In-Situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy

in-situ transmission electron microscopy (TEM) tensile tests on as-cast and aged 63Sn37Pb solder alloys were conducted, and the fracture behavior in nanometer scale ahead of the crack tip was inspected and discussed. Results show that the fracture was completed by connecting the discontinuous cracks or voids. Dislocation behavior was concentrated along the grain boundaries for as-cast samples, and displayed mainly as dislocation climb. The crack was intergranular dominated under the lower strain rate. While remarkable mutual dislocation emission was detected in the aged solder. Transgranular cracks were dominant in the fractured area, and they propagated by linking up with the nanometer scale cracks ahead of the crack tips under the effective promotion of the inverse dislocation emission. At the same time, the partial interphase or intergranular cracks in the thinned area were also found. Under this condition, a new critical stress intensity factor K _c ^′ to define the mutual dislocation emission was proposed.     

In-situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy

In-situ transmission electron microscopy (TEM) tensile tests on as-cast and aged 63Sn37Pb solder alloys were conducted, and the fracture behavior in nanometer scale ahead of the crack tip was inspected and discussed. Results show that the fracture was completed by connecting the discontinuous cracks or voids. Dislocation behavior was concentrated along the grain boundaries for as-cast samples, and displayed mainly as dislocation climb. The crack was intergranular dominated under the lower strain rate. While remarkable mutual dislocation emission was detected in the aged solder. Transgranular cracks were dominant in the fractured area, and they propagated by linking up with the nanometer scale cracks ahead of the crack tips under the effective promotion of the inverse dislocation emission. At the same time, the partial interphase or intergranular cracks in the thinned area were also found. Under this condition, a new critical stress intensity factor K′_C to define the mutual dislocation emission was proposed.     

Deformation inhomogeneity and representative volume in Pb/Sn solder alloys

To verify whether the deformation of the bulk test specimens represents that of small solder joints, we have used the concept of “representative volume.” The representative volume is defined as the smallest volume which represents the average inelastic and inhomogeneous deformation of the entire body. We have performed the first set of experiments to determine this representative volume by measuring the microscopic deformations of a large Pb/Sn eutectic alloy specimen as a function of applied strain. The results show that the representative volume of the specimen is very large compared to the size of the solder joints. Formerly with IBM T.J. Watson Research Center, Yorktown Heights, NY 10598     

Morphology and kinetics of cellular dissolution of the Pb−Sn alloy

Dissolution of lamellar precipitates by cell boundary migration in a Pb-5.5 at. pct Sn alloy has been studied in the temperature range between 44° and 83°C by optical microscopy. Morphological observation shows that it is a cellular mode of transformation. The cell boundary, acting as a short circuit path of diffusion, recedes, thereby dissolving the precipitates. The orientation of the transformed matrix is found to follow the same relationship as that in cellular precipitation. Measurements of the rate of dissolution are in agreement with a kinetic analysis that assumes a boundary diffusion limited process. A critical temperature, defined as the temperature at which the receding boundary ceased moving, was found to exist slightly below the solvus temperature of the alloy. By balancing driving forces at both sides of the cell boundary at the critical temperature, we evaluated the upper limit of the interlamellar surface tension of Pb?Sn alloy as 232 erg per sq cm. This value is six to ten times smaller and is believed to be more realistic than those values calculated from the precipitation kinetics.     

The microstructural details of β-Sn precipitation in a 5Sn-95Pb solder alloy

Solders of Pb-rich compositions, such as 5Sn-95Pb, are commonly used in electronic packaging applications, and this demanding use necessitates that the microstructure-property-processing relationships of the solder be understood fully. In this study, the microstructure of 5Sn-95Pb solder was characterized using a variety of metallographic techniques. The effect of cooling rate on the precipitation of β-Sn from supersaturated α-Pb was determined. On slow cooling, β-Sn precipitates discontinuously with resultant β-Sn lamella alternating with the α-Pb. Upon rapid cooling, the β-Sn precipitates with a nominally homogeneous distribution. These discrete precipitates were found to have a platelike shape with a (111)_Pb habit plane and an orientation relationship of (111)_Pb‖(010)_Sn and [011]_Pb‖ [001]_Sn. Regions of the β-Sn precipitates that curved away from this habit plane formed ledges. Upon heating, the precipitates were found to dissolvevia a ledge mechanism. Prolonged aging of both the fast and slow cooled 5Sn-95Pb resulted in a coarsening of the β-Sn precipitates with a resultant decrease in strength. Furthermore, the strength of the aged alloy was observed to be independent of cooling rate. D.R. FREAR, formerly Graduate Research Assistant, Lawrence Berkeley Laboratory     

Damage effect on the fracture toughness of nodular cast iron: Part-II. Damage zone characterization ahead of a crack tip

In order to understand the fracture toughness of nodular cast iron, the damage zone was studied by Scanning Electron Microscope (SEM) observations of the polished surface of a CT 25 specimen before and after ductile tearing. Damage is defined as decohesion at the graphite/matrix interface. It is shown that the damage zone is very large in nodular cast iron (almost throughout the whole remaining ligament ahead of the crack tip), so linear elastic fracture mechanics (LEFM) are not valid for small specimens. The size of the damage zone was calculated analytically by introducing a damage initiation criterion which was based both on observations of the debonding of the interface between matrix and graphite nodules and on measurements of the pressure sensitivity of cast iron. To take into account the actual boundary conditions, the damage zone was also calculated by numerical modeling using the modified Gurson’s model and by considering the nodular cast iron as a porous material. The calculated results led to good agreement with the damage zone observations. Plane stress and plane strain calculations yielded nearly the same size plastic zone. This result is opposite to those obtained for fully dense materials.     

The equilibrium concentration of hydrogen atoms ahead of a mixed mode I-Mode III crack tip in single crystal iron

Calculations of the equilibrium hydrogen concentration profiles about a mixed mode I-mode III crack in single crystal iron were performed. Both material anisotropy and the tetragonal nature of the distortion induced in the iron crystal structure by interstitial hydrogen were incorporated. Results show that, unlike the case of a spherical distortion, a strong coupling exists between the strain field of the interstitial hydrogen and the stress field of the crack for orientations of the crack plane that are not coincident with the cube axes of the lattice. As a result, the predicated enhancement of hydrogen in the crack tip region increases with increasing levels of mode III loading for those orientations. The results may help reconcile conflicting observations concerning the potential role of shear stresses in hydrogen embrittlemet and preferential cracking of grains ahead of loaded crack tips in sustained load cracking experiments.     

62Sn36Pb2Ag组装焊点长期贮存界面化合物生长动力学及寿命预测

Sn基合金焊接接头是电子产品不可或缺的关键部位,是实现电子元器件功能化的基础,电子整机失效往往由于焊点的损伤所导致,焊点的寿命预测对电子产品的可靠性研究具有重要意义.金属间化合物（IMC）厚度是衡量焊点质量的重要参数,以IMC层厚度为关键性能退化参数,以62Sn36Pb2Ag组装的小型方块平面封装（QFP）器件焊点为研究对象,采用扫描电子显微镜对在94、120和150°C三种温度贮存不同时间后的焊点微观形貌进行表征,测量了IMC层的厚度,基于阿伦尼乌斯方程建立了双侧界面金属间化合物生长动力学模型.并以其作为关键性能退化函数,通过对初始IMC厚度进行正态分布拟合获得失效密度函数,进而获得可靠度函数对焊点的长期贮存失效寿命进行了预测.研究结果有望对长期贮存焊点的寿命预测方式提供新的思路,为62Sn36Pb2Ag钎料的可靠应用提供试验和数据支撑.     

The effect of microstructure on the fatigue crack propagation behavior of an Al-Zn-Mg-Cu alloy

The effect of slip distribution on the fatigue crack propagation behavior in vacuum of a high purity Al-5.9Zn-2.6Mg-l.7Cu alloy in various age-hardened conditions has been investigated. The crack propagation resistance was observed to be significantly higher for underaged microstructures containing shearable precipitates in comparison to overaged conditions with nonshearable precipitates. The improved crack propagation resistance is attributed in part to an increased amount of reversed slip in the plastic zone at the crack tip due to a higher degree of planar slip for conditions with shearable precipitates. The observed increase in fatigue crack propagation resistance with decreasing precipitate size for microstructures containing a constant volume fraction of shearable precipitates cannot be explained on the basis of such slip reversibility alone. The variation in ductility for the different microstructures has also to be taken into account. It was found that the enhanced crack propagation resistance can be correlated to the increased ductility with decreasing precipitate size. This explanation was supported by the experimental observation that microstructures containing different volume fractions and sizes of shearable precipitates but exhibiting the same ductility showed approximately the same resistance against fatigue crack propagation. formerly with German Aerospace Research Establishment (DFVLR), Cologne, Germany. formerly with Ruhr-University, Bochum, Germany.     

Concerning the role of mechanical twinning on the stress-strain behavior of Cu-4.9 At. pct Sn alloy

The effects of mechanical twinning on both the tensile and compressive stress-strain behavior of Cu-4.9 at. pct Sn were investigated at 77, 193, and 298 K using polycrystalline specimens with a mixed <111>-<100> wire texture. In tension, twinning occurred mainly in the <111> structure component while in compression it occurred preferentially in the <100> component in agreement with the relevant Schmid factors for twinning. The volume fraction of twins was larger in the compression than in the tension specimens. The stressstrain curves in tension and compression differed significantly with the compression curves lying below the tensile curves. This difference can be rationalized in terms of the effect of twinning on the stress-strain behavior. The start of twinning is accompanied by a decrease in the work hardening rate, since at low strains twins tend to be parallel in any given region of the structure. At larger strains the tendency for twins to form on intersecting planes should act to increase the flow stress due to grain subdivision. Microhardness data obtained from twinned and untwinned regions of compression specimens support this hypothesis.     

Formation of Deformation Textures in Face-Centered-Cubic Materials Studied by In-Situ High-Energy X-Ray Diffraction and Self-Consistent Model

The evolution of deformation textures in copper and α brass that are representative of fcc metals with different stacking fault energies (SFEs) during cold rolling is predicted using a self-consistent (SC) model. The material parameters used for describing the micromechanical behavior of each metal are determined from the high-energy X-ray (HEXRD) diffraction data. At small reductions, a reliable prediction of the evolution of the grain orientation distribution that is represented as the continuous increase of the copper and brass components is achieved for both metals when compared with the experimental textures. With increasing deformation, the model could characterize the textures of copper, i.e., the strengthening of the copper component, when dislocation slip is still the dominant mechanism. For α brass at moderate and large reductions, a reliable prediction of its unique feature of texture evolution, i.e., the weakening of the copper component and the strengthening of the brass component, could only be achieved when proper boundary conditions together with some specified slip/twin systems are considered in the continuum micromechanics mainly containing twinning and shear banding. The present investigation suggests that for fcc metals with a low SFE, the mechanism of shear banding is the dominant contribution to the texture development at large deformations.     

Roles of In-Situ Forming Hard Particles in a Zr-Based Bulk-Metallic Glass during Monotonic and Cyclic Loading

Zr₆₇Ni₂₀Cu₅Al₈ (in at. pct) is a bulk-metallic glass (BMG) with in-situ forming hard particles. Some crystalline particles can be observed in the polished beam samples under optical microscopy (OM). The hardness measurements show that these crystalline particles are harder than the matrix of Zr₆₇Ni₂₀Cu₅Al₈. The energy-dispersive spectroscopy (EDS) analyses demonstrate that there is little difference in the compositions between the crystalline particles and the matrix. The Zr₆₇Ni₂₀Cu₅Al₈ BMG under compressive loading exhibits yield strength of 1580 MPa with an up to 7.7 pct fracture strain. The fatigue behavior of Zr₆₇Ni₂₀Cu₅Al₈ is investigated under four-point-bending loading. The fatigue results show that the fatigue limit of Zr₆₇Ni₂₀Cu₅Al₈ is approximately 361 MPa, based on the stress range. It is generally found that the fatigue cracks initiate from the hard particles. The influence of the hard inclusions on the fracture and fatigue behavior of Zr-based BMGs is discussed.     

Influence of atomising gas pressure on 63A solder powder in supersonic atomisation

Abstract

The atomising gas pressure is one of several important process parameters that affect the characteristics of the powder particles. The work analyses qualitatively the influence of the atomising gas pressure on 63A solder alloy fine powders. Also studied was the mechanism of the effect of the atomising pressure on the base of the air dynamics through the atomising 63A solder alloy experiment with different atomising pressures on the supersonic nozzle. The results indicate that 63A solder alloy fine powders may be attained, which can satisfy the SMT application requirement when the atomising gas pressure is at 0.7 MPa.     

Diffusion of63Ni in the intermetallic B82-alloys in the systems Ni/Sn and Ni/In

The self-diffusion of⁶³Ni has been investigated in single crystals of the ordered intermetallic compounds Ni₆₁Sn₃₉, Ni₆₂Sn₃₈ and Ni₆₄In₃₆. The activation energies for diffusion perpendicular and parallel to the hexagonal c-axis are nearly the same and the values are between 2.14 and 2.34 eV. Moreover the D₀-values are direction dependent and range from 0.5 to 0.8 cm²/s for Ni/Sn and from 65 to 110 cm²/s for Ni/In. The experimental values of D_⊥/D_∥ are between 1.13 and 1.68 for Ni/Sn and between 0.87 and 0.79 for Ni/In, depending on temperature. These values are to be compared with theoretical values for the different diffusion mechanisms. The majority mechanism should be a site change of the tracer atom between the Ni-chains in c-direction and the double tetraeder interstices (DTI) with a further jump to another chain. Moreover two other mechanisms as minority mechanisms are possible. With rising temperature and filling the DTI's the part of minority mechanisms increases.     

Effect of phase morphology on fatigue crack growth behavior of α-β titanium alloy—A crack closure rationale

Effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an α-β titanium alloy. Fatigue crack growth tests with on-line crack closure measurements are performed in the microstructures varying in primary α (elongated/equiaxed/Widmanstätten) and matrix β (transformed/metastable) phase morphologies. The microstructures comprising metastable β matrix are observed to yield higher FCG resistance than those for transformed β matrix, irrespective of primary α phase morphology (equiaxed or elongated). But, the effect of primary α phase morphology is dictated by the type of β phase (transformed or metastable) matrix. It is observed that in the microstructures with metastable β matrix, the equiaxed primary α as second phase possesses higher FCG resistance as compared to that of elongated α morphology. The trend is reversed if the metastable β matrix is replaced by transformed β phase. The fatigue crack path profiles are observed to be highly faceted. The detailed fractographic investigations revealed that tortuosity is introduced as a result of cleavage in α or β or in both the phases, depending upon the microstructure. The crack closure concept has been invoked to rationalize the phase morphology effects on fatigue crack growth behavior. The roughness-induced and plasticity-induced crack closure appear to be the main mechanisms governing crack growth behavior in α-β titanium alloy.     

A study of crack tip blunting and the influence of blunting behavior on the fracture toughness of ultra high strength steels

Cross-sections of strained but not fractured compact tension J_IC speciments have been examined to investigate crack-tip blunting behavior as a function J level for four different microstructures. The microstructures were the as-quenched microstructures of HP9-4-20 and HO9-4-10 steels and the microstructures obtained by tempering these steels at 565°C. Smooth blunting was observed for the as-quenched microstructures while the fatigue cracks for tempered microstructures blunted to geometries characterized by two or three corners or vertices. The blunting geometries were clearly defined at J levels well below J_IC. For the case of smooth blunting voids tended to form directly ahead of the crack tip and crack extension by fracture occurred when the ligament between the blunting crack tip and the void directly ahead of the crack tip failed by shear fracture at an angle of about 45°C to the plane of the crack. Blunting to vertices was characterized by the growth of large voids very close to the corners or vertices of the blunting rack tip; it appears that the blunting geometry was maintained by the coalescence of these voids with the blunting crack tip. The results further suggest that if two microstructures have the same constrained ductility and identical inclusion distributions and one blunts smoothly and the other to vertices the microstructure which blunts to vertices can have substantially higher toughness.     

温度与应力比对6061铝合金疲劳裂纹扩展行为的影响

为了研究温度与应力比对航空铝合金疲劳裂纹扩展行为的影响,利用电液伺服疲劳试验机对6061铝合金材料开展了不同温度(室温、-70、150 ℃)、应力比(0.1、0.5)条件下的疲劳裂纹扩展速率试验,获得不同条件下的疲劳裂纹扩展速率曲线,揭示温度与应力比对疲劳裂纹扩展的影响规律。结果表明,在相同应力比下,室温与高温150 ℃下的疲劳裂纹扩展速率曲线(da/dN-ΔK)基本一致,低温-70 ℃下的疲劳门槛值与疲劳裂纹扩展速率明显提高,这表明低温环境下6061铝合金材料具有较高的抗疲劳裂纹扩展性能;在相同温度下,随着应力比的增大,疲劳门槛值降低,疲劳裂纹扩展速率升高。讨论了温度与应力比对疲劳裂纹扩展行为影响的可能原因。