat generation, temperature, and friction coefficient, and propose a relationship map between them for the first time at the end.