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This paper reports the results of a study on the effect of the cooling rate during solidification on the isothermal shear-fatigue life of 60Sn/40Pb solder joints. Solder joints are made with three different initial microstructures by quenching, air-cooling, and furnace-cooling. The test results show that the quench-solidified solder joints have isothermal fatigue lives of about twice long as those of the furnace cooled solder joints tested at 20°C and 65°C with the straining rates of about 10−4 per s. These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate.

The behavior during creep in shear of eutectic indium-tin joints on copper and nickel substrates was examined in order to determine the effect of creep deformation on the microstructure of the alloy. Primary creep was absent in all the samples tested. The stress exponent at 20° C was much higher for samples on nickel than for those on copper, indicating a different creep deformation mechanism. Also, the microstructure within the indium-tin joints differed significantly with a change of substrate. Samples with a nickel substrate showed a colony microstructure that was not observed in those with a copper substrate. Thus, eutectic morphology is important in determining deformation mechanism during creep. The microstructure within the joints in tested and untested regions of the samples was effectively the same. This is quite distinct from the behavior of eutectic lead-tin observed in previous work. Lead-tin formed a recrystallized band of material along a region of concentrated shear deformation. While the deformation pattern in the indium-tin samples appeared to be the same as for lead-tin, the microstructural changes did not occur. This implies that the behavior observed in lead-tin is not universal to eutectic alloys.

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.

The reliability and integrity of pre-tinned copper-clad printed circuit (PC) boards are serious concerns in the manufacture of electronic devices. The factors that influence the wetting during soldering of Cu are discussed. The results suggest that pre-tinning with a Pb-rich solder, such as 95Pb-5Sn, is preferred to pre-tinning with eutectic solder, since the latter can develop exposed intermetallics during aging that wet poorly. The results also confirm that the use of flux leads to carbon contamination in the solder.

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and e phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to e phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when the Sn is consumed; e phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases.

The kinetics of wetting were studied on several different prepared surfaces of copper (Cu) to simulate the microstructure observed in pre‐tinned Cu‐clad printed circuit boards. The results illustrate the effectiveness of pre‐tinning in maintaining the solderability of Cu surfaces. Pre‐tinning with Pb‐rich solder (95Pb‐5Sn) is particularly effective since solderability is preserved even after a relatively long ageing treatment. On the other hand, pre‐tinning with eutectic solder risks the loss of solderability during ageing or baking due to surface exposure of an ε‐phase intermetallic with poor wetting properties. The results also confirm the presence of carbon in pre‐tinned specimens due to the use of flux. The effect of carbon on solderability is not yet known.

The effect of gold (Au) on the reliability of 0.65 mm pitch surface mount solder joints between plastic quad flat packs and Cu‐Ni‐Au FR‐4 printed circuit boards was investigated. Cu‐Ni‐Au is a desirable printed circuit board finish for multi‐chip modules or printed circuit boards that would otherwise require a selective Au finish, for example for edge connectors or wire bondable parts. However, Au is known to embrittle solder when it is present in sufficiently high concentrations, creating a concern that solder joint fatigue life in service will also be adversely affected. This paper reports the results of mechanical shock, mechanical vibration and thermal cycling testing of fine pitch solder joints containing varying amounts of Au. Tests were performed on as‐soldered joints and on joints that had been heat‐treated to evolve the microstructure towards equilibrium. The tests were designed to accelerate in‐service conditions in a typical industrial environment. Under these conditions, the Au concentrations tested did not promote solder joint failures. Microstructural characterisation of the distribution and morphology of the Au‐, Ni‐ and Cu‐Sn intermetallics in the joint before and after accelerated testing was also performed. On the basis of these observations it is recommended that the Au concentration in solder joints between plastic quad flat packs and Cu‐Ni‐Au FR‐4 printed circuit boards not exceed 3.0 wt.%.

A multiphase diffusion model was constructed and used to analyze the growth of the e- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in thee andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the e phase(≈De) is well fit by an Arrhenius expression with D0 = 5.48 × 10−9 m2/s andQ = 61.9 kJ/mole, while that in the η phase (≈Dη) has D0= 1.84 × 10−9 m2/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that thee phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted e-phase thickness is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.

The morphological evolution of coherent inclusions in elastic media is studied in two-dimensions. The inclusions are simple dilations with isotropic surface energy in a system with homogeneous elastic constants of negative anisotropy. The equilibrium sizes at which a circular inclusion transforms to a rectangle or square, and at which a square splits into a doublet or quartet of separated inclusions are computed analytically. A finite-element model is then constructed to simulate the evolution of an arbitrary distribution of inclusions along the minimum-energy path. In the model, the circle evolves into a square, which splits into a doublet by hollowing from its center, or, if this is forbidden, by drawing in a perturbation on its surface. The sizes at which shapes spontaneously transform are compared to the equilibrium values. Finally, the simulation is used to study the evolution of a random distribution of inclusions. The first metastable state assumed by the distribution depends on the elastic interaction, surface energy and areal fraction of the inclusion phase through a single dimensionless parameter that groups these three effects. The results are compared to prior theoretical and experimental work on coarsening patterns in three dimensions.

The grain size and grain growth kinetics in sputter deposited Al-2% Cu films on silicon substrates were determined by TEM for various film thicknesses and anneal times, temperatures and methods. Grain sizes were found to be typically lognormally distributed. The as- deposited grain size (d{sub o}) dependence on film thickness (TH) was found to be d{sub o} = C TH{sup {1/2}}, due to competitive grain growth during film formation. Annealed grain size (d) after Rapid Thermal Annealing (RTA) for time (t) at temperature (T) is described by the general equation d {minus} do = C TH{sup 0.7} {l brace}t exp ({minus}{Delta}E{sub a}/kT){r brace}{sup 1/8}, where {Delta}E{sub a} = 0.85 ev for 0.4 {mu}m films and {Delta}E{sub a} = 1.1 ev for 0.8 {mu}m films. Grain growth is largely saturated for these anneals. Grain growth is shown to be more extensive during RTA anneals than furnace annealing and more extensive in 0.4 {mu}m films than 0.8 {mu}m films for equivalent RTA cycles. The results are discussed in terms of models, simulations and previous results of grain growth in thin metal films. 21 refs., 4 figs.

Ni/Au metallization layers are used with increasing frequency to protect Cu substrates in ball grid array microelectronic packaging. The external Au layer provides oxidation and corrosion resistance during storage prior to assembly, while the intermediate Ni layer acts as a diffusion barrier that inhibits the formation of a thick Cu-Sn intermetallic layer during aging. During soldering with eutectic Pb-Sn, the Au dissolves into the molten solder and forms fine, needle-shaped AuSn{sub 4} intermetallic precipitates that are retained in a dense distribution in the bulk of the solder joint after it has solidified. However, recent research by Mei et al. has revealed a new and potentially problematic phenomenon that seems to be peculiar to the Ni/au metallization. They found that after extensive aging (150 C for 2 weeks in their case), the Au-Sn intermetallic redeposited onto the solder-substrate interface. The reconstituted interface was significantly weakened and failed by brittle fracture along the surface between the redeposited Au-Sn and the Ni{sub 3}Sn{sub 4} layer that formed during reflow. While the interfacial redeposition of Au-Sn intermetallics has been observed previously, the mechanism remains unknown. The present work was undertaken to identify the mechanism of this phenomenon and explore methods for controlling it.

The selection of composition and processing of an iron--nickel--titanium (Fe--Ni--Ti) alloy having a favorable combination of strength and toughness at cryogenic temperatures is discussed. Criteria on alloy composition lead to the choice of low interstitial Fe--/sup 12/Ni--0.25Ti. The alloy has a naturally favorable microstructure, but cryogenic toughness must be imparted through grain refinement. Several alternate procedures are identified; four of these are discussed in detail, along with the resulting tensile properties at 77/sup 0/K, Charpy impact energies at 77 and 5/sup 0/K, and behavior in fracture toughness tests at 77/sup 0/K. Alloys are obtained having yield strengths to 150 ksi with excellent impact toughness to 5/sup 0/K.

A wide range of microstructures were obtained in iron-manganese alloys by varying the manganese and chromium contents. When a body centered cubic ..cap alpha.. structure was produced, increasing amounts of manganese were found to be detrimental to low temperature toughness. At manganese levels greater than 12 percent where appreciable amounts of epsilon and ..gamma.. phases formed, the ductile-brittle transition temperature dropped rapidly. In terms of the epsilon + ..gamma.. phases present, the ductile-brittle transition temperature decreased at a rate of 1.3/sup 0/C/ volume percent epsilon + ..gamma... Increasing the epsilon + ..gamma.. content to achieve good low temperature toughness, however, also caused a decrease in the yield strength. Increases in the yield strength were achieved without appreciable increase in the ductile-brittle transition temperature by greater manganese and chromium additions.

Intrinsic and extrinsic factors that influence the mechanical properties of as welded weldments in an Al–Cu–Li alloy, 2090, have been investigated. Electron beam (EB) and gas tungsten arc (GTA) welds were produced in 2090–T3 and 2090–T8. The tensile properties of as welded and solutionised and aged weldments at 293 and 77 K are presented. The results show that differences in thermomechanical processing between the base metal and the fusion zone lead to inhomogeneities in the weldment microstructure and, consequently, differences in properties. In the as welded condition, the EB and GTA fusion zones lack the strengthening precipitates present in the base metal. As a result, increased base metal constraint on the as welded fusion zone confines the plastic deformation to within the weld and therefore the measured mechanical properties of the weldment are those of the weld material. After equivalent thermal processing, the properties of the EB and GTA weldments are as good as, or better than, those o...

A two-step heat treatment was developed as a means of maximizing the basal texture of Zircaloy-4 sheet. The heat treatment, consisting of a brief beta heat treatment and subsequent controlled cooling, resulted in a Kearns number (fn) of 0.83 with a microstructure of uniformly fine grains and high hardness. The texture component, as determined by the crystallite orientation distribution function (CODF), was (0002)(10−10). The result was discussed in terms of phase transformation, either diffusional or martensitic, incurred by controlled cooling. The rate of stress corrosion cracking (SCC) of the alloy was evaluated with constant deflection tests in an iodine environment. The exponentn of the relationshipda/dt = C · (Kln was found to correlate with fn for KI, ranging from 4 to 17 MPa√m.

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.

The cryogenic mechanical properties of a 12.7-mm (0.5-in.) Vintage III 2090-T81 (Al-3.05Cu-2.16Li-0.11Zr by weight) plate material were studied. The results indicate that the through-thickness anisotropy in the yield strength of Vintage I 2090-T81 is reduced in the Vintage III material by utilizing a partial recrystallization step during the thermomechanical processing but at some cost to the elongation and cryogenic fracture toughness. Sharp discontinuities in the microstructure resulting from this thermomechanical treatment give rise to distinct differences in the variation of the work hardening behavior with temperature between those regions which undergo recrystallization and those which do not. The lower elongations observed in Vintage III 2090-T81 result from a decrease in work hardenability that appears to be associated with increased slip heterogeneity. The reduced cryogenic toughness is due, in part, to the disruption of the monolithic unrecrystallized grain structure previously observed in Vintage I 2090-T81. The results suggest that a fully unrecrystallized structure is beneficial for cryogenic toughness in the longitudinal orientation.

The reliability and integrity of pretinned, copper-clad printed circuit boards are serious concerns in the manufacture of electronic devices. The nature of wetting during the soldering of copper is critical in the reliability of these solder joints. Some preliminary experiments on the solderability of pretinned copper suggest that pretinning with a lead-rich solder, such as 95Pb-5Sn, is preferred to pretinning with a eutectic solder, because the latter can develop exposed intermetallics during aging or baking that wet poorly.

A transmission electron microscopy (TEM) investigation of the electron beam (EB) and gas tungsten arc (GTA) fusion zones of 2090 indicates that in both the as-welded and aged conditions, the EB and GTA fusion zones lack the volume fraction and the homogeneity of strengthening precipitates found in the base metal. In the underaged and peak-aged conditions, the δ′ phase is the primary strengthener, the volume fraction ofT 1 present being too low to be effective. TheT 1, precipitates are found either in the vicinity of other inclusions or at the dendrite boundaries. As the strength increases with postweld aging, the elongation decreases to 1 pct. The presence of the boundary phases and Cu- and Cl-containing inclusions at the boundaries leads to poor elongation. The joint efficiencies of the peak-aged EB and GTA weldments (EBWs and GTAWs, respectively) are 75 and 55 pct at 293 K and 75 and 50 pct at 77 K, respectively. Both EBWs and GTAWs have relatively low elongations.

There is a need in the aerospace industry for structural, superplastic aluminum alloys that are formable at strain-rates greater than 10[sup [minus]3] s[sup [minus]1] in order for the economic benefits of superplastic forming to be realized. The standard, structural, superplastic aluminum alloy in the aerospace industry is 7475, which has an optimum forming strain-rate near 10[sup [minus]4] s[sup [minus]1]. Thus, research has been focused on modifying the microstructures of wrought Al-Li alloys such as 2090 and 8090 into superplastically formable (SPF) microstructures with improved properties, but the results have not been completely successful. Superplastic alloys with high strengths have been produced from the Al-Mg-Sc system. These alloys are strengthened by thermomechanical processing which (1) precipitates small, coherent Al[sub 3]Sc particles and (2) increases the dislocation density of the material. The Mg is in solid solution and improves the work hardening capability of these alloys. Because superplastic forming is carried out at relatively high temperatures, recovery processes eliminate the dislocation strengthening resulting from the rolling and overage the precipitates. Thus, additional precipitation strengthening is required to make these alloys attractive for use in aerospace applications. Lithium provides the most promising choice since it forms the ordered coherent precipitate [delta][prime] (Al[sub 3]Li),more » lowers the density, and increases the stiffness of aluminum alloys. In addition, at low concentrations, Li should be in solution at the SPF temperatures and should have little effect on the superplastic formability of the alloys. This led to research on alloys from the Al-Mg-Li-Sc system. This paper describes the preliminary high-temperature tensile results of an Al-Mg-Li-Sc alloy and relates them to other superplastic Al alloys.« less

Two rhombohedral carbonates, ferroan dolomite (ankerite) and magnesian calcite from lower Jurassic ammonites, have been studied by transmission electron microscopy. The samples show small domains with the morphology of platelets parallel to (11\(\bar 2\)0) which have exceptionally well ordered superstructures and characteristic c-reflections. Diffraction patterns and contrast analysis of atomic resolution images indicate that the domains observed in ankerite are rich in Ca [Ca0.75(Mg, Fe)0.25CO3] and those in calcite rich in Mg, and there is strong evidence that cation ordering is the cause for the observed superstructures which are proposed mainly on diffraction evidence. Rhombohedral carbonates have a close similarity to NaCl, with CO3-groups substituting for Cl. Cation ordering patterns can therefore be discussed in analogy to f.c.c. alloys. We make use of ordering waves to describe superstructures and to qualitatively interpret kinetic conditions for their formation.

One of the primary concerns in the manufacture of advanced microelectronic devices is to ensure that metallic contacts and interconnects do not fail by electromigration or stress-induced voiding. This article discusses the possibility of implementing two kinds of beneficial treatments within normal manufacturing processes—treatments that modify grain structure to minimize failures through diffusional flux divergence and treatments that control solute and precipitate distributions in order to avoid undesirable, overaged microstructures at the end of manufacturing.

This research reports an investigation into the influence of mechanically induced martensitic transformation on the rate of fatigue crack growth in 304-type austenitic stainless steels. Two steels of different composition, 304L and 304LN, were used to test the influence of composition; two test temperatures, 298 and 77 K, were used to study the influence of test temperature; and various load ratios were used to determine the influence of the mean stress. It was found thadecreasing the mechanical stability of the austenite by changing composition or lowering temperature reduces the fatigue crack growth rate and increases the threshold stress intensity for crack growth. However, this beneficial effect diminishes as the load ratio increases, even though increasing the load ratio increases the extent of the martensite transformation. Several mechanisms that may influence this behavior are discussed, including the perturbation of the crack tip stress field, crack deflection, work hardening, and the relative brittleness of the transformed material. The perturbation of the stress field seems to be the most important; by modifying previous models, we develop a quantitative analysis of the crack growth rate that provides a reasonable fit to the experimental results.

Distinct morphologies of electromigration-induced voids and failures are shown for Al, Al-2%Cu, and Al-2%Cu-l% Si narrow (1–6 µm) unpassivated thin film conductors. SEM and TEM images typically show large non-fatal voids and narrow slit-like open circuit failures for all film conditions and accelerated test conditions. Evidence for transgran-ular slit failures is shown for 1.33 µm wide conductors. A simple model for void growth is presented which accounts for the void morphologies seen. The observed morphologies and the results of void growth modelling suggest that slit voids nucleate after other voids and rapidly produce failure. These conclusions are discussed in terms of ‘classical’ models for electromigration failure processes and resistance and noise power monitoring techniques.

The influence of the heat-affected zone (HAZ) thermal cycles on the microstructure and mechanical properties of 2090 Al alloy has been investigated. Emphasis is placed on the effects of the HAZ peak temperatures on the T3 and T8 tempered 2090. The HAZ thermal cycles were simulated using a Gleeble, and tensile tests were conducted at 293 and 77 K. The difference in properties between the HAZ and the base metal is primarily caused by the dissolution of strengthening phases. At 578 K, degradation in strength occurs due to dissolution of δ’ phase and, at 773 K, due to dissolution of T1 phase. The thermal cycle to 648 K leads to an observable growth of T1 phase, but the volume fraction is too low to be an effective strengthener. As a result, at the 293 K test temperature, the base metal (BM) tempers become irrelevant with the peak temperatures above 578 K, and the ultimate tensile strength (UTS) curves of both the BM-T3 and BM-T8 coincide and follow the same trend up to 853 K. At the 77 K temperature, the strength and elongation of the BM and the thermally cycled specimens in the T3 and T8 tempered conditions follow the trend seen at 293 K and exhibit higher strengths and elongations.

The precipitation, dissolution, and reprecipitation processes of Al2Cu (θ phase) in Al-2 wt pct Cu thin films were studied. The films were characterized in the as-deposited condition, after annealing at 425 °C for 35 minutes, and after rapid thermal annealing (RTA) at 345 °C, 405 °C, and 472 °C. In the as-deposited samples, the precipitates had a fine even distribution throughout the thin film both at aluminum grain boundaries and within the aluminum grains. Annealing below the solvus temperature caused the grain boundary precipitates to grow and precipitates within the center of aluminum grains to diminish. Annealing above 425 °C caused the θ-phase precipitates to dissolve. Upon cooldown, the θ phase nucleated at aluminum grain boundaries and triple points in the form of plates.In situ heating and cooling experiments documented this process in real time. Analytical microscopy revealed that there is a depletion of copper at the aluminum grain boundaries in regions free of precipitates. The θ-phase precipitates nucleated and grew at the grain boundariesvia a collector plate mechanism and drew copper from the areas adjacent to the aluminum grain boundaries.

The performance of very large-scale integrated circuits is significantly determined by the reliability of interconnection materials. Electromigration, or diffusion of the interconnection material induced by the high current density used in the connections, gives rise to voids which are a major culprit in device failures. Materials solutions are possible, but the device fabrication processes are more technologically advanced than the performance of interconnection materials. A variety of reliability tests can be conducted, but if these tests are accelerated over real-world conditions, they may introduce new failure modes, producing essentially useless statistics. For advances in device performance to continue efficiently, alloy design, failure analysis, reliability testing and lifetime characterization must be thoroughly applied.

The effect of cellular dislocation substructures on the flow stress of AKDQ steel sheet was studied in interrupted-temperature tensile tests performed at temperatures between {minus}72C and room temperature. Subsequent work hardening from a given stress level was unaltered by the prestrain temperature, although the dislocation cell structure was dramatically altered. It was therefore concluded that the presence of a dislocation cell structure, as seen in TEM, does not dominate tensile work-hardening behavior in this material in a regime defined approximately by temperatures at and below room temperature, and strain rates at and above 1 {times} 10{sup {minus}4}s{sup {minus}1}. In this regime, the stress-strain behavior can be adequately described by a single-structure-parameter model, which implies the existence of a single structural feature, independent of the cell structure, which dominates the flow stress. The extent of development of this parameter is a function of the previous strain history, and the current deformation conditions. At higher temperatures (100C), the behavior is more complex, and a single structural parameter is no longer adequate.

This paper contrasts recent discussions of the theory of strengthening by coherent precipitates. The most important difference is in the assumed condition at maximum hardness. In one treatment, the maximum hardness is reached when dislocations first begin to loop precipitates, and is hence controlled by the largest particles in the precipitate size distribution. In the other treatment, the maximum hardness occurs when precipitates of average size are looped, and is hence controlled by the average precipitate size. More general theories of hardening by distributions of particles of different strength support the former treatment, but are incomplete. The experimental data is ambiguous. Experiments are needed to resolve the issue.

Thermal fatigue failures of solder joints in electronic devices are a great concern in the electronics industry. Since the fatigue load is often in shear the details of thermal fatigue failure in shear are of particular interest. Recent work indicates that similar failure mechanisms operate in both thermal fatigue in shear and unidirectional creep in shear. Additionally, since the operative temperatures during thermal fatigue represent high solder homologous temperatures, creep deformation is certainly involved. These factors and the relative ease of conducting creep experiments encourage the study of solder joints under shear creep conditions. This work presents steady state shear creep rate vs shear stress data for several solder compositions, including the binary eutectic alloy and Pb-Sn alloyed with small amounts of Bi, Cd, In, and Sb, in a joint configuration. These data indicate that conventional creep mechanisms operate in the temperature and shear strain rate ranges studied. Extensive microstructural information is also reported. The microstructural evolution under creep conditions indicates that the instability of the as-cast binary Pb-Sn eutectic microstructure initiates creep failure. Changes of the as-solidified microstructure with the third element addition are reported as are the microstructural responses of each of these alloys to creep deformation. Themore » efficacy of postponing the microstructural instability with the addition of small amounts of ternary elements is discussed. 27 refs., 13 figs., 1 tab.« less

Isothermal fatigue data were collected for the compositions 5Sn--95Pb, 20Sn--80Pb, 40Sn--60Pb, 50Sn--50Pb and 63Sn--37Pb within the binary Sn--Pb system. All of these compositions are commercially available and include those most commonly used. Because Sn-rich solders are rarely used, they were not investigated here. The fatigue life was defined by a 30% load drop. The solders were tested in a double shear configuration joined to copper at 75{degree}C. The displacement rate chosen was 0.01 mm/min, which corresponds to a strain rate of 1.5 {times} 10{sup {minus}4} s{sup {minus}1} for our specimen configuration, over a 10% plastic strain range. Additionally, the microstructural changes during fatigue are presented. The various solder compositions studied exhibit strikingly different as-solidified microstructures. These differences are discussed in terms of their effect on the isothermal joint failure mechanism and joint isothermal fatigue life. 24 refs., 10 figs.

Metastable austenitic steels are often used as structural materials in superconducting magnets, in which they are subject to high magnetic fields at 4.2 K. The most common structural alloy is 304 stainless steel. Recent work on the fracture toughness of 304 indicates that the toughness may change significantly at 4.2 K in a high magnetic field. In this study, an increase in the fracture roughness of 304 specimens tested at 4.2 K in an 8T magnetic field is observed relative to the fracture toughness of specimens tested in 0 T. Part of this increase is ascribed to martensite (α′) transforming ahead of the crack tip during the JIc test.

Publisher Summary This chapter presents initial observations using the novel mechanical testing technique of in situ nanoindentation in a transmission electron microscopy (TEM). Four classes of material systems are studied with this new technique: (1) bulk single crystal, (2) bulk polycrystalline material, (3) a soft thin film on a harder substrate, and (4) a hard thin film on a softer substrate. The direct observations of each material system during indentation provided unique insight into the interpretation of ex situ nanoindentation tests, and to the intrinsic mechanical behavior of nanoscale volumes of solids. The in situ nanoindentation results are presented, which provide the first direct evidence of dislocation nucleation in single crystal silicon at room temperature. In contrast to the observation of phase transformations during conventional indentation experiments, the unique geometry employed for the in situ experiments resulted in dislocation plasticity. The wedge geometry leads to a higher ratio of shear stress to hydrostatic pressure than that which is associated with conventional nanoindentation experiments, and therefore leads to a change in the mode of plastic deformation.

A newly discovered group of alloys, called Gum Metals, approaches ideal strength in bulk form, exhibits significant plastic deformation prior to failure, and shows no indications of conventional-dislocation activity. Two conditions must be met for a material to exhibit this "ideal" behavior: (1) the stress required to trigger conventional-dislocation plasticity in the material must exceed its ideal strength, and (2) the material must be intrinsically ductile when stressed to ideal strength. Gum Metals satisfy both criteria, explaining their remarkable mechanical properties.

The remanence fields of fatigued ferritic steel specimens were measured using a scanning microscope based on a high transition temperature superconducting quantum interference device (SQUID). The results show an overall increase of remanence until dislocation density saturates and an additional local remanence increase after saturation during cyclic loading. Because of the combined magnetic and spatial resolution of the SQUID microscope, these local changes of dislocation structures can be detected before a crack actually initiates, and identify the sites where crack nucleation will occur.

This research concerns the influence of electron current on the diffusion of Sn and Cu in simple Cu-Sn and Cu-Sn-Cu diffusion couples. The diffusion couples are designed to permit in situ studies of the progress of diffusion. Initial tests were done in 60/spl deg/C air with a current density of 1 /spl times/ 10/sup 4/A/cm/sup 2/. The results showed Cu movement into Sn in the direction of the electron current. With accompanying grain boundary sliding of the Sn grains. There is also evidence for Sn flow in the direction opposite the electron current.

In the usual case the strength of a crystalline material is determined by the motion of defects such as dislocations or cracks that are present within it. Materials scientists control strength by modifying the microstructure of the material to eliminate defects or flaws and inhibit the motion of dislocations. There is, however, an ultimate limit to the strength that can be obtained in this way. The mechanical stresses that are not relieved by plastic deformation or fracture are supported by elastic deformation, which is, essentially, the stretching of the interatomic bonds. These bonds have finite strength. There is a value of the stress at which bonding itself becomes unstable and the material must fracture or deform, whatever its microstructure. This elastic instability sets an upper bound on mechanical strength that cannot be exceeded, however creative a scientist may be.

The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (μXRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot “streaking”, which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that μXRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures.

The ideal tensile strengths of the $\mathrm{B}2$-type $(\mathrm{CsCl})$ transition-metal aluminides $\mathrm{FeAl}$, $\mathrm{CoAl}$, and $\mathrm{NiAl}$ have been investigated using an ab initio electronic structure total energy method. The three materials exhibit dissimilar mechanical behaviors under the simulated ideal tensile tests along [001], [110], and [111] directions. $\mathrm{FeAl}$ is weakest in tension along [001] whereas CoAl and NiAl are strongest in the same direction. The weakness of $\mathrm{FeAl}$ along [001] direction is attributed to the instability introduced by the filling of antibonding $d$ states.

Sn-rich solders that have become candidates for use in Ph-free solder joints have anomalous creep hehavior when tested in the form of solder joints. 'Their steady-state creep rates separate into two regimes with different stress exponents (n). The lowstress exponents range from -3-6, while the higlistress exponents are 7-12, unusually hieh values. Strikingly, the high-stress exponent has a strong temperature depeudence near room temperature, increasing significantly as the temperature drops from 95 to 60T The anonidlous creep behavior of the solders seems to be due to the dominant Sn constituent. Pure Sn joints have stress exponents, n, that change with stress and temperature almost exactly like those of the Sti-rich solder joints. These results have the consequence that conventional constitutive relations for steady-state crccp must be used with caution in treating Snrich solder joints, and qualification tests that are intended to verify performance should be carefully designed. In particular, the results suggest a need for rcsearch on the creep properties of Sn, which remain poorly understood.

We have calculated the phonon spectra of aluminum as a function of strain using density functional perturbation theory for $⟨110⟩$, $⟨100⟩$, and $⟨111⟩$ uniaxial tension, as well as relaxed $⟨112⟩{111}$ shear. In all four cases, phonon instabilities occur at points away from the center of the Brillouin zone and intrude before the material becomes unstable according to elastic stability criteria. This is the first time the ideal strength of a metal has been shown to be dictated by instabilities in the acoustic phonon spectra. We go on to describe the crystallography of the unstable modes, all of which are shear in character. This work further suggests that shear failure is an inherent property of aluminum even in an initially dislocation-free perfect crystal.

The Aerodyne Aerosol Mass Spectrometer (AMS) has been designed to measure size-resolved mass distributions and total mass loadings of volatile and semivolatile chemical species in/on submicron particles. This paper describes the application of this instrument to ambient aerosol sampling. The AMS uses an aerodynamic lens to focus the particles into a narrow beam, a roughened cartridge heater to vaporize them under high vacuum, and a quadrupole mass spectrometer to analyze the vaporized molecules. Particle size is measured via particle time-of-flight. The AMS is operated in two modes: (1) a continuous mass spectrum mode without size information; and (2) a size distribution measurement mode for selected m/z settings of the quadrupole. Single particles can also be detected and sized if they have enough mass of a chemical component. The AMS was deployed at a ground sampling site near downtown Atlanta during August 1999, as part of the Environmental Protection Agency/Southern Oxidant Study Particulate Matter “Supersite” experiment, and at a suburban location in the Boston area during September 1999. The major observed components of the aerosol at both sites were sulfate and organics with a minor fraction of nitrate, consistent with prior studies and colocated instruments. Different aerosol chemical components often had different size distributions and time evolutions. More than half of the sulfate mass was contained in 2% of the ambient particles in one of the sampling periods. Trends in mass concentrations of sulfate and nitrate measured with the AMS in Atlanta compare well with those measured with ion chromatography-based instruments. A marked diurnal cycle was observed for aerosol nitrate in Atlanta. A simple model fit is used to illustrate the integration of data from several chemical components measured by the AMS together with data from other particle instruments into a coherent representation of the ambient aerosol.

In situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this article, we detail the development of a novel specimen goniometer that allows real-time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer-scale positioning of a sharp, conductive diamond tip onto the edge of an electron-transparent sample. This allows application of loads to nanometer-scale material volumes coupled with simultaneous imaging of the material's response. The emphasis in this report is qualitative and technique oriented, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.

The ideal strength of a material can be defined as the stress that causes an infinite defect-free perfect crystal to become mechanically unstable. The ideal strength is of interest because it sets a firm upper bound on the mechanical strength the material can attain. It is also approached experimentally in situations where there are few mobile defects. The present paper is concerned with the ideal strength of iron. We specifically compute the ideal tensile strength of iron for tension along , the weak direction, and the ideal shear strength for relaxed shear in the {112} and {110} systems. We also consider the influence of pressure on the strength. The computation is done ab initio using the Projector Augmented Wave Method within the framework of density functional theory and the generalized gradient approximation in order to account for the magnetism of the material. The fact that iron can have a high tensile strength is puzzling. Because Fe has a stable fcc phase (at least at moderate temperature) simple models suggest that it should be very weak in tension on . That it is not turns out to be due to the difference in magnetic character between the bcc and fcc phases, which has he consequence that iron behaves like a “typical” bcc metal in both its tensile and shear behavior.

A scanning microscope based on a high transition temperature Superconducting Quantum Interference Device (SQUID) was used to detect incipient fatigue failure in ferritic stainless steel. The results show the development of localized peaks in remanent magnetization prior to the formation of visible fatigue cracks, that is, in region 2 of fatigue damage. Because of the combined magnetic and spatial resolution of the SQUID microscope, these local changes can be detected well before there is any dramatic change in the overall magnetization of the specimen. An interesting secondary result is that, while the SQUID microscope also detects small cracks, until these have a significant crack opening (more than a few microns) the dominant contribution to the local magnetization peak is the damage associated with the crack (presumably, the damage that created the crack) rather than the crack itself.