Inhibition of Electromigration in Eutectic SnBi Solder Interconnect by Plastic Prestraining

Plastic prestraining was applied to a solder interconnect to introduce internal defects such as dislocations in order to investigate the interaction of dislocations with electromigration damage. Above a critical prestrain, Bi interfacial segregation to the anode, a clear indication of electromigration damage in SnBi solder inter- connect, was effectively prevented. Such an inhibiting effect is apparently contrary to the common notion that dislocations often act as fast diffusion paths. It is suggested that the dislocations introduced by plastic prestraining acted as sinks for vacancies in the early stage of the electromigration process, but as the vacancies accumulated at the dislocations, climb of those dislocations prompted recovery of the deformed samples under current stressing, greatly decreasing the density of dislocation and vacancy in the solder, leading to slower diffusion of Bi atoms.

Dynamic stretching–electroplating metal-coated textile for a flexible and stretchable zinc–air battery

A metal electrode is a significant component of a zinc–air battery(ZAB),but the metal material is usually not elastic,which severely restricts the application of flexible and stretchable ZABs in the field of wearable electronic devices.Herein,we report a flexible and stretchable metal-coated textile prepared by a dynamic stretching–electroplating based on a wavy spandex textile substrate.Benefiting from the unique woven and wavy structure,the metal-coated textile shows a high stretchability of 100%and stable conductivity.In situ scanning electron microscope observation during stretching showed that the tensile strain of the metal-coated textile is mainly attributed to the deformation of the microfiber network at the bottom position of the wave structure.In addition,a sodium carboxymethyl cellulose–polyacrylic acid–potassium hydroxide composite hydrogel has been used as the electrolyte.This hydrogel shows excellent ionic conductivity,mechanical properties,and water retention properties,which makes it suitable for the semi-open system of ZAB.Furthermore,a flexible and stretchable sandwich-structure ZAB,assembled using the above-mentioned electrodes and electrolyte,operates stably even under rapid stretching/releasing cycle deformation.Because of its facile preparation and low cost,this flexible and stretchable ZAB is suitable for fabrication of large-area batteries to obtain higher output current and power to drive wearable electronic devices.

Bake hardening behavior of TRIP and DP steels

The bake hardening （BH） behavior of transformation-induced plasticity （TRIP） and dual-phase （DP） steels after different prestrains was studied. The experimental results indicate that TRIP steel exhibits good BH ability while DP steel does not, and prestrain displays a strong effect on the BH values of both steels. The comparison of microstructures of the two steels showed that the hard second phase in the matrix might be harmful to the BH ability. For deformed specimens, baking resulted in a loss of uniform elongation, but there was no obvious decrease in uniform elongation for unprestrained specimens.

The Reverse Transformation Behavior of TiNi Shape Memory Alloy Wires Prestrained at Different Temperatures

The reverse transformation behavior of TiNi alloy wires prestrained at different temperatures is studied in this paper. Experimental results show that prestrain at different temperatures obviously affects the reverse transformation behavior of the TiNi alloy wire. A single peak appears on the DSC curves of wires prestrained at 253-313K (in the martensite state). However deformed at 333K, three consecutive peaks appear on the DSC curves of wires with a smaller prestrain and a single peak appears on the DSC curves of the wires with a larger prestrain. The recovery strain ratio of the wires prestrained at 253-313K are very similar. However, the ratio of the wire predeformed at 333K is obviously smaller than that of the wire prestrained in the martensite state.

Reverse Transformation Behavior of TiNi Shape Memory Alloys Prestrained in the Parent Phase

The reverse martensitic transformation of TiNi alloy wires prestrained in the parent phase was studied. Experimental results shou, that the reverse transformation of the TiNi allogys prestrained in the parent phase is significantly different from that of the TiNi alloys prestrained in the martensite phase. Three continual peaks appear on the DSC curves of wires with a small prestrain and one high temperature peak appears on the DSC curves of wires with a large prestrain.

A reliability study of springback on the sheet metal forming process under probabilistic variation of prestrain and blank holder force

This work deals with a reliability assessment of springback problem during the sheet metal forming process. The effects of operative parameters and material properties, blank holder force and plastic prestrain, on springback are in- vestigated. A generic reliability approach was developed to control springback. Subsequently, the Monte Carlo simula- tion technique in conjunction with the Latin hypercube sam- pling method was adopted to study the probabilistic spring- back. Finite element method based on implicit/explicit al- gorithms was used to model the springback problem. The proposed constitutive law for sheet metal takes into account the adaptation of plastic parameters of the hardening law for each prestrain level considered. Rackwitz-Fiessler al- gorithm is used to find reliability properties from response surfaces of chosen springback geometrical parameters. The obtained results were analyzed using a multi-state limit reli- ability functions based on geometry compensations.

Mechanical behavior of electric resistance welded pipes in pipe-making process

The changes in the mechanical behavior of electric resistance welded（ERW） pipes before and after the cage roll forming process were investigated through tensile experiments. It is found that the Bauschinger effect does not exist in the pipe product, while the work hardening effect introduced by pipe-making is the direct cause of the mechanical changes. The prestrain introduced during different pipe-making processes are accumulative to the work hardening effect. And the increment of the yield strength for making Ф 244.48 × 8.94 pipe is approximately 45 MPa, higher than that of hot-rolled plates. It is verified that the strain ε=t/D-t is an efficient index representing the work hardening effect from the engineering viewpoint.

Mechanics of nonbuckling interconnects with prestrain for stretchable electronics

The performance of the flexibility and stretchability of flexible electronics depends on the mechanical structure design,for which a great progress has been made in past years.The use of prestrain in the substrate,causing the compression of the transferred interconnects,can provide high elastic stretchability.Recently,the nonbuckling interconnects have been designed,where thick bar replaces thin ribbon layout to yield scissor-like in-plane deformation instead of in-or out-of-plane buckling modes.The nonbuckling interconnect design achieves significantly enhanced stretchability.However,combined use of prestrain and nonbuckling interconnects has not been explored.This paper aims to study the mechanical behavior of nonbuckling interconnects bonded to the prestrained substrate analytically and numerically.It is found that larger prestrain,longer straight segment,and smaller arc radius yield smaller strain in the interconnects.On the other hand,larger prestrain can also cause larger strain in the interconnects after releasing the prestrain.Therefore,the optimization of the prestrain needs to be found to achieve favorable stretchability.

Cross-linking density and aging constitutive model of HTPB coating under prestrain thermal accelerated aging

In order to study the cross-linking density and aging constitutive relationship of HTPB coating during storage,the thermal accelerated aging tests at 0%,3%,6%and 9%prestrains were carried out.The crosslinking density of HTPB coating at different aging stages were tested using low-field^1 H NMR and the variation of cross-linking density was analyzed.The aging model of cross-linking density considering the chemical aging and the physical stretching factors was established.The uniaxial tensile tests were carried out on HTPB coating at different aging stages and the cross-linking density was introduced into Ogden hyperelastic constitutive model as a characterization parameter of correction coefficient.Combined with uniaxial tensile test results,a prestrain aging constitutive model of HTPB coating was established.The results show that the cross-linking density of HTPB coating increases rapidly at first and then slowly with the increase of thermal accelerated aging time without prestrain.Under prestrain conditions,the crosslinking density of HTPB coating decreases at the early stage,and increases rapidly at first and then slowly at the middle and late stages of thermal accelerated aging.The correlation coefficients of aging model of cross-linking density and aging constitutive model with test results are R>0.9500 and R>0.9900 respectively,which can be used to accurately describe the cross-linking density and aging constitutive relationship of HTPB coating under prestrain accelerated thermal aging conditions.

EFFECT OF RESIDUAL STRESS ON MICROPLASTIC DEFORMATION BEHAVIOR

The effect of residual stress on microplastic deformation behavior andmicroyicid strength is investigated.The results show that the residual stress does not affectthe characteristics of mieroplastic deformation,but has an influence on the microyieldstress.It also points out that its effects can not be judged by the traditional view of simplysuperimposing the external stress onto the residual stress.

An overview of strain ageing in low carbon steels

Strain ageing in steel was first observed during the 19th century when the maximum load carrying capacity of a test piece was increased after it had been retested following a previous series of testing in a plastic range. Over the last decades, a great deal of research has been performed to gain deeper insights into the phenomenon of strain ageing. The synergistic development of theoretical hypothesis and new experimental evidences has promoted the understanding of the mechanisms underlying strain ageing. The content of this paper is organized into four components. First, we have compared the main theories developed in recent years. Second, we have summarized the metallurgical effects on the strain ageing behavior observed in experiments. Moreover, we have made an attempt to correlate the experimental results and the above-mentioned theories. Third, we have proposed a feasible solution to control strain ageing in low carbon steels. Finally, we have elaborated the potentiality of the atom probe technique in facilitating direct characterization of the distribution of solute atoms.

THE DAMAGE STATES AND TENSILE FAILURE BEHAVIOR OF TORSIONAL PRESTRAINED STEELS

In this paper,the damage state of a torsional prestrained steel is examined by means of the concepts of continuum damage mechanics and then the tensile properties and fracture ductility of two kinds of steels under various torsional prestrained conditions are investigated from both macroscopic and microscopic points of very slight as contrasted with tensile damage;(2)after torsional prestraining,both yielding strength and ultimate tensile strength become higher for 20 steel and lower for 40Cr steel;(3)when the torsional prestrain exceeds a critical value,that is about 70% of pure torsional shear fracture strain,the ductile-brittle transition of tensile fracture behavior may initiates.Moreover,the advantages and applicable conditions of torsional prestrain strengthening technique are also discussed.

Artificial neural network model of constitutive relations for shock-prestrained copper

Data from the deformation on Split Hopkinson Bar were used for constructing an artificial neural network model. When putting the thermodynamic parameters of the metals into the trained network model, the corresponding yielding stress can be predicted. The results show that the systematic error is small when the objective function is 0.5 , the number of the nodes in the hidden layer is 6 and the learning rate is about 0.1 , and the accuracy of the rate error is less than 3%. [

Effect of prestrain and baking on the secondary deformation properties of cold-rolled ultra-high-strength steel sheets

Two experimental steels with tensile strength above 980 MPa were prepared to investigate the effect of prestrain and baking on their mechanical and fracture behaviors. The experimental results reveal that,for both experimental steels,with increases in the prestrain level,the bake hardening value increases before reaching a maximum point,and then decreases with further increases in the prestrain level. The results of a "bending-baking-secondary bending"test indicate that the secondary bendability deteriorates at a high level of prestrain. The yield strength of the experimental steels was found to increase and the elongation to decrease after high levels of prestrain and bake hardening. Fracture morphology images indicate that a high prestrain level is associated with shallow dimples and more and larger local cleavage areas.

Theoretical Analysis of the Buckling Behaviors of Inhomogeneous Shape Memory Polymer Composite Laminates Considering Prestrains

The mismatch in thermal expansion coefficients between the fiber-rich and resin-rich regions of a shape memory polymer composite(SMPC)laminate,along with the residual strain during SMPC fabrication,results in buckling deformation of the inhomogeneous laminate.This paper presents a macroscopic model for buckling of an inhomogeneous SMPC laminate under initial biaxial prestrains.Both linear and nonlinear buckling analyses are carried out using the energy method.The influences of prestrain biaxiality,temperature,and ply angle on the buckling wavelength,critical buckling prestrain,and buckling amplitude are calculated.The results demonstrate that the critical buckling wavelength of the SMPC laminate is independent of the prestrain,while the amplitude is almost independent of temperature.In addition,the optimal fiber stacking configuration with the maximum critical buckling prestrains of inhomogeneous SMPC laminates is determined by a genetic algorithm.

Influence of Pre-Transformed Martensite on Work-Hardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work-hardening behavior under the uniaxial tensile condition. The X-ray diffractometer （XRD） was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre-transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work-hardening curve of prestrained specimens observably changes with true strain. The work-hardening exponent n of stainless steel decreases with the increase of pre-transformed martensite. The achievement is a significant contribution to the process design during pressing.

Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging

The multi-principal-component concept of high-entropy alloys(HEAs) generates numerous new alloys.Among them,nanoscale precipitated HEAs have achieved superior mechanical properties and shown the potentials for structural applications.However,it is still a great challe nge to find the optimal alloy within the numerous candidates.Up to now,the reported nanoprecipitated HEAs are mainly designed by a trialand-error approach with the aid of phase diagram calculations,limiting the development of structural HEAs.In the current work,a novel method is proposed to accelerate the development of ultra-strong nanoprecipitated HEAs.With the guidance of physical metallurgy,the volume fraction of the required nanoprecipitates is designed from a machine learning of big data with thermodynamic foundation while the morphology of precipitates is kinetically tailored by prestrain aging.As a proof-of-principle study,an HEA with superior strength and ductility has been designed and systematically investigated.The newly developed γ’-strengthened HEA exhibits 1.31 GPa yield strength,1.65 GPa ultimate tensile strength,and 15% tensile elongation.Atom probe tomography and transmission electron microscope characterizations reveal the well-controlled high γ’ volume fraction(52%) and refined precipitate size(19 nm).The refinement of nanoprecipitates originates from the accelerated nucleation of the γ’ phase by prestrain aging.A deeper understanding of the excellent mechanical properties is illustrated from the aspect of strengthening mecha nisms.Finally,the versatility of the current design strategy to other precipitation-hardened alloys is discussed.

Effect of Prestrain on Microstructures and Properties of Si-Al-Mn TRIP Steel Sheet With Niobium

Paint baking treatment was carried out in a silicon oil bath at 170℃ for 20 min for Si-Al-Mn TRIP Steel sheet with different prestrains, and effect of prestrain on microstructures and properties was studied before and after baking. The results show that with the increasing of prestrain amount during prestraining and baking, the volume fraction of retained austenite decreases, and the volume fraction of martensite and bainite increases as well as yield strength increases; as prestrain ranges from 0 to 4%, the baking-hardening （BH） value increases; while the prestrain ranges from 4% to 16%, the BH value decreases; when the prestrain amount is 4%, the highest BH value is about 70 MPa for Si-Al-Mn TRIP steel sheet with niobium, which displays excellent baking-hardening behavior.

Microscopic Response of TRIP Steels to Prestrain During Plastic Deformation

In order to uncover the mechanism of elastic modulus degradation during plastic deformation, uniaxial ten- sile test of transformation-induced plasticity （TRIP） steels under different prestrain levels was carried out. The real elastic modulus unloaded at each prestrain was calculated by linearly fitting. The microstructure evolution with plas-tic strain and the fracture morphology were monitored by using a scanning electron microscope （SEM）. Dislocation density and its distribution were detected under a transmission electron microscope （TEM）. Microscopic mechanism of the elastic modulus degradation of TRIP steels was discussed in detail. Experimental results indicated that the in- vestigated TRIP600 steel was of severe elastic modulus degradation during plastic deformation. The new-born mar tensite distributed among the retained austenite, resulting in the combination of good ductility and high strength for TRIP steels. It was the change of dislocation movement that induced the variation of atomic binding force and finally led to the variation of elastic modulus.

Prediction of cyclic large plasticity for prestrained structural steel using only tensile coupon tests

Cold-formed steel members,which experience complicated prestrain histories,are frequently applied in structural engineering.This paper aims to predict cyclic plasticity of structural steels with tensile and compressive prestrain.Monotonic and cyclic tests on hourglass specimens with tensile and compressive prestrain are conducted,and compared with numerical simulations using the Chaboche model.Two approaches are taken in the simulation.The first requires only the monotonic tensile test data from the prestrained steels,and the second requires both the monotonic tensile test data from the virgin steel and the prestrain histories.The first approach slightly overestimates the compressive stress for specimens with tensile prestrain,while the second approach is able to accurately predict the cyclic plasticity in specimens with tensile and compressive prestrain.