High Homologous Temperatures Research Articles

Suppression of microstructural coarsening and creep deformation in a lead-free solder

Bi-43% Sn eutectic solder [melting point (m.p.)∼139 °C] undergoes extensive microstructural coarsening and severe creep deformation under stresses at temperatures near 100 °C. It is shown that a relatively uniform dispersion in the solder of essentially insoluble particles such as iron can be accomplished by use of a magnetic field, and that the presence of these dispersoid particles significantly reduces microstructural coarsening and resultant accelerating tertiary creep rates. The useful service ranges of this Bi-Sn lead-free solder can thus be raised to much higher homologous temperatures, allowing its use at or exposure to elevated temperatures (near 100 °C) which are typical for the widely used, lead-containing, eutectic Pb-63-Sn solder alloy (m.p.∼183 °C), but with a greatly reduced creep rate.

Progress in the design of new lead-free solder alloys

Various alloy design approaches have been employed to develop new lead-free solder alloys that can not only substitute for the lead-tin solders, but also offer significantly improved mechanical properties. Three new alloys are described in this article. In Sn-3.5Ag-1Zn (melting point ~217°C), the solidification structure and the eutectic precipitate morphology are6 refined by the addition of zinc. As a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. In Bi-43Sn+2.5Fe, a eutectic alloy (melting point ~137°C), dispersion hardening by magnetically distributed iron particles retards both high-temperature deformation and microstructural coarsening, thus widening the useful service range of Bi-Sn eutectic alloys to much higher homologous temperatures than are typical for the Sn-Pb eutectic alloy. Lastly, Sn-Zn-In based alloys (melting point ~185°C) have been developed for consideration as a drop-in replacement for the neareutectic Sn-Pb alloy(melting point ~183°C).

Thermo-elasto-viscoplasticity of isotropic porous metals

A rate and temperature dependent elastic-plastic model for isotropic, moderately porous metallic materials is formulated. This model is intended for material rate-sensitivities in the entire range spanning from highly rate-dependent behavior at high homologous temperatures to nearly rate-insensitive behavior at low homologous temperatures. The predictive capabilities of the constitutive model are verified by comparing results from finite element calculations against results from physical experiments. Specifically, example calculations are presented for: (a) isothermal hot compression of a tapered disk made from an initially porous material. This calculation illustrates the effect of secondary tensile stresses on hot workability of metals, (b) Tension tests, under isothermal conditions at low homologous temperatures, on axisymmetric notched bars made from initially porous materials. This calculation illustrates the effects of nonuniform multiaxial tensile stress states on void growth. Predictions from the computational procedures for both examples agree well with experimental results. The new state variable rate and temperature dependent constitutive model for microporous materials and the associated computational procedures form a basis for the simulation and design of deformation processing operations. This new capability should be useful for the prediction of formation of defects during both cold-working when the material rate sensitivity is low, as well as hot-working when the material is highly rate sensitive. The computational capability should also be useful in simulating the late stages of densification of powder metallurgical products in complex forming operations.

Mechanical behaviour and microstructure of fatigued aluminium single crystals

For many decades the fatigue behavior of metallic materials has been a subject of intensive studies. Most profoundly investigated are Cu single crystals. A prominent feature of mechanical behavior in cyclically deformed pure Cu, Ni and other fee metals is the occurrence of a plateau in the cyclic stress-strain curve. A characteristic microstructure developing in these metals under cyclic loading exhibits bands of localized strain referred to as persistent slip bands (PSBs). Scarce experimental data on the mechanical response of Al to cyclic loading indicate that at room temperature the behavior of this material differs from that of Cu and Ni. This motivated the present study aiming to obtain more data and to get a better insight into the properties of Al under tension-compression loading. High purity Al single crystals oriented for single slip were tested under cyclic tension-compression conditions at two temperatures (T = 298 K and T = 78 K). For aluminum, the room temperature corresponds to a relatively high homologous temperature of about 0.3. Given the large stacking-fault energy of Al, its fatigue behavior at this temperature can be placed in the category of high-temperature behavior of pure fcc metals. At the liquid nitrogen temperature the dislocationmore » motion tends to become more planar which leads to dislocation structures resembling those found in Cu and other pure fcc metals cyclically deformed at room temperature. A plateau in the cyclic hardening curve associated with the occurrence of the PSBs was found in the fatigued specimens, which is at variance with the cyclic hardening behavior for relatively homogeneous room temperature deformation in tension-compression.« less

An examination of the metals deforming by Harper-Dorn creep at high homologous temperatures

At low stresses and high homologous temperatures, metals exhibit Newtonian viscosity with a stress exponent of 1. This behavior is attributed to either Harper-Dorn creep arising from a dislocation process or Nabarro-Herring creep resulting from the stress-directed diffusion of vacancies. By analysing experimental data where the occurrence of Harper-Dorn creep is unambiguous, a relationship is derived for the transition between the two processes. It has been suggested recently that several experiments, using the zero creep technique on wires and foils, are consistent with Harper-Dorn creep. Using the transition relationship, it is demonstrated that these experiments are instead within the regime of Nabarro-Herring creep.

A rheological view of high-strain-rate superplasticity in alloys and metal-matrix composites

Recent studies have indicated that superplasticity can occur at extremely high strain rates, yielding tensile elongations of the order of 1250 percent at a strain rate of 1/s. This phenomenon, presently designated 'high strain rate superplasticity' (HSRS), has been reported in alloys, metal-matrix composites (MMCs), and mechanically-alloyed materials; all materials exhibiting HSRS have extremely fine, about 1-micron grain size, and the phenomenon is observed at high homologous matrix temperatures that are in some cases higher than the matrix solidus temperature. Attention is presently given to the rheological factor in HSRS. It is suggested that the presence of a liquid phase or a low melting point region due to solute segregation, as at the whisker-matrix interface of an MMC, is responsible for HSRS. 24 refs.

On the origins of ghost boundaries during in situ strain-induced grain-boundary migration

Grain boundaries in high-purity (99.999%) aluminum with misorientations of 8°–15° have been studied in situ under conditions of applied stress. At both high and low homologous temperatures ( T ≈ 0.8 T mp and T ≈ 0.32 T mp, respectively), substantial grain-boundary-dislocation motion and grain-boundary migration was observed. In conjunction with grain-boundary migration was the formation of “ghost” boundaries (indicating prior positions of the boundary) during both high- and low-temperature experiments. Experimental observations suggest that one cause of “ghost” boundary formation was a discontinuous change in the thickness of the region swept out by the migrating boundary, which was apparently due to grain-boundary diffusion from the grain boundary to the grain surface during strain-induced grain-boundary migration. Evidence that grain boundary diffusion was involved received further support from the formation of epitaxial hillocks at the grain boundary during migration. Finally, a simple model was derived which showed that grain-boundary diffusion during migration was capable of producing a discontinuous thickness change such as that observed experimentally.

The fracture resistance of metal/ceramic/intermetallic interfaces

This paper reports that there are major differences in interface fracture with the homologous temperature, t/t{sub m}, used to produce the interface. Low homologous temperatures (T/T{sub m} {lt} 0.3) processes such as evaporation result in interface mechanical properties dominated by impurities, especially organics present on the original surface prior to deposition. Consequently, thin bond layers, such as Cr and Ti, are often used to enhance the fracture resistance. The presence of these layers apparently results in the formation of carbides and hydrides that 'clean' the surface. The use of high homologous temperatures (T.T{sub m} {gt} 0.5) generally results in 'strong' bonding for a wide range of metals, ceramics, intermetallics and glasses. Such diffusion bonds are emphasized in this article.

Physicomechanical properties of porous titanium nickelide

The insufficiently high degree of perfection of interparticle contacts at high homologous sintering temperatures of titanium nickelide is related to specifics of its superelastic behavior and is explained by rupture of the contact bonds formed in pressing during preparation of the material.

A TEM and EDX study of cavities formed in tin by xenon ion implantation

A detailed transmission electron microscopy and EDX study has been carried out on cavities formed in tin by xenon ion implantation. Computer simulation of dark field images has identified thickness fringe contrast due to pyramidal {112} faceting on the otherwise {001} surfaces. Quantitative EDX measurements have shown that all the detectable xenon is associated with the cavities as a consequence of the high homologous implantation temperature, and that the larger of these are considerably underpressurized with respect to equilibrium pressure. These cavities have been shown to shrink when annealed, due to thermal emission of vacancies.

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10-4 to 2.8 X 10-2 s-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10-3 s-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec−1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec−1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec−1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m ≈ 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.

Electroplasticity—the effect of electricity on the mechanical properties of metals

Investigations of the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5Tm) reveal a polarity effect and yield an electron wind force in some agreement with theory. An external directcurrent electric field has been reported to influence the creep rate of unalloyed metals at high homologous temperatures. During superplastic deformation of the 7475 Al alloy, such a field has been found to decrease the flow stress, reduce strain hardening, increase strain-rate hardening, reduce grain boundary cavitation and reduce grain growth. The effects of the field were polarity dependent and extended to the center of 1–2 mm thick specimens. No significant effect of the field on the flow stress occurred at low homologous temperatures. This suggests that the field influences atomic mobility through vacancy generation and/or migration. The occurrence of an uneven electron density at the interfaces between phases and at grain boundaries has been proposed as a factor, but this idea needs further consideration.

Creep in Shear of Experimental Solder Joints

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 case 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 studies. 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. The efficacy of postponing the microstructural instability with the addition of small amounts of ternary elements is discussed.

Analysis of deformed YBa2Cu3O7??

In this study, polycrystalline YBa2Cu3O7−δ (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10−4 sec−1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa2Cu3O7−δ and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.

High Rate Superplastic Deformation Mechanisms in IN90211 Mechanically Alloyed Aluminum

ABSTRACTIN90211 has exhibited superplastic elongations above 500% at high homologous temperatures (0.76–0.82 Tm). A high strain rate and flow stress for optimum elongation was measured (1–5/sec, 20–60 MPa, 425–485 °C). The apparent strain rate sensitivity of m≈0.25 differs from the usual m≈0.5 observations of superplastic deformation. An analysis of the data at several strains indicates a highly temperature dependent threshold stress is present, with either a n=2 or n=3 assumption for the stress exponent. The magnitude of the threshold stresses in IN90211 are smaller than usually observed in a dispersion strengthened matrix (1–20% instead of ≈50% of the Orowan stress). Experimental evidence from creep experiments supports the n=3 deformation mechanism as the rate limiting step of deformation.

Fracture toughness of columnar freshwater ice from large scale DCB tests

A series of 42 fracture toughness tests were performed on laboratory-grown S2 columnar freshwater ice at high homologous temperatures (−2 to 0° C). The floating double cantilever beam specimen used and the monitoring of the crack mouth opening displacement in addition to the applied load provided a means for obtaining an apparent fracture toughness, an effective elastic modulus, a lower-bound estimate of the crack speed, and a side-loaded flexural strength of the ice. An expression for the apparent fracture toughness as a function of the applied load, specimen geometry, and ice thickness was developed using a finite-element program. This allowed comparison with previously published values for the toughness of freshwater ice. The small range of scatter in apparent fracture toughness values as well as the ability to measure other mechanical properties of the ice indicates the usefulness of such tests.

Creep-fatigue interactions in solders

Solders in surface-mount-technology (SMT) joints are subjected to high homologous temperatures that cause the solder to deform and accumulate damage by both time-dependent and instantaneous mechanisms. The effects of frequency, stress range, mean stress, waveform (hold times on and off load), and mechanical history are investigated in bulk uniaxial solder in the as-cast condition. Strain is measured on a per-cycle basis as well as cumulatively, to separate the mechanisms of strain storage and damage storage. At hold times less than about 10 s, a significant part of the strain is recoverable, time dependent, and nondamaging. By this strain storage mechanism, life, measured either by cycles to failure or by time on load, can be increased by at least a factor of 5. At lower frequencies the damage is stored as a function of time on load and stress (creep damage). The effective stress for a cyclic creep test is defined in terms of the stress sensitivity of the solder in creep. A technique for measuring damage for life predictions is developed and examined. This technique involves the grain boundary embrittlement of partially fatigued specimens and the statistical analysis of the voids on the surfaces. >

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC‐ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep‐fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.

Superplasticity of a Zn-1.1 WT.% Al alloy at high temperatures

The Zn-1.1 wt.% Al alloy exhibits superplastic properties in the temperature range from 292 K (0.42 Tm) to 635 K (0.92 Tm). The best properties were observed at the temperature of 520 K (0.75 Tm). The parameter m reaches the value m = 0.73 at the true strain-rate ϵ = 7 × 10 −5s −1, and the maximum ductility is A = 1020% at the initial strain-rate ϵ 0 = 1.7 × 10 −4s −1. This value is the highest one observed in low-alloyed ZnAl alloys and shows that materials with great volume fraction of the second phase with very good superplastic properties can be reached at comparably high homologous temperatures also in low-alloyed materials. The extreme values of m and A were observed in the material with the initial mean grain size d = 17 um.