Plastic Mismatch Research Articles

Grain refinement, strain hardening and fracture in thermomechanically processed ultra-strong microalloyed steel

The current research in microalloyed steel has witnessed a continuous improvement in strength-ductility combination. The intention is to reduce the gauge thickness of automotive body sheets without compromising formability, thereby enhancing fuel efficiency. In this light, the present work has devised a novel thermomechanical controlled processing (TMCP) for a low-carbon automotive-grade (Nb+V)-stabilized microalloyed steel. The microstructural modification by TMCP has resulted in an excellent combination of yield strength (782 MPa), ultimate tensile strength (1059 MPa) and ductility (21%). The critical factor translating the investigated alloy towards advanced high strength steel is the ferrite grain refinement by dynamic recrystallization (DRX) and deformation-induced ferrite transformation (DIFT). The DRX predominated during TMCP at the intercritical temperature domain of 800–700 °C, imparting the average recrystallized grain size of 2 ± 0.8 µm. In contrast, the DIFT starts well above the upper intercritical temperature regime (i.e., around 810 °C), but it contributes less to the ferrite grain refinement. The thermodynamic reason has been analyzed keeping in view the changes in grain boundary energy and dislocation energy. The plastic deformation additionally leads to precipitation of cubic (Nb, V)-rich carbides in ferrite. The incoherency, coarse precipitate size (70–80 nm), and a poor phase fraction (⁓0.5%) are the key factors that the strengthening mechanism is governed by ferrite grain refinement majorly, followed by dislocation and solid solution strengthening. The segregation of Mn, Nb and S leads to MnS-type inclusion formation, which acts as potential sites for crack initiation due to plasticity mismatch with ferrite matrix during the tensile deformation.

Achieving superplastic TWIP steel welded joint via vacuum electron beam welding

A crack-free high-Mn TWIP steel welding joint with comparable strength and superplastic was obtained via vacuum electron beam welding (VEBW) where the microstructural evolution, mechanical behaviors and strengthening mechanism of welding joint were investigated. The results indicated that the welding joint with a typical nail-shaped welding cross-section was divided into three zones including the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ). Massive fine AlN-type precipitates with the hexagonal crystal structure were observed in welding join. The orientation relationship between the γ-austenite matrix and AlN precipitates was determined as [001]γ//[2110]AlN. In FZ, small-sized cellular grains or columnar grains with consistent growth directions were combined into large-sized columnar grains with obvious <001> preferential orientation. Compared with the base mantal, the yield strength, ultimate strength and elongation of the VEBWed specimen were slightly reduced by 6.03%, 4.7% and 13.23%, respectively. However, the impact toughness was obviously decreased from the 87.90 J–68.97 J after the VEBW process, which was mainly attributed to the brittle AlN precipitates, inhomogeneous microstructure and back stress induced by the plastic mismatch between gradient grains. The strength contributions of different regions to overall strength including of BM, HAZ and FZ in welding joint were calculated. Here, the dislocation strengthening and precipitation strengthening were the dominant strengthening mechanisms of FZ, which was attributed to the high level of density and massive fine AlN precipitates induced by micro-elements segregations during the VEBW process.

Analysis of fracture toughness and fatigue crack growth rates of electron beam-welded dissimilar nickel-based superalloys: Mode-I cracks oriented normal to the weld

This paper discusses the experimentally obtained results of fracture toughness and fatigue crack growth rates of dissimilar metal-welded joints of Inconel 718 to Nimonic 80A. The dissimilar metal-welded joints were prepared in butt joint configuration using electron beam welding. Mode-I cracks were oriented normal to the weld. The base metals have different yield strength values (plastic mismatch) and ultimate tensile strength values but have the same elastic modulus and thermal expansion coefficients. The experiments were conducted using single-edge notched bend specimen design. The fracture toughness ( J) and fatigue crack growth rates of dissimilar metal-welded joints were affected by the direction of crack growth, such as cracks moving from stronger metal to weaker metal or vice versa. In comparison to Nimonic 80A (base metal), the J value of dissimilar metal-welded joint with crack direction from Nimonic 80A (stronger) to Inconel 718 (weaker) was lower. Moreover, the J value of dissimilar metal-welded joint was higher than that of Nimonic 80A (base metal) when crack traveled from Inconel 718 (weaker) to Nimonic 80A (stronger). In the fatigue crack growth rate test, the crack grew faster as it approached the weld interface from Nimonic 80A toward Inconel 718 due to crack acceleration. Conversely, the crack grew slowly in the opposite direction due to crack shielding. The onset of these effects began sooner for higher loads than lower loads.

Fracture toughness and fatigue crack growth rate studies on rotary friction weldments of nickel-based superalloys

In this paper, fracture toughness (FT) and fatigue crack growth rate (FCGR) studies were carried out on dissimilar metal welded joints (DMWJs) of Inconel 718 and Nimonic 80A. The DMWJs were made using rotary friction welding (RFW). The base metals (BMs) showed plastic mismatch. Single edge notched bend specimen was used for FT and FCGR studies. The crack was initiated and propagated along the weld. BMs showed higher toughness than the DMWJs. The crack growth resistance offered by the DMWJs were inferior to BMs. Post-weld heat-treated DMWJ showed improved FT and crack growth resistance than the untreated DMWJ.

Effect of Direct energy surface modification on fatigue life of rotary friction welded Ti6Al4V ELI fatigue coupons

Fatigue crack initiation in engineering components is predominantly associated with tensile surface residual stresses or stress raisers caused by discontinuities or mechanical notches. During welding, surface residual stresses arising from plastic mismatch due to the temperature gradient can accelerate the initiation of cracks during cyclic fatigue testing. Direct energy surface treatment is currently considered a viable post-processing methodology for modifying surfaces within the weld zone to improve fatigue life. This paper discusses results obtained during experimental research, investigating the effect of direct energy surface treatment on surface residual stresses, microstructure, and fatigue life of cylindrical rotary friction welded Ti-6Al-4 V samples. Direct energy post-processing technique aided in improving the fatigue life of friction welded specimens by modifying the surface microstructure, introducing a uniform refined and homogenous structure in the welded region. This near-surface refined microstructure inhibited crack initiation in the welded region, shifting initiation sites to an area adjacent to the modified surface region.

Investigation on fatigue crack propagation law of the crack perpendicular to interface for Zr/Ti/steel composite plate

In this paper, the effects of load ratio, elastic mismatch and plastic mismatch on the fatigue crack growth behavior of zirconium-titanium-steel composite plate are discussed in detail. Firstly, considering the crack closure effect caused by load ratio, different effective driving force models of crack tip are studied. The results show that Daniel's two parameter driving force model has the best normalization effect. Then, based on the material inhomogeneity theory, the equation of actual stress intensity factor at crack tip is obtained, and the crack growth rate equation considering elastic mismatch is obtained. Furthermore, the influence of plastic mismatch is studied. Finally, by dividing the influence area of the elastic-plastic mismatch and combining the respective crack growth criteria, the crack growth rate equation considering the load ratio and elastic-plastic mismatch is obtained.

Mechanical properties and fracture behaviors of medium-Mn steels with and without delta-ferrite for different intercritical annealing times

The mechanical properties and fractured behaviors of 0.2C–6Mn–3Al-(0/0.6)Si medium Mn steels, denoted as 0Si and 0.6Si (0Si without δ-ferrite and 0.6Si with abundant δ-ferrite), are investigated. As the annealing time increases, δ transforms from a hard phase to a soft phase. Due to the hard δ and the weak transformation-induced plasticity (TRIP) effect in 0.6Si, the product of strength and elongation (PSE) is slightly lower in 0.6Si than in 0Si, when the annealing time is less than 7 min. However, the PSE is much higher in 0.6Si than 0Si, resulting from the soft δ and the appropriate TRIP effect in 0.6Si, when the annealing time is higher than 10 min. Microcracks of 0Si initiated at the γ(α')/α interface, and unstable γ could enhance the nucleation and increase the number of microcracks. Microcracks of 0.6Si propagated along the (γ(α') + α)/δ interface, resulting in delamination on fractograph. When the hard δ transformed into soft δ, soft δ abates the plasticity mismatch of the interface, thereby leading to a decrease in the number of interfacial microcracks and delaminations.

Microstructure and Formation Mechanism of Ultrasound-Assisted Transient Liquid Phase Bonded Magnesium Alloys with Ni Interlayer.

Ultrasound-assisted transient liquid phase bonding (U-TLP) has been regarded as a promising brazing process to join magnesium alloys with a Sn and Zn interlayer; however, the formation of brittle magnesium intermetallic compounds (Mg2Sn, MgZn, and MgZn2) compromises the mechanical properties of the joints. In this study, Mg alloy U-TLP joints with a Ni interlayer were evaluated based on shear strength and hardness measurement. Microstructural evolution along with ultrasonic duration time and intermetallic compound formation were characterized using X-ray diffraction and electron microscopy methods. The results show that incremental ultrasonic durations of up to 30 s lead to the microstructural evolution from the Mg2Ni layer, eutectic compounds (Mg2Ni and α-Mg) to α-Mg (Ni), accompanied by shear strength increases. The maximum value of the shear strength is 107 MPa. The role that ultrasound vibration played in brazing was evaluated, and showed that the MgO film was broken by the acoustic softening effect when the interlayer and base metal were solid. As the MgO and Mg substrate have different stress reduction τ, this plastic mismatch helps to break the oxide film. Additionally, the diffusion between the solid Mg substrate and Ni interlayer is accelerated greatly by the acoustic pressure based on the DICTRA dynamic calculation.

The Performance of TiAlSiN Coated Cemented Carbide Tools Enhanced by Inserting Ti Interlayers

To enhance the cutting performance of TiAlSiN coated cemented carbide tools by inserting Ti interlayers and to explore their mechanism, TiAlSiN/Ti multilayer coatings with different Ti thicknesses, including 0 nm, 25 nm, 50 nm, 100 nm, and 150 nm, were deposited onto cemented carbide (WC-10 wt%, Co) substrates by high power impulse magnetron sputtering (HiPIMS). The microstructure, hardness, grain orientation, residual stress, adhesion, and toughness of those coatings were measured, and the cutting performance against Inconel 718 was analyzed. Meanwhile, finite element method (FEM) indentation simulations were performed to gain detailed insight into the effects of Ti interlayer thickness on mechanical properties of TiAlSiN/Ti multilayer coatings. Results demonstrated that mechanical properties of TiAlSiN multilayer coatings were significantly changed after the Ti interlayer was introduced, and the multilayer coating #M2 with 25 nm Ti layer showed the excellent toughness and adhesion without sacrificing hardness too much. As Ti interlayer thickness increased, both toughness and adhesion decrease owing to the plastic mismatch between individual layers, and these changes were discussed detailedly with finite element method. Moreover, the result of the cutting experiment also revealed that the tool flank wear Vb can be reduced by the multilayer structure. This improvement is believed to be due to the increasing toughness, which alleviated the damage caused by the continuous impact load of hard phases generated by Inconel 718 during cutting.

The coordination behaviors of alpha and beta phases with different grain sizes in Ti-6Al-4V subjected to surface severe plastic deformation

The structure characteristics of alpha and beta phases on both sides of phase boundary in Ti-6Al-4V subjected to high energy shot peening were investigated via high-resolution transmission electron microscope. The results indicated that the incoherent phase boundary was the considerably active dislocation resource and stress concentration in phase boundary owing to plastic mismatch of two phases offered larger driving force for dislocation nucleation and motion. Correspondingly, twins were completely suppressed nearby phase boundary. Because of the diversity of grain refinement levels of alpha and beta phases, the coordination deformation behaviors were analyzed based on the ratio dα/dβ, in which dα and dβ were average grain sizes of alpha and beta phases respectively. As dα/dβ was above 10 and below 30, beta phase with extremely tiny grain sizes promoted dislocation nucleation and pileup of alpha phase with larger sizes. In contrast, as dα/dβ was above 3 and below 10, alpha phase which was severely work-hardening promoted dislocation multiplication of beta phase. As dα/dβ was below 3, the phase with higher orientation factor primarily plastically deformed and promoted dislocation motion and pileup of the other phase, further resulting in crystal rotation of equiaxed ultrafine grains and grain refinement of nanograins.

Identifying the significance of Sn addition on the tribological performance of Ti-based bulk metallic glass composites

Tailoring the properties of an engineering material with low coefficient of friction (COF) and high wear resistance has been a long goal in the tribological research. It is particularly true for the bulk metallic glass composites (BMGCs), which are promising for a large range of advanced applications. Although the materials design and mechanical properties have been extensively investigated, knowledge of their tribological behaviors is still lacking. Ball-on-block wear tests were thus conducted to investigate the effect of Sn addition on the tribological behaviors of Ti45Zr25Nb6Cu5Be17 BMGCs under different normal loads. Nano-indentation and nano-scratch tests were carried out to study the phase-specific wear properties of the crystalline dendrite and amorphous matrix in BMGCs respectively. Preferred wear and delamination in the dendrite phase as a result of the plastic mismatch between the two constituent phases is responsible for the abrasive wear in the Ti47Zr25Nb6Cu5Be17 and Ti46Zr25Nb6Cu5Be17Sn1 BMGCs. While, mechanically driven tribolayer during sliding enables oxidative wear and improved tribological properties in the Ti45Zr25Nb6Cu5Be17Sn2 BMGC. Based on the synergistic mechanical and chemical effects, the underlying relationship between the intrinsic dendrite properties and the wear behaviors of BMGC was revealed.

Plastic mismatch effect on plasticity induced crack closure: Fatigue crack propagation perpendicularly across a plastically mismatched interface

AbstractIn the present work, comprehensive investigation of both theoretical analysis and numerical simulation was carried out to investigate the plastic mismatch effect on plasticity induced crack closure (PICC) behavior and effective fatigue crack tip driving force. During the process of crack tip approaching interface, crack tip load and crack tip load ratio will change, resulting in the change of PICC degree. When the crack propagates towards higher strength side, Kop/Kmax increases; when the crack propagates towards lower strength side, Kop/Kmax decreases firstly and then increases. The two mechanisms of “interface plastic mismatch effect on nominal fatigue crack tip driving force” and “interface plastic mismatch effect on PICC degree” were compared. The second mechanism must be considered when building crack tip driving force model for describing fatigue crack crossing plastically mismatched interface, because it is more physically factual and maybe more important than the first mechanism.

Microstructure, mechanical and tribological properties of nickel-aluminium bronze alloys developed via gas-atomization and spark plasma sintering

This work investigated the effect of sintering temperatures (600–750°C) on mechanical and tribological behaviors of nickel-aluminium bronze (NAB) alloys developed by gas atomization and spark plasma sintering. Results indicated an increase of the volume fraction of B2-type NiAl precipitates with increasing sintering temperature, leading to an improvement of the yield strength and the wear resistance. Specifically, detailed microstructural analyses of sintered NAB alloys at 750°C showed the presence of ultrafine grains with an average size of 367nm, nanoscale twins with a volume fraction of 11.7% and dislocations with a density of 1.3 ± 0.1 × 1014m−2. A much higher yield strength of 620MPa was obtained in NAB, if compared to that of conventional cast counterparts (280–440MPa). Estimations of strengthening mechanisms suggested the predominant mechanism of grain boundary strengthening (335MPa) for NAB alloys with contributions from precipitate strengthening (54MPa), dislocation strengthening (75MPa), twin boundary strengthening (89MPa) and solid solution strengthening (50MPa). Moreover, dislocations was blocked at twin boundaries to form complex dislocation barriers and networks, further contributing to the high strength. The high wear resistance of NAB could be ascribed to the reduction of the local stress around crack tips due to the high elastic modulus mismatch (ENiAl/ECu), and to the crack extension toughening near the interface of the matrix and the precipitate caused by the high plastic mismatch (σNiAl/σCu).

Threshold stress intensity factor range of a mechanically-long and microstructually-short crack perpendicular to an interface with plastic mismatch

In this study, we investigated long fatigue crack growth perpendicular to a soft-hard material interface comprising the same Young’s moduli but different yield strengths. We determined the degree of fatigue crack growth retardation attributable to the interface under the constant stress intensity factor range (ΔK) using the plasticity-induced crack closure analysis with the Dugdale model. We subsequently determined the threshold stress intensity factor range (ΔKth) under a constant stress amplitude (σa). Under a constant ΔK, the retardation degree was primary dependent on the two materials’ yield strength ratio. Under a constant σa, ΔKth was dependent on both the yield strength ratio and the distance between initial crack tip and interface.

Experimental investigations of fracture and fatigue crack growth of copper–steel joints arc welded using nickel-base filler

This paper experimentally investigates fracture toughness and fatigue crack growth behaviour of mode-I cracked bi-metallic compact tension specimens. The specimens were made of weak-copper (UNSC11000) and strong-alloy steel (En31) arc-welded with nickel-filler. Residual stresses induced in constituent materials during welding, due to their different coefficients of thermal expansion (thermal mismatch) were measured at important locations in all the materials near the weld zone. Several notched specimens with positions of crack tip in critical zones of copper, steel and nickel-weld were subjected to fracture tests. The specimens were fatigue precracked prior to fracture testing. Bi-metallic specimens having an equal initial crack length were subjected to constant-amplitude fatigue crack growth rate tests until their fracture occurred. Residual stress intensity factor as a function of crack length was computed theoretically for inclusion in the analysis. Thermal and mechanical mismatches between constituent materials, the latter represented by elastic and plastic property differences, were strongly found to affect the behaviour of crack tip in the vicinity of the weld under both monotonic and fatigue load. The combined effect of “elastic–plastic mismatch” across the material interfaces over the crack tip was described by crack driving force, Jinterface. Under monotonic load (i) fracture toughness of bi-metallic specimens with crack in copper side facing the weld backed by steel was found to be higher than that of monolithic copper with crack arrest noticed in all the specimens when the crack tip was near the weld. Furthermore, Jinterface was found to be positive in this case which implied that the crack driving force, Jtip, was less than the remotely applied crack-driving force, Japplied. (ii) Fracture toughness of bi-metallic specimens with crack in steel side facing the weld backed by copper was found to be lesser than that of monolithic steel with the specimens failing quickly in the form of short brittle fracture within steel and the part of the weld. In addition, Jinterface was negative in this case with Jtip being more than Japplied. Under constant amplitude fatigue load (i) for crack growth from copper to steel side, crack growth rates in copper side retarded in the vicinity of the weld due to shielding caused by the stronger weld and back up steel. The crack deflected away from mode-I plane before penetrating into weld and back up steel. (ii) For crack growth from steel to copper side, crack growth rates in steel accelerated in the vicinity of the weld due to the amplification caused by the “elastic–plastic” property gradient from higher to lower. Furthermore, the crack penetrated through the weld and back up copper with no deflection. The results were also compared with those of monolithic metals subjected to similar fracture and fatigue crack growth rate tests. The shielding and amplification trends in the fatigue crack growth tests were not found to be altered by residual stresses. This implied that the combined effect of “elastic–plastic” mismatch was dominant over that of residual stresses in both directions of crack-transition.

Effect of a graded layer on the plastic dissipation in mixed-mode fatigue crack growth along plastically mismatched interfaces

Prior work by the authors has proposed a dissipated energy theory of fatigue crack growth in ductile solids under mode I loading based on the total plastic dissipation per cycle ahead of the crack. The approach has since been extended to a general bimaterial interface geometry under mixed-mode I/II loading, with application to fatigue debonding of layered materials. An inherent assumption of this prior work is that a perfect crack exists along the interface between the two materials. The current work extends the approach to incorporate a grading of material properties between the two layers, as may occur in a variety of welding, soldering or layered manufacturing applications. Attention is restricted to elastic perfectly-plastic layers with identical elastic properties and a mismatch in yield strength across a linearly graded interface, with the crack on the boundary of the weaker material. A dimensionless plastic dissipation is extracted from 2-D plane strain finite element models over the full range of yield strength mismatches, graded layer thicknesses and mixed-mode loading conditions. Results reveal that for all modes of loading, the effect of a graded layer is to increase the total plastic dissipation per cycle, which is bounded by the extremes in plastic mismatch for a perfect crack interface. While the graded layer has a measurable effect, the plastic dissipation for all strength mismatches is dominated by the mode of loading.

The effect of thermal and plastic mismatch on stress distribution in diamond like carbon film under different interlayer/substrate system

The poor adhesion for diamond or diamond like carbon (DLC) on steel substrates is attributed to carbon solution and high stress level resulted from the large difference in coefficients of thermal expansion between the film and the substrate. The adhesion is improved by the addition of a metal interlayer with carbide formation and acted as the diffusion barrier. However, the addition of the interlayer might cause larger stress along the interface which will impair the adhesion. Moreover, for DLC deposited on nonferrous substrate, the choice of the interlayer might coupled with the substrate effect. Previous studies have shown that Young's modulus (elastic) mismatch and yield stress (plastic) mismatch between film and substrate has effect on mechanical properties and crack growth stress. The difference in coefficient of thermal expansion (CTE) between interlayer and substrate is the source of thermal stress. These mismatches and the associated stress distribution might provide another perspective about interlayer other than carbide formation. A finite element program which simulate the thermal cooling and indentation process has been utilized to study the stress distribution caused by these mismatches. The former is simulated as deposition process while the latter is related to wear-protection operation. The results show both interlayer Cr and Ti has effects on magnitude and distribution on thermal stress and indentation stress. Such effect is also substrate dependent. To minimize the thermal stress, the thermal coefficient of expansion of the interlayer should lie between the film and the substrate. This provide a guideline for the choice of the interlayer for DLC deposited on different substrates.

Influence of plasticity mismatch and porosity on mechanical behavior of nanoscale Ag/W multilayers

Ag/W multilayers with periodicity ranging from 15 to 200 nm were deposited by direct current magnetron sputtering. The microstructure, hardness and elastic modulus were investigated by X-ray diffraction, Rutherford backscattering, X-ray fluorescence, scanning electron microscopy and nanoindentation. The results show that multilayers with periodicity less than 50 nm have columnar porous structure, which leads to low modulus and brittle fracture. Multilayers with periodicity larger than 50 nm have continuous laminated structure, and they are relatively ductile. All the multilayers have abnormal low hardness far less than a rule of mixture value, which has been attributed to porous structure and the deformation localization due to the plasticity mismatch between Ag and W.

Finite-Element Simulations of Cracks Near Interfaces: Effects of Thermal, Elastic, and Plastic Mismatch

Cracks situated parallel to, and very near, the interface in layered, ductile-brittle composite specimens were investigated with finite-element analysis. Elastic, plastic and thermal properties previously obtained from experiments were utilized in the model. A routine was employed for automatic crack extension and remeshing, enabling simulation of incremental crack propagation. The elastic, thermal and plastic contributions to crack propagation behavior were investigated, along with the variation of these with crack length and crack-tip position. Thermal residual stresses are shown to have a large influence on crack path, although this is mitigated to some extent by plasticity. The implications on the inherent reliability of joints and layered materials containing brittle constituents are discussed.

Crack initiation and propagation close to the interface in a ferrite–austenite joint

The safety assessment of welded structures and joints still remains an important industrial problem. In this study, a simple diffusion bonded bi-material joint has been made in order to analyze the plastic mismatch effect. It consists of an assembly of ferritic and austenitic steels which are representative of nuclear pressure vessel components. Tests were performed on various specimens including smooth and notched tensile bars, Charpy V-notch specimens and single-edge notch bend specimens. Homogeneous and bimaterial specimens were tested. On deeply notched specimens, the effect of the distance between the notch root and the interface on fracture initiation and crack propagation direction was studied. Tests were modeled using elasto-plastic finite element simulations. These simulations were post-processed in order to determine crack initiation based on the Rice and Tracey criterion according to the “local approach of fracture” procedure.