Pre-straining Treatment Research Articles

Effect of pre-strain on microstructure evolution and fracture behavior of undermatching X80 pipeline steel girth weld

This work aims to study the effect of pre-strain on the fracture behavior of X80 pipeline girth weld joint, a comprehensive analysis was conducted on the microstructure and mechanical properties of the girth weld before and after pre-strain treatment. The mechanical properties were evaluated through tensile testing, Charpy impact testing, and digital image correlation (DIC) strain analysis. Furthermore, the microstructure and fracture morphology of the girth weld were observed using optical electron microscopy (OM) and scanning electron microscopy (SEM). The results show that the application of pre-strain treatment leads to dislocation accumulation at the grain boundary of X80 pipeline girth weld, resulting in stress concentration and subsequent formation of damage holes. This process disrupts the continuity of chain M-A island and initiates small cracks at the grain boundary, ultimately causing a significant decrease in impact toughness and impact work from 177 J to about 10 J.

The strain regulation of atomic structure and properties on ultra-stable monolayer butylammonium-based halide perovskites: Insight from density functional theory

The two-dimensional (2D) monolayer butylammonium (BA)-based halide perovskites are applied for perovskite solar cells (PSCs) due to ultra-stable and highly crystalline. However, the systematic analysis has not been well conducted for 2D monolayer (BA)2BX4 (BPb, Sn; X = I, Br, Cl) perovskites. Here we systematically investigated the photoelectric and mechanical properties using density functional theory (DFT), further revealing the effect and mechanism of the direct and doping-modulated strain induced by external strain(stress) and pre-strain treatment technology. (BA)2PbI4, (BA)2SnI4 and (BA)2SnBr4 have excellent properties for PSCs. Based on the direct strain regulation, (BA)2SnBr4 exhibits smaller bandgaps and more stable atomic evolution than (BA)2PbI4, and better mechanical properties and smaller electrostatic macroscopic average potential difference (ΔP) than (BA)2SnI4. Based on the doping-modulated strain regulation on 2D (BA)2(Pb1-xSnx)X4, lower Pb-doping ratio induces the nonlinear dependence of bandgaps, which mainly originating from the local compressive strain effect on Sn crystal structure induced by larger Pb-atomic radius. Especially, the double-decker structure (Sn2Pb2-p) obtains excellent stability due to stress relaxation induced by in-inorganic layer homogeneity. We anticipate this work will provide the sufficient guidance for designing and fabricating green 2D or 2D/3D halide junction PSCs to improve both PCE and stability.

Improved creep aging response in Al-Cu alloy by applying pre-aging before pre-straining

The improvement of post-form properties without compromising creep formability has been a critical issue in creep age forming of aluminum alloy component. A pretreatment process incorporating artificial pre-aging at 165 °C for 6 h/12 h/24 h followed by pre-strain (3%–9%) has been developed. This method not only evidently improves the strength but also accelerates the creep deformation during creep aging of an Al-Cu alloy. A strength increase of 50 MPa with a slight decrease of ductility relative to the 9% pre-strained alloy is acquired in the alloy artificially pre-aged for 24 h regardless of the pre-strain level (3%–9%). Artificial aging for 24 h prior to 3% pre-strain enables an increase of creep strain by 30%. The creep strain in the alloy artificially pre-aged for 24 h and pre-strained by 6% is comparable to that in the alloy pre-strained by 9%. The strength and ductility in the alloy artificially pre-aged for 6 h/12 h and pre-strained by 3% are even slightly higher than those in the alloy purely pre-strained by 9%. The characterizations by transmission electron microscopy reveal that pre-aging at 165 °C could promote the accumulation of dislocations during pre-straining due to the pinning effect of pre-existing Guinier-Preston zones (GP zones)/θ″ phases and thus expedite the creep deformation in respect to the pure pre-straining treatment. The enhanced precipitation of θ′ phases at these pinned dislocations contributes to the improved strength after creep aging. The results demonstrate applying artificial pre-aging before pre-straining is an efficient strategy to elevate the creep aging response in Al alloys.

The Effect of the Pre-Strain Process on the Strain Engineering of Two-Dimensional Materials and Their van der Waals Heterostructures

Two-dimensional (2D) materials and their van der Waals stacked heterostructures (vdWH) are becoming the rising and glowing candidates in the emerging flexible nanoelectronics and optoelectronic industry. Strain engineering proves to be an efficient way to modulate the band structure of 2D materials and their vdWH, which will broaden understanding and practical applications of the material. Therefore, how to apply desired strain to 2D materials and their vdWH is of great importance to get the intrinsic understanding of 2D materials and their vdWH with strain modulation. Here, systematic and comparative studies of strain engineering on monolayer WSe2 and graphene/WSe2 heterostructure are studied by photoluminescence (PL) measurements under uniaxial tensile strain. It is found that contacts between graphene and WSe2 interface are improved, and the residual strain is relieved through the pre-strain process, which thus results in the comparable shift rate of the neutral exciton (A) and trion (AT) of monolayer WSe2 and graphene/WSe2 heterostructure under the subsequent strain release process. Furthermore, the PL quenching occurred when the strain is restored to the original position also indicates the pre-strain process to 2D materials, and their vdWH is important and necessary for improving the interface contacts and reducing the residual strain. Thus, the intrinsic response of the 2D material and their vdWH under strain can be obtained after the pre-strain treatment. These findings provide a quick, fast and efficient way to apply desired strain and also have important significance in guiding the use of 2D materials and their vdWH in the field of flexible and wearable devices.

Effects of pre-strain and aging treatments on the mechanical property and corrosion resistance of the spray formed ultra-high strength Al-Zn-Mg-Cu alloy

Mechanical property and corrosion resistance are essential performances in ultra-high strength Al-Zn-Mg-Cu alloys for the aerospace and weapon industries. In this study, the pre-strain and aging treatments were used to enhance such performances of the spray formed ultra-high strength Al-Zn-Mg-Cu alloy. After 2% pre-strain with peak aging (PA) and retrogression and re-aging (RRA) treatments, the ultimate tensile strength, yield strength, and elongation can reach to 774 MPa, 735 MPa, 10.7% and 774 MPa, 747 MPa, 9.4%, respectively. The coarser precipitates of the samples with 4% pre-strain can lead to the slight lower strength compared with the samples with 2% pre-strain. The proper combination of pre-strain and aging treatments can also effectively enhance the stress corrosion resistance of the studied alloy by the following reasons. First, the higher strain concentration degree and dislocation density introduced by pre-strain treatments have high internal energy and provide more stored energy for the growth of the passive film. Second, the spacing of grain boundary precipitates under double aging (DA) and RRA states is larger than that under PA state. The width of uniform precipitation free zone (PFZ) with 2% pre-strain is larger than that with 4% pre-strain. Third, after pre-strain treatments, the cracks perpendicular to the deformation direction can be produced in Al 7 Cu 2 Fe phase, which further breaks up such particles. According to the results of Kelvin probe microscopy, the Al 7 Cu 2 Fe phase with smaller dimension can obtain the lower difference of potential to Al matrix. • The strength and corrosion resistance of the alloy were enhanced by pre-strain. • The pre-strain produced cracks in Al 7 Cu 2 Fe particles and further broke up them. • The pre-strain improved the strain concentration degree and dislocation density. • The effect of precipitation free zone and precipitate is better under 2% strain. • The improvement mechanism of pre-strain treatment was revealed.

Pre-strain accommodation enabled multi-dimensionally and hierarchically elastomeric MoSe2/MXene and AC/MXene electrodes for stretchable sodium ion capacitors

Stretchable supercapacitors with simultaneously high energy/power densities and long life can enrich integrated power options for ever-increasing wearable electronics. However, supercapacitors with high electrochemical performances while maintaining high stretchability are still challenging. Herein, we report stretchable sodium ion capacitors (S-SICs) based on multi-dimensionally and hierarchically elastomeric composite electrodes, and stretchable high voltage gel polymer electrolyte (3.0 V). The elastomeric composite electrode is constructed by stretchable 3D electrospun polyurethane (PU) fibrous mat and conductive 1D Ag nanowire network. 2D MoSe2/MXene and 3D AC/MXene as anodic and cathodic materials are embedded in this conductive and stretchable mat. This hierarchical structure enables multi-dimensional conductive connections under tensile condition. The pre-strain treatment on PU fibrous mat further increases the relatively stretching distance before losing conductive connection among hierarchical components. The electrodes exhibit superior sodium ion storage performances (420 mAh g−1 at 5 A g−1), as well as good stretchability and stability (95% after 1000 cycles at 2 A g−1). Based on this, the S-SICs with high operating voltage demonstrate high energy/power densities (max. 5.44 mWh cm−3, 493.55 mW cm−3) and good stretchability. This prototypical exhibition of S-SICs indicates a promising design direction of stretchable supercapacitors with high electrochemical performances and tolerance of stretchability.

Pre-strain-dependent creep ageing behavior of an Al Cu alloy

Pre-strain treatment is widely used in combination with many forming technologies to achieve better formability and higher performance. The pre-strain-dependency of the creep age behavior was systematically investigated in this paper. When pre-straining is introduced, the ageing strengthening of the AlCu alloy is significantly enhanced, which saturates when pre-strain amount exceeds 9%. The formability is also greatly improved due to the multiplication of the creep strain, along with an alteration in creep mechanism from diffusion creep to dislocation creep. Applying stress was still found to be detrimental to the ageing response but the degree is much smaller. Stress exceeding the yield strength was found to generate the same effect with certain amount of pre-strain. The microstructure including dislocations and precipitates was characterized with X-ray diffractometer, scanning/transmission electron microscopy and differential scanning calorimetry to explain the macroscopic evolution of the studied alloy. The typical bi-modal distribution of precipitates, where θ' heterogeneously grows around dislocation and θ" homogenously disperses in the matrix, was detected due to the pre-strain-induced massive dislocation. Increasing the pre-strain amount leads to a larger proportion of θ’ and a higher strength, which also saturates when pre-strain amount exceeds 9%. The present study revealed the relationship between the macroscopic behavior and the microstructural evolution in a large range of pre-strain amount and stress level, which provides valuable guidance in the development of the creep age forming technology.

Effect of pre-strain treatment on mechanical properties of a Ni-Co base disk superalloy

The effect of pre-strain treatment on mechanical properties of a Ni-Co base disk superalloy was studied. Results show that the pre-strain treatment, creep at 725 °C/630 MPa, improves the yield strength of the alloy at 23 °C and 760 °C. However, the creep life (at 760 °C/480 MPa) of the alloy was reduced by pre-strain treatment. The microstructure indicated that a lot of microtwins and Suzuki segregations were introduced by the pre-strain treatment, besides some unfavorable factors on mechanical properties, i.e., ripening of precipitates. The interactions of microtwin and Suzuki segregation with dislocations were discussed in this study, and it was deemed that the impediment of twin boundary and Suzuki segregation to the motion of dislocation may be responsible for the enhancement of the mechanical properties. Meanwhile, the microtwin and the Suzuki segregation have good stability at service temperature. If the problem of reducing the creep properties caused by pre-strain treatment would be solved, the introduction of microtwin and Suzuki segregation appropriately can be considered as a novel strengthening method in designing advanced superalloys.

Cyclic behavior of SMA slip friction damper

This paper proposes a new type of self-centering (SC) damper which exploits the shape memory alloy (SMA) bolts and variable friction mechanism, termed the SMA slip friction damper (SMASFD). The theoretical equations governing the force–displacement relationship, the equivalent stiffness and the equivalent viscous damping are first derived. Then, the application potential of such damper is briefly discussed. In the experimental study, the results of the experimental validations of the SMA bolts and the friction mechanism are presented, followed by the results of proof-of-concept test on the fabricated SMASFDs. According to the testing results, the damper exhibited a symmetric tension–compression flag-shaped hysteresis characterized by excellent SC capacity and good damping capability. The prestrain treatment was applied to the SMA bolts in one test, which was found effectively increased the initial stiffness of the damper. To provide a further understanding on the damper and examine the stress and strain demands of the components, high-fidelity finite element models were established for numerical simulations. Both the analytical method and numerical simulation were validated by the experimental data.

Strain effect on microstructural properties of SUS 304L joint by PAW+GTAW

JIS SUS 304L stainless steel was joined by a combination welding process of plasma arc welding and gas tungsten arc welding (PAW+GTAW). Then pre-strain treatment on 304L welded joint of 9 % was carried out using uniaxial quasi-static tensile at room temperature. Effect of strain on microstructure evolution in joint was analyzed by field emission scanning electron microscope (FESEM) and on mechanical properties was also studied. The results indicated that the pre-strain rate of 304L joint showed inhomogeneity including 3 % in the weld metal and 13 % in the heat-affected zone (HAZ), which induced martensitic transformation occurring in HAZ. Tensile strength of the joint increased from 700 MPa as welded to 804 MPa after pre-strain treatment at room temperature, and it reached 1700 MPa from 1480 MPa as welded at low temperature of −196 °C. Impact energy in the HAZ was the least among the whole 304L weld, but it was still 94 J at −196 °C after pre-strain. The fracture morphology showed large numbered of cleavage steps with elongated parabolic dimples.

Pre-strain mediated fast natural aging in a dilute Mg-Zn-Ca-Sn-Mn alloy

A remarkable natural aging (NA) hardening response induced by pre-strain treatment, i.e. an evident increment of flow stress from ~138 MPa to ~163 MPa within only 48h, was observed in a dilute Mg–1.0Zn–0.45Ca–0.33Sn–0.2Mn (wt.%, ZXTM1000) alloy sheet. The pre-strain deformation introduces profuse basal <a> dislocations and supersaturated non-equilibrium vacancies, favoring the solute diffusion process and therefore resulting in a heterogeneous solute distribution for energy minimization. The retarded dislocation movement by co-segregation of Zn and Ca atoms is responsible for the strengthening effect. This work sheds light on the appreciable NA phenomenon previously unexpected in dilute Mg alloys assisted by the introduction of crystal defects, and the associated microstructure evolutions at ambient temperature provide a novel strategy for the design of high-performance low-cost Mg alloys.

Effect of Combined Pre-Straining and Pre-Aging on the Precipitation Behavior and Age Hardening Response for Al-Mg-Si Alloys

Pre-strain (PS) and pre-aging (PA) treatments are often applied during the preparation of Al-Mg-Si automotive aluminum alloy. In this study, the effect of combined PS and PA on the precipitation behavior and age hardening response for Al-Mg-Si alloys was investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test, Vickers hardness test, and differential scanning calorimetry (DSC). It was found that the dislocations introduced by PS treatment and the cluster (2) formed during PA treatment effectively inhibited the cluster (1), which further strengthened the inhibition of natural aging hardening at room temperature (RT). The strength increment of the alloys was kept below 10.0 MPa during two weeks RT storage. The dislocations provided heterogeneous nucleation for the precipitates forming and the cluster (2) transformed into β″ strengthening phase during bake hardening treatment. With the acceleration response of the dislocations and the cluster (2), the age hardening response of Al-Mg-Si alloys obviously improved with the denser and larger β″ strengthening phase formed.

Thermomechanical pretreatment of Al-Zn-Mg-Cu alloy to improve formability and performance during creep-age forming

Abstract Thermomechanical treatment is often conducted to improve the formability and performance of high-strength aluminum alloys simultaneously. This paper presents an improved creep-age forming (CAF) processing technology that combines thermomechanical pretreatment and CAF. The proposed processing technique is expected to enhance the forming efficiency and comprehensive properties of Al–Zn–Mg–Cu alloys subjected to CAF. Creep-aging behavior and properties of the 7150 aluminum alloy undergoing retrogression and re-aging (RRA) with 3 % pre-strain treatment was experimentally investigated in this study. Results obtained reveal greater creep deformation in thermomechanically pretreated samples than in those pretreated using RRA or 3 % pre-strain exclusively. Mechanical properties, electrical conductivity, and exfoliation corrosion of creep-aged samples with thermomechanical pretreatment were closely match those of the traditional RRA-treated sample. Microstructure observation discovered η2 phase in the creep-aged RRA pretreated sample in addition to conventional GP / η’ phase, η phase and coarse and discrete grain-boundary precipitates. Low concentration of solutes in matrix in RRA pretreated sample impeded dislocation motion less and pre-strain-induced dislocations increased the creep strain significantly. Moreover, hardening effect of η2 phase and dislocation hardening compensated the strength loss due to precipitation coarsening. The proposed thermomechanical pretreatment process provides an appropriate initial temper for CAF of Al–Zn–Mg–Cu alloys, thereby improving the creep formability and comprehensive properties remarkably. The findings of this study could aid in the development of CAF technology for Al–Zn–Mg–Cu alloys, especially in terms of manufacturing aviation components.

Influence of pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel

In the present work, influences of room temperature tensile pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel (SS) were investigated. The tensile properties of the 316LN SS with various tensile pre-straining amounts of 15%, 25% and 35% were measured at both room temperature (RT) and liquid helium temperature (4.2 K). Results indicated that both 0.2% proof strength (Rp0.2) and ultimate tensile strength (Rm) at both RT and 4.2 K of the pre-strained 316LN SS significantly increased compared to those of the as-received material. The Rp0.2 and Rm at 4.2 K for 35% pre-strained specimens increased around 86% and 18%, respectively, compared to those of the as-received material. However, the elongation at fracture (A) at both RT and 4.2 K decreased for the pre-strained 316LN SS compared to those of the as-received material. The fractured morphologies of the pre-strained 316LN SS were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dimples in the surface fractured at both RT and 4.2 K of the as-received and pre-strained 316LN exhibited a ductile fracture mode. Moreover, fractographic results indicated that the pre-strain treatment considerably decreased the ductility at both RT and 4.2 K. TEM analyses revealed that pre-strain treatment at RT led to dislocation accumulation and stress-induced martensite phase occurred at 4.2 K, which interpreted the cryogenic tensile properties of the pre-strained 316LN SS.

Effect of pre-straining treatment on high temperature creep behavior of Ni-based single crystal superalloys

This study investigated a novel hypothesis on the influences of residual or pre-dislocation structures on creep life of Ni-based single crystal superalloys. Different types of anisotropic dislocation structures were created by pre-straining the samples at a relatively low temperature under different constant bias load conditions. After such pre-straining treatment, the samples were subjected to creep testing at higher temperatures under uniaxial tension. It was found that the pre-straining treatment was effective in creating different anisotropic dislocation structures without affecting the γ′ phase cuboid morphology and that the samples containing different anisotropic dislocation structures have all significantly reduced creep lives. The difference in the anisotropy of the dislocation structures has little influence on the reduction of the creep life or the rafting morphology of the γ′ phase cuboids. By using a Re-free alloy, the influence of the TCP phase precipitation on creep life is also clarified. These observations suggest that interruption to high temperature straining is the main contributor to the reduced creep lives. This finding raises a serious concern on the validity, or reliability, of creep life assessment based on continuous laboratory creep tests for real life applications, where the services are almost always interrupted into intermittent sessions.

Intrinsically Stretchable Nanostructured Silver Electrodes for Realizing Efficient Strain Sensors and Stretchable Organic Photovoltaics.

In this study, a new hybrid electrode featuring a high gauge factor of >30, decent stretchability (100% of the original conductivity can be retained after 50 cycles of stretching under a 20% strain without prestrain treatment), high transmittance (>70%) across 400-900 nm, and a good sheet resistance (<50 Ω sq-1) was successfully exploited. These superior properties were revealed to originate from the reversible phase separation endowed by the nanogranular-like morphology formed in Ag. Owing to such discrete nanomorphology, the free volume within this Ag electrode is susceptible to the applied tensile strain and the ensuing change in conductivity enables the realization of an efficient strain sensor. Besides, a representative PTB7-th:PC71BM organic photovoltaic (OPV) using this electrode (with the assistance of a wrinkled scaffold to reinforce the stretchability of the active layer) can exhibit a power-conversion efficiency (PCE) of 6% along with high deformability, for which 75% of its original PCE is retained after 50 cycles of stretching under a 20% strain. Meanwhile, a representative all-polymer OPV consisting of a PTB7-th:N2200 blend, in which the N2200 has a better mechanical stretchability than that of PC71BM, can maintain over 96% of its original PCE after 50 cycles of stretching (under a 20% strain) without employing the wrinkled scaffold. Such promising performance in stretchable OPVs is among the state-of-the-art results reported to date.

Effect of pre-strain on creep aging behavior of 2524 aluminum alloy

As an advanced manufacturing technology, creep age forming (CAF) has been developed to form large aluminum panels. However, industrial processes often involve complex thermo-mechanical treatments. For instance, it is necessary to impose a plastic strain after solution heat treatment to relieve quench stresses. In this work, the effect of tensile pre-strain before CAF on creep aging behavior of 2524 aluminum alloy is investigated by constant-stress creep aging test, Transmission Electron Microscope (TEM) and mechanical properties test. It is found that the magnitude of creep strain is greatly increased with the increase of pre-strain. Moreover, the peak strength of the creep aged AA2524 is enhanced and the time to reach the peak strength is decreased with the increases of pre-strain. TEM observations revealed that the precipitation process of AA2524 is accelerated due to pre-strain treatment. With the increase of pre-strain, the average size of S (Al2CuMg) phases is reduced. Meanwhile, the density and homogeneity of S phases are improved. Thus, it can be concluded that the pre-strain can evidently improve the formability, mechanical properties and microstructures of AA2524 in creep age forming process.

Effects of high temperature pre-straining on natural aging and bake hardening response of Al–Mg–Si alloys

The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al–Mg–Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.

Pre-strain effect on fatigue strength characteristics of SUH660 plain specimens

Precipitation particles in precipitation-strengthened materials are considered to be cut by pre-strain treatment, which affects the fatigue strength. In this study, fatigue tests were performed on precipitation-strengthened stainless steel SUH660 to investigate the effect of pre-strain on fatigue crack initiation and propagation characteristics. Fatigue test results showed that pre-strained specimens have a shorter fatigue crack propagation life compared to non-strained specimens; this is the opposite of results observed in carbon steel. The accelerated fatigue crack growth observed in pre-strained specimens was first assumed to be caused by precipitate cutting. A dislocation accumulation model for the fatigue crack tip in the precipitation-strengthened material was then suggested. Buff-polished specimens were also used for the fatigue tests, and the results showed that the work-hardened layer had a significant impact on the fatigue strength and fatigue life.

Influence of a pre-strain treatment on the stresses generated by the growth of an oxide scale on Ni 76Cr 16Fe 8 alloys

The influence of pre-deformation on the growth stresses and related parameters were studied at P o 2 = 10 5 and 1 Pa at 740°C. Two Ni 76Cr 16Fe 8 alloys with different carbon contents were analysed. Thermogravimetric and deflection experiments were carried out. Oxide scale analyses were performed by means of X-ray photo-electron spectroscopy and scanning electron microscopy. Differences in the oxide scales growing at P o 2 = 10 5 and 1 Pa were detected. The variations of Δ D = D(O 2)- D)(argon) (where D is the deflection value) can be related to the growth of different oxide layers. Influences of predeformation and of carbon content on the oxide scale thickness were clearly observed. The calculated oxide growth stresses are compared with the oxide yield point.