Rate-dependent indentation behavior of solder alloys

Finite element (FE) simulations of visco-plastic indentation in Sn-37Pb eutectic solder alloy are performed to investigate the influence of loading rate on its creep characteristic. The resulting indentation load-displacement curves are rate-dependent and have varying creep penetration depths during the same hold time. Creep indentation hardness H, defined from the concept of work of indentation, varies with volume strain occurring during the creep hold time, which is a measure of creep strain rate _cr. Thus, creep stress sensitivity can be determined from the H versus _cr curve. This analysis can be verified by the good agreement between the derived value and the predefined value, and then be used to analyze the Berkovich indentation load-displacement curves of Sn-3.5Ag-0.75Cu lead-free solder. Such indentation tests and physical analysis provide a cheaper and more convenient method to determine the mechanical properties of the upcoming lead-free solder alloys.     

Surface damage accumulation in Sn–Ag solder joints under large reversed strains

During service, solder joints may encounter repeated reversed stress states. Since realistic solder joint specimen geometry employed for creep/stress relaxation studies does not facilitate stress reversal, shear test specimens consisting of 3/8×3/8×1 copper blocks joined with eutectic Sn–Ag solder of realistic thickness of about 400 m were used for this study. The mechanical behavior and microstructural features of the solder joint subjected to repeated reversed straining were investigated by imposing large shear strains. Damage accumulation on the surface of the solder joints due to a few such large shear strain reversals is very similar to that observed on the surface of thermomechanically fatigued joints made with the same solder.     

The mechanical properties and microstructures of copper and brass joints soldered with eutectic tin-bismuth solder

The mechanical properties and microstructures ofcopper and brass soldered with eutectic tin-bismuth solder have been determined and the joints examined using metallographic techniques. Joints made with copper were stronger than those made with brass. At the copper/solder interface a uniform layer 2m thick of Cu_5.2Sn₅ was formed and at the brass/solder interface a uniform layer 2 m thick of (Cu, Zn)_2.9Sn and an irregular layer 2 to 5m thick of (Cu, Zn)_5.7Sn₅ were formed. Copper joints fractured etthocopper/solder interface and brass joints fractured in the internmetalic layer. Copper joints soldered with eutectic Sn-Bi were stronger than copper joints soldered with eutectic Sn-Pb and the reverse was true for brass joints. Results are also given for the effect of thermal shock on copper and brass joints soldered with Sn-Bi and Sn-Pb solders, and also for We fatigue and creep behaviour of joints soldered with eutectic Sn-Bi solder.     

Single-crystal growth and characterization of Cu2O and CuO

We have prepared a large number of crystals of cuprous oxide (Cu2O) by various procedures. Photoluminescence spectra of these crystals were studied to examine the concentration of defects, especially copper vacancy VCu to seek favourable conditions for growing Cu2O crystal. High-quality single crystals of Cu2O were prepared by the floating-zone melting method in air. Several synthetic crystals (specimens FA, FZ and GZ) and also a natural crystal were studied by X-ray analysis, inductively coupled plasma spectroscopy analysis, optical absorption, photoluminescence, photoconductivity and cyclotron resonance absorption, photoluminescence, photoconductivity and cyclotron resonance absorption to characterize their optical and electrical qualities. The best values of mobility and scattering time of photocarriers at T = 4.2 K are estimated to be h1.8 × 105 cm2 V–1 s–1 and h60 ps for positive holes, and 1.3 × 105 cm2 V–1 s–1 and 70 ps for electrons in Cu2O. Further, we report preliminary experimental results on transport property of crystals also of cupric oxide (CuO) purified by the floating-zone melting method.     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

Macroscopic volume changes of PVF2 undergoing uniaxial tension and creep

Uniaxial tension and creep tests were performed on apolar cristalline PVF₂ at room temperature and above the high glassy transition temperature (T_g=40°C and T_g=60°C), for different strain-rates (tension) or applied stresses (creep). Volume changes were simultaneously measured to study cavitation damage occuring in this polymer when strained. Damage mechanisms were explicated. Influence of experimental conditions such as strain-rate or temperature was studied: onset of cavitation is delayed and volume changes more pronounced when molecular motions in amorphous phase are hindered, for lower temperature or higher strain-rates. This work brings elements for a better comprehension of PVF₂ global mechanical behaviour as damage is very linked to yield or creep studies.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

The effect of Ag additions on the microstructure of aluminium–lithium alloys

Minor quantities of Ag have been added to Al–Li–Cu–Mg–Zr alloys. Their microstructure has been studied by means of optical metallography, transmission electron microscopy and X-ray diffraction. In the high Li, low Cu:Mg ratio alloys the main phases found were , , S and T1, while fewer T2 and Al7Cu2Fe precipitates were also observed. The addition of up to 0.5 wt% Ag diminishes the and T1 precipitates size. This is attributed to a small increase of Li solubility in the matrix. In the low Li, high Cu:Mg ratio alloy the addition of 0.2 wt% Ag resulted in the precipitation of phase simultaneously with , , S and T1 phases. Due to the low Li concentration an unusual growth of the / precipitates at the expense of the precipitates was also observed. © 1998 Kluwer Academic Publishers     

Rejuvenation procedures to recover creep properties of nickel-base superalloys by heat treatment and hot isostatic pressing techniques

The rejuvenation procedures to recover the creep properties of nickel-base superalloys by atmospheric pressure heat treatment and hot isostatic pressing techniques have been reviewed in detail. It is very important that such treatments be applied at an optimum stage in the service life of a turbine blade. In other words, the rejuvenation procedures must be applied early enough to prevent catastrophic failures or irreparable damage and late enough to give a cost-effective benefit. The optimum stage at which to undertake a rejuvenation procedure to extend the creep lives of superalloys is immediately prior to the tertiary stage. By using these techniques it is not possible to extend the creep lives of superalloys indefinitely because of the accumulation of some permanent damage incurred during service conditions.Nomenclature ERF Economic repair factor - P _r Price of repaired and rejuvenated part - P _n Price of new part - L _n Potential operational life of new part - L _r Potential operational life of repaired/rejuvenated part - N Cavity density or number of cavities per unit area (mm^–2) - n _v Number of cavities per unit volume (mm^–3) - Creep strain - ₁ Maximum principal stress (MPa) - ¯ von Mises effective shear stress (MPa) - t _f Time to failure - t _t Time to commencement of tertiary creep - Creep damage tolerance parameter - _f Strain at fracture (or failure) - T _m Absolute melting temperature - ₀ Friction stress - r Spherical radius of cavities - 2x Intercavity spacing - Grain boundary width - P _I Cavity gas pressure - P _H External hydrostatic pressure - Atomic volume - k Boltzmann constant - T Absolute temperature - Surface energy of the cavity - D _b Grain boundary diffusion coefficient - _d Ductility recovery parameter - Strain to reach the same acceleration after recovery annealing - ₀ Strain necessary for standard material to reach a given acceleration of the secondary-creep rate in the tertiary region - _t Strain needed to have produced the reduced cavity volume after rejuvenation annealing - Creep rate - Secondary or minimum creep rate - ₁ Strain previous to the regenerative annealing period - n Total number of strain/regenerative anneal cycles - _v Recovery parameter for cavity volume - V ₀ Original total cavity volume at the start of the recovery - V _t Cavity volume after recovery annealing for a timet     

Electron diffraction study of superlattices and orientation variants in Ca3SrAl6SO16

Samples having the nominal composition of Ca₃SrAl₆SO₁₆ were sintered at 1380 °C and analysed by electron diffraction. The frequent appearances of forbidden and satellite reflections in this compound imply the presence of a number of basal and nonbasal superlattices so that the microstructure of this cement clinker was characterized by various superstructures including one-, two- and even three-dimensional superstructures along the 0 0 1, 1 1 0, 1 1 2, 1 1 4 or 2 2 1, 0 1 3 and 1 2 3 directions with repeat periods of two or three times of that of basic one, respectively, and intergrowth of these. Various domain structures with 90°, 120° and 48.2°, etc., orientation relationships were also detected in these superstructures and the total number of these orientation variants related to the symmetry elements lost in the process of phase transformation, can be predicted according to the conclusions of Van Tendeloo and Amerlinckx, or they are equal to the number of those unique planes in the matrix.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Phase equilibria in the Ag4SSe-ZnTe system

The Ag₄SSe-ZnTe phase diagram has been determined on the basis of X-ray diffraction, differential thermal and metallographic analyses, as well as microhardness and density data. It has been divided into two subdiagrams of eutectic type by an intermediate A-phase with most probable composition of 2Ag₄SSe.ZnTe. The unit-cell parameters of the low-temperature -2Ag₄SSe·ZnTe modification have been determined (a=3.330 , b=3.010 , c=2.895 , =95.04°, =107.83°, =92.62°). It has been supposed that in the ZnTe-rich part a second intermediate B-phase with a composition of Ag₄SSe·2ZnTe is formed, which is stable in the range of 320÷500 °C.     

High-temperature creep of the particlehardened commercial Al-Li-Cu-Mg alloy 8090

Creep of the particle-hardened commercial Al-Li 8090 alloy has been studied at temperatures of 425 and 445 K. The measured stress sensitivity of the minimum creep rates changes abruptly at a given applied stress with stress exponents being around 4–6 at low stresses and 30–40 at high stresses. Creep activation enthalpies were determined by both temperature cycling and by comparing creep rates at two temperatures at a given applied stress, the results from both gave the same unrealistically high values. The internal stresses, _i, developed during creep were determined using the strain-transient dip test. These increased linearly with the applied stress, _a, at low stresses and were effectively constant at high stresses. The minimum creep rate was found to be a simple function of the effective stress, _a-_i, with a stress exponent of between 5 and 6, at all applied stresses. The dislocation and precipitate structure of the alloy was examined before and after creep using thin-film electron microscopy. The initial structure consisted of pancake grains with a well-developed {1 1 0}1 1 2 type texture. The grains contained well-developed sub-cells and and S precipitates. The structure developed during creep consisted of dislocation pairs, single dislocations and dislocations loops. There was evidence to suggest that slip took place on both {1 0 0} and {1 1 1} planes. The dislocation loops were most likely to have been Orowan in character and around the rodlike S precipitate, with the coherent precipitate being sheared by pairs of dislocations. The measured internal stresses result from inhomogeneity of plastic deformation. These stresses increase continuously with applied stress up to the observed macroscopic yield stress, and then become constant. The internal stresses are likely to have arisen from the Orowan loops around S and the behaviour of sub-grain boundaries. The increases in internal stress may have resulted from an increased loop density with increasing applied stress. This rate of increase is likely to slow down if S particles are sheared or fractured at high applied stresses.     

Effect of annealing on the resistivity of quench-condensed Cu and Cu(Fe) thin films

Pure Cu and Cu(Fe) thin films containing 0.1 and 1.0 at % Fe were prepared by low-temperature deposition onto a liquid-helium-cooled substrate. The Cu(Fe) films were annealed sequentially at approximately 17, 70, and 270 K. After each annealing stage the resistivity was measured down to 1.5K. The Cu(Fe) films exhibited a region in which the resistivity was proportional to ln T; in this region the logarithmic slope of the resistivity curve was only weakly affected by annealing. Below 10 K annealing produced a significant decrease in the impurity (Fe) contribution to the resistivity. The results are interpreted as due to increased interactions between Fe atoms produced by an increase in conduction-electron mean free path.This work was supported by the Materials Research Center, University of North Carolina, under Grant Number GH-33632 from the National Science Foundation.     

Investigation of the creep and micropolar creep of various materials under static and dynamic loads

Creep is investigated under a uniform stress state with allowance for the micropolar creep of the following materials: lignostone (T=293°K), steel ON2M (T=773° K), and an AIMgSi aluminum alloy (T= 293° K) understate and cyclic tension, i.e., atA = ^a/^m = 0, 0.25, and 0.5. It is established that for lignostone, the microstrains I₁₁ are two orders lower than 2₁₂ in torsion, and the values of 2l₁₂ are only a half order lower than ₁₁ in tension. The creep strain of lignostone is described using nonlinear theory of viscoelasticity. For alloys and metals under static creep, the value of I_ij is two or three orders lower than _ij A qualitative change in microipolar creep occurs in the case of dynamic loading: whenA = 0.5,the ratio of values of fatigue creep rate 21₁₂/₁₂ = 0.2.A subject investigated in 1990 with funding provided by the Ministry of National Education.Translated from Problemy Prochnosti, No. 12, pp. 18–23, December, 1993.     

The influence of interfacial dislocation arrangements in a fourth generation single crystal TMS-138 superalloy on creep properties

The morphologies of (001) / interfacial dislocation networks are studied through TEM observations. The lattice misfit has an important relation with creep property for superalloy during high temperature creep deformation. The fourth generation superalloy TMS-138 possesses superior creep properties based on its fine interfacial dislocation networks. The networks have two typical characteristics: closely spaced dislocations and stable square morphology during creep deformation. Such arranged dislocations can effectively prevent the slipping dislocations in the phase from moving through the / interface and improve drastically the creep resistance in the fourth generation superalloy TMS-138.     

Ageing characteristics of cast Zn-Al based alloy (ZnAl7Cu3)

Microstructure and ageing characteristics of a cast Zn-Al based alloy (ZnAl7Cu3) were studied using X-ray diffraction, electron scanning microscopy and back-scattered diffraction techniques. Two stages of phase transformation, i.e., decomposition of zinc rich phase and four phase transformation, + T + , were detected during ageing at 150°C. Electron back-scattered diffraction technique was applied in distinguishing both zinc rich and phases.     

The effects of residual α phase on the 1370°C creep performance of yttria-doped HIPed silicon nitride

The creep behaviour at 1370°C (2500°F) of yttria-doped, hot isostatically pressed silicon nitride was examined as a function of residual phase content. The pre-test silicon nitride materials had either 30% or 40% phase content. The creep resistance was found to increase as the residual phase content decreased. For equivalent times and stresses, the higher -containing silicon nitride accumulated more creep strain and exhibited faster creep rates. The residual phase decreased as a function of time at 1370°C and converted to phase; it was also found that the to phase transformation rate was enhanced by stress. In the absence of stress, the kinetics of the to phase transformation at 1370°C followed a first-order reaction. If a first-order reaction was assumed for the to phase transformation in the presence of stress at 1370°C, then the magnitude of the reaction rate constant for this transformation was twice as large for tensile stresses equal to or greater than 130 MPa than for the reaction rate constant describing the transformation with no applied stress. © 1998 Chapman & Hall     

Observations on the mechanical properties of nickel oxide scales

The mechanical properties of NiO scales produced by the complete oxidation of high-purity (grade-1) Ni and commercial-purity (grade-A) Ni have been investigated at 700 to 1000 C. The modulus of elasticity of both grades of oxide decreased with increasing temperature, whereas the modulus of rupture for grade-A oxide exhibited a maximum at 850 C and that for grade-1 oxide decreased with increasing temperature. At 700 C, elastic deformation to fracture occurred with both oxides, whereas, at temperatures 850 C, plasticity was also observed. The plasticity of grade-1 oxide was 3 times greater than that of grade-A oxide.Creep behaviour of the oxides was studied at 900 and 1000 C. Primary and secondary creep was observed and, in both oxides, the creep rates increased with increasing temperature and load. The creep rate of grade-1 oxide was 10 to 20 times greater than that for grade-A oxide.