Showing papers on "Creep published in 2004"

Properties of lead-free solder alloys with rare earth element additions

Abstract: Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has prompted the development of “Pb-free” solders, and has enhanced the research activities in this field. In order to become a successful solder material, Pb-free alloys need to be reliable over long term use. Although many of these alloys possess higher strength than the traditional Sn–Pb ones, there still exist reliability problems such as electromigration and creep. Also, the solderability of many Pb-free alloys is inferior to that of Sn–Pb and any improvement or replacement will be welcomed by industry. In order to develop new Pb-free solders with better properties, trace amounts of rare earth (RE) elements were selected by some researchers as alloying additions into Sn-based solders. These solder alloys are mainly Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu. In general, the resulting RE-doped solders are found to have better performances than their original ones. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep resistance in some RE-doped alloys gives creep rupture time increases by over four times for Sn–Ag and seven times for Sn–Cu and Sn–Ag–Cu. Like other Sn-based alloys, their creep rates are controlled by dislocation pipe diffusion in the Sn matrix. Also, it was found that the creep rate of these Sn-based alloys can be represented by a single empirical equation. With the addition of RE elements, solders for bonding on difficult substrates such as on semiconductors, diamond, and optical materials have also been developed. This report summarizes the effect of RE elements on the microstructure, mechanical properties, wetting behavior of certain Pb-free solder alloys. As an illustration of the advantage of RE doping, interfacial studies were carried out for electronic interconnections with RE-doped Pb-free alloys. It was found that the intermetallic compound (IMC) layer thickness and the amount of interfacial reaction were reduced in a Ball Grid Array (BGA) package. These results indicate that RE elements would play an important role in providing better electronic interconnections.

Effect of silica fume on mechanical properties of high-strength concrete

Abstract: This paper presents the results of experimental work on short- and long-term mechanical properties of high-strength concrete containing different levels of silica fume. The aim of the study was to investigate the effects of binder systems containing different levels of silica fume on fresh and mechanical properties of concrete. The work focused on concrete mixes having a fixed water/binder ratio of 0.35 and a constant total binder content of 500 kg/m3. The percentages of silica fume that replaced cement in this research were: 0%, 6%, 10% and 15%. Apart from measuring the workability of fresh concrete, the mechanical properties evaluated were: development of compressive strength; secant modulus of elasticity; strain due to creep, shrinkage, swelling and moisture movement. The results of this research indicate that as the proportion of silica fume increased, the workability of concrete decreased but its short-term mechanical properties such as 28-day compressive strength and secant modulus improved. Also the percentages of silica fume replacement did not have a significant influence on total shrinkage; however, the autogenous shrinkage of concrete increased as the amount of silica fume increased. Moreover, the basic creep of concrete decreased at higher silica fume replacement levels. Drying creep (total creep − basic creep) of specimens was negligible in this investigation. The results of swelling tests after shrinkage and creep indicate that increasing the proportion of silica fume lowered the amount of expansion. Because the existing models for predicting creep and shrinkage were inaccurate for high-strength concrete containing silica fume, alternative prediction models are presented here.

Mechanical Behavior of Materials

Abstract: 1. Stress and strain 2. Elasticity 3. Mechanical tensile testing 4. Strain hardening of metals 5. Plasticity 6. Strain-rate and temperature dependence of flow stress 7. Slip 8. Dislocation geometry and energy 9. Dislocation mechanics 10. Mechanical twinning 11. Hardening mechanisms 12. Discontinuous and inhomogeneous deformation 13. Ductility and fracture 14. Fracture mechanics 15. Viscoelasticity 16. Creep and stress rupture 17. Fatigue 18. Residual stresses 19. Ceramics 20. Polymers 21. Composites 22. Mechanical working Appendix I. Miller indices Appendix II. Stereographic projection.

Fundamentals of Creep in Metals and Alloys

Abstract: 1.0 Introduction A. Description of Creep B. Objectives 2.0 Five-Power-Law Creep A. Macroscopic Relationships B. Microstructural Observations C. Rate-Controlling Mechanisms D. Other Effects on Five-Power-Law Creep 3.0 Diffusional Creep 4.0 Harper Dorn Creep A. The Size Effect B. The Effect of Impurities 5.0 Three-Power-Law Viscous Glide Creep 6.0. Superplasticity A. Introduction B. Characteristics of Fine Structure Superplasticity C. Microstructure of Fine Structure Superplastic Materials D. Texture Studies in Superplasticity E. High Strain Rate Superplasticity (HSRS) F. Superplasticity in Nano and Submicrocrystalline Materials 7.0 Recrystallization A. Introduction B. Discontinuous Dynamic Recrystallization (DRX) C. Geometric Dynamic Recrystallization D. Particle Stimulated Nucleation (PSN) E. Continuous Reactions 8.0 Creep Behavior of Particle Strengthened Alloys A. Introduction and Theory B. Small Volume Fraction Particles That Are Coherent and Incoherent with Small Aspect Ratios 9.0 Creep of Intermetallics A. Introduction B. Titanium Aluminides C. Iron Aluminides D. Nickel Aluminides 10.0 Creep Fracture A. Background B. Cavity Nucleation C. Growth

A simple phenomenological approach to nanoindentation creep

Abstract: Nanoindentation is frequently used to measure elastic modulus and hardness of structural materials such as ceramics, metals and thin films. The assumption behind conventional nanoindentation analysis methods, where the unloading data is analysed, is that the material behaves in an elastic-plastic manner. However, many materials can also exhibit a visco-elastic and visco-plastic response which is commonly termed “creep”. In a nanoindentation test, this is usually observed as an increase in depth during a hold period at maximum load in the load-displacement data. Creep is not accommodated in conventional nanoindentation analysis methods. The present work shows how conventional linear spring and dashpot elements can be used to model the creep response of a wide range of materials using the hold period force-displacement data. The method shown can be readily incorporated into a computer program and can be used with any conventional nanoindentation test instrument using either spherical or sharp indenters.

Inelastic Analysis of Solids and Structures

Abstract: An Introduction to the Incremental-Iterative Solution of Nonlinear Structural Problems.- Fundamental Notions of Metal Plasticity.- A General Procedure for Stress Integration and Applications in Metal Plasticity.- Creep and Viscoplasticity.- Plasticity of Geological Materials.- Large Strain Elastic-Plastic Analysis.

Thermomechanical finite-element model of shell behavior in continuous casting of steel

Abstract: A coupled finite-element model, CON2D, has been developed to simulate temperature, stress, and shape development during the continuous casting of steel, both in and below the mold. The model simulates a transverse section of the strand in generalized plane strain as it moves down at the casting speed. It includes the effects of heat conduction, solidification, nonuniform superheat dissipation due to turbulent fluid flow, mutual dependence of the heat transfer and shrinkage on the size of the interfacial gap, the taper of the mold wall, and the thermal distortion of the mold. The stress model features an elastic-viscoplastic creep constitutive equation that accounts for the different responses of the liquid, semisolid, delta-ferrite, and austenite phases. Functions depending on temperature and composition are employed for properties such as thermal linear expansion. A contact algorithm is used to prevent penetration of the shell into the mold wall due to the internal liquid pressure. An efficient two-step algorithm is used to integrate these highly nonlinear equations. The model is validated with an analytical solution for both temperature and stress in a solidifying slab. It is applied to simulate continuous casting of a 120 mm billet and compares favorably with plant measurements of mold wall temperature, total heat removal, and shell thickness, including thinning of the corner. The model is ready to investigate issues in continuous casting such as mold taper optimization, minimum shell thickness to avoid breakouts, and maximum casting speed to avoid hot-tear crack formation due to submold bulging.

Analysis of nanoindentation creep for polymeric materials

Abstract: This article presents studies on nanoindentation creep for polymeric materials at room temperature. A semi-empirical method has been established from a generalized Kelvin model. This model includes the elastic-viscoelastic-viscous effects on indentation creep. It is shown that this method can fit the experimental creep data well, to give the exact changes of creep behavior during nanoindentations for numbers of amorphous polymeric materials. A method has also been proposed to determine the elastic modulus based on this model; the elastic modulus determined from this method is independent of the holding and unloading processes of nanoindentations. Creep deformation is further used to derive creep compliance and retardation spectrums for the polymeric materials studied, which are very useful to predict other mechanical properties of polymeric materials.

The effect of long-term thermal exposures on the microstructure and properties of CMSX-10 single crystal Ni-base superalloys

Abstract: The long-term thermal stability of CMSX-10 single crystal superalloys has been studied as a function of heat treatment and thermal exposure. The CMSX-10 alloys were subjected to long-term unstressed heat treatments at temperatures of 950 and 1050 °C for periods of 1000 and 10,000 h. The microstructures and creep properties of the samples before and after long-term thermal exposures were characterized. The γ′ coarsening was observed after all exposures and the formation of topologically close packed (TCP) phases was observed in both samples exposed at 1050 °C and the 10 kh exposure at 950 °C. High temperature creep tests were carried on these samples at 950 °C at an engineering stress of 310 MPa. The creep lives were found to deteriorate with the increase in exposure time and temperature as a result of the microstructural degradation. The coarsening of the γ′ precipitates was observed to be the main factor in the degradation of the properties.

Stabilization of martensitic microstructure in advanced 9Cr steel during creep at high temperature

Abstract: In order to improve the long-term creep strength of 9%Cr steel, the stabilization of martensitic microstructure in the vicinity of prior austenite grain boundaries during creep has been investigated by the addition of boron and by a dispersion of nano-size MX nitrides. Creep tests were carried out at 923 K for up to about 3×10 4 h. Boron is enriched in the M 23 C 6 carbides during aging and creep, especially in the vicinity of prior austenite grain boundaries. This reduces the coarsening rate of M 23 C 6 carbides, which effectively stabilizes the martensitic microstructure in the vicinity of prior austenite grain boundaries. A dispersion of nano-sized MX nitrides but no M 23 C 6 along boundaries also gives rise to excellent pinning force for migrating boundaries during creep, as shown by approximately two orders of magnitude longer time to rupture than ASME-P92. The stabilization of martensitic microstructure retards the onset of tertiary or acceleration creep, which results in lower minimum creep rate and longer time to rupture.

Size effects of nanoindentation creep

Abstract: The size effects on indentation creep were studied on single-crystal Ni3Al, polycrystalline pure Al, and fused quartz samples at room temperature. The stress exponents were measured by monitoring the displacement during constant indentation loads after correction for thermal drift effects. The stress exponents were found to exhibit a very strong size effect. In the two metals Al and Ni3Al, the stress exponent for very small indents is very small, and for Al, this even approaches unity, suggesting that linear diffusional flow may be the controlling mechanism. The stress exponents in these two metals rise rapidly to over 100 as the indent size gets larger, indicating a rapid change of the dominating mechanism to climb-controlled to eventually glide-controlled events. In fused quartz, the stress exponent also exhibits a sharply rising trend as the indent size increases. The stress exponent is also close to unity at the smallest indents studied, and it rises rapidly to a few tens as the indent size gets larger.

The influence of time, temperature, and grain size on indentation creep in high-purity nanocrystalline and ultrafine grain copper

Abstract: Microhardness measurements have been carried out on high purity Cu samples with average grain sizes ranging from ∼10 to ∼200nm, over temperatures from liquid nitrogen to ambient, and dwell-times of the indenter in the sample from 5 s to 39 h. The Vickers hardness diminishes approximately linearly with the logarithm of the dwell-time. At short dwell-times the hardness increases significantly with decreasing grain size and with decreasing temperature, but the influence of these variables substantially diminishes at longer times. Investigation by transmission electron microscopy shows that rapid grain growth under the indenter most likely is responsible for the strong and long-lasting indentation creep.

The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part I—experimental analysis and creep characterization

Abstract: The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. in viscoelastic pipes, typically made of polyethylene (pe), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. this is a result of the retarded deformation of the pipe-wall. this effect has been observed in transient data collected in a high-density pe pipe-rig, at imperial college (london, uk). several transient tests were carried out to collect pressure and circumferential strain data. the pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. the creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. a goo...

Coarsening behavior of lath and its effect on creep rates in tempered martensitic 9Cr–W steels

Abstract: The coarsening behavior of martensite lath has been investigated by means of transmission electron microscopy for tempered martensitic 9 wt.% Cr–(0, 1, 2, 4 wt.%) W steels during creep at 823–923 K. During creep, the recovery of excess dislocations, the agglomeration of carbides and the coarsening of laths take place. The coarsening of laths with absorbing excess dislocations is the major process in the creep acceleration. The coarsening rate of lath decreases with increasing W concentration, which is correlated with the rate of Ostwald ripening of M23C6 carbides. The progressive local-coalescence of two adjacent laths boundaries near the Y-junction causes the movement of Y-junction, resulting in the coarsening of lath.

Tensile properties of Sn–Ag based lead-free solders and strain rate sensitivity

Abstract: The tensile properties of Sn-35 mass%Ag, Sn-35 mass%Ag-075 mass%Cu and Sn-3 mass%Ag-2 mass%Bi lead-free solders were investigated and compared with those of a Sn–Pb eutectic solder The tensile strength of each solder decreases with decreasing strain rate, and with increasing test temperature The ductility of each lead-free solder is relatively constant in the strain rate ranging from 167×10 −4 s −1 to 167×10 −2 s −1 and in the test temperature ranging from −40 to 120 °C From the results of the strain-rate-change tests, stress exponents were investigated The stress exponents for the lead-free solders are higher than those of the Sn–Pb eutectic solder at 25, 80 and 120 °C, and are relatively stable at these temperatures The activation energies for the creep of the lead-free solders were also investigated

Viscoelastic measurement techniques

Abstract: Methods for measuring viscoelastic properties of solids are reviewed The nature of viscoelastic response is first presented This is followed by a survey of time and frequency-domain considerations as they apply to mechanical measurements Subresonant, resonant, and wave methods are discussed, with applications

Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory

Abstract: The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature ~not exceeding 100°C!. The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying ~Pickett effect! which overpowers the effect of steady-state moisture content ~i.e., less moisture—less creep !. Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient ~exponential-type! integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the consti- tutive parameters and a satisfactory agreement with typical test data is achieved.

High temperature behavior of Ni-base weld metal: Part II – Insight into the mechanism for ductility dip cracking

Abstract: The ductility dip cracking susceptibility of two Ni-base filler metals, Filler Metal 52 (59%Ni–29%Cr–9%Fe–1%Mn–1%Al) and Filler Metal 82 (72%Ni–20%Cr–1%Fe–3%Mn–3%Nb) was evaluated using the strain-to-fracture test The analysis of these results and detailed weld metal microstructural characterization (SEM and TEM) was presented in Part I Part II of this work provides new insight into the creep-like, grain boundary sliding mechanism that leads to elevated temperature intergranular cracking in these weld deposits The effect of precipitation on grain boundary tortuosity and sliding, and its influence on ductility-dip cracking resistance is discussed in the context of current theories of high temperature creep Finally, the effect of composition, including both precipitate forming elements and impurity elements, on ductility-dip cracking is discussed

Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

Abstract: [001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135 MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ⟨011⟩ interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ⟨100⟩ edge dislocations climbing in the γ′ phase. Climb of a ⟨100⟩ dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ⟨011⟩ dislocations in the matrix. Moreover the γ′ disloc...

Simulation of shrinkage induced cracking in cement composite overlays

Abstract: A random lattice is used to model moisture transport in cement-based composites. Model geometry, and the scaling of the elemental diffusivity terms, are based on a Voronoi discretization of the material domain. Steady-state and transient potential flow problems are simulated, and compared with theory, to demonstrate model accuracy and objectivity with respect to lattice random geometry. A novel routine is described for computing moisture flux values at the random lattice sites. Stress production, and potential shrinkage crack development, are driven by the associated drying processes. The random lattice modeling of moisture movement is coupled with rigid-body-spring networks (RBSN), which account for elasticity, creep, and fracture properties of the material. The RBSN is based on the same Voronoi discretization of the material as used to model moisture movement. Relative humidity contours, stress contours, and crack plots are produced for a cement composite overlay restrained by a mature concrete substrate. That example is based on a set of simulation results reported in the literature.