Hardening Mechanism Research Articles

Визначення здатності до дисперсійного твердіння нової штампової сталі з регульованим аустенітним перетворенням

The chemical composition of the stamping steel for high operating temperatures (above 700 °C) with controlled austenitic transformation during operation (CATO) was adjusted to implement its hardening by the dispersion hardening (aging) mechanism. The base steel was chosen as 3Х3Н8М7Ф, in which quenching and subsequent aging did not lead to such hardening. It was taken into account that in order to implement dispersion hardening, CATO steels should have a predominantly austenitic (rather than martensitic) structure in the hardened state, which was not provided for the base steel. As a result of changes in the content of Mn, Ni and C in the base steel, it was determined that the required conditions are met by CATO steel of grade 4Х3Н3Г6М7Ф (95 % austenite after quenching with a predominantly ferrite base in the annealed state). The experiments have established the ability of this steel to harden with ageing. Strength growth was determined by high-temperature tensile tests immediately after aging (without intermediate cooling of samples to room temperature), which is a feature of testing steels with CATO. The highest strength growth (compared to the quenched state) is provided by heat treatment in the following mode: quenching 1150 °C, 2 hours, oil and subsequent aging 725 °C, 2 hours (at a test temperature of 750 °C, s0,2 increases to 674 MPa, sВ to 697 MPa). This hardening is due to the release of dispersed particles of the Laves phase of Fe2Mo and carbide of type VC during aging. In the aged state, the steel retains its austenitic structure at high temperatures, and when cooled below 200 °C, it undergoes a γ → α transformation according to martensitic kinetics and acquires a hardness of 49 HRC. The achieved high-temperature (700...900 °C) strength characteristics of 4Х3Н3Г6М7Ф steel are twice as high as those of the high-temperature die steel 5Х3В3МФС (DI23) and are not inferior to the heat-resistant alloy ХН35ВТЮ (EI787). This makes it possible to effectively use it instead of commercially available heat-resistant martensitic die steels at operating temperatures above 700 °C. Keywords: die steels CATO, alloying, quenching, austenitic structure, dispersion hardening, high-temperature strength.

Heterostructure microstructure and laves phase evolution mechanisms during inter-layer hammering hybrid directed energy deposition (DED) process

Directed energy deposition (DED) is suitable for fabrication of large Inconel 718 components, however, large columnar dendrites and several laves phase impair the mechanical properties. In this work, the formation mechanism of heterostructure microstructure with coarse grain (CG) and fine grain (FG) alternating distribution was explored, and the evolution process of Laves phase during inter-layer hammering hybrid directed energy deposition (HDED) was revealed. The results show that the heterostructure microstructure composed of CG region and FG region with a ratio of 1:1 was obtained due to the work hardening and recrystallization mechanisms, and the average grain size of CG region and FG region are 68.9 μm and 12.0 μm, respectively. The Laves phase was first fractured into small pieces by hammering and then resolved into the matrix during the subsequent heat input process, which promotes the precipitation of γ″ phase from the matrix. The strength and plasticity of the specimens fabricated by HDED are synchronously improved. The dislocation strengthening and solid solution strengthening are the mainly contributions to the strength improvement, and the grain refinement and heterostructure microstructure increased the plasticity of the hybrid manufactured specimens.

Shock-induced melting and crystallization in titanium irradiated by ultrashort laser pulse

Modification of titanium microstructure after propagation of a melting shock wave (SW) generated by a femtosecond laser pulse is investigated experimentally and analyzed using hydrodynamic and atomistic simulations. Scanning and transmission electron microscopy with analysis of microdiffraction is used to determine the microstructure of modified subsurface layers of titanium. We found that two layers are modified beneath the surface. A top surface polycrystalline layer of nanoscale grains is formed from shock-molten material via rapid crystallization. In a deeper subsurface layer, where the shock-induced melting changes into plastic deformation due to attenuation of SW, the grain structure of solid is considerably affected, which results in a grain size distribution differing from that in the intact titanium. Molecular dynamics simulation of single-crystal titanium reveals that the SW front continues to melt even after its temperature drops below the melting curve Tm(P). The enormous shear stress of ∼12 GPa generated in a narrow SW front leads to free slip of atomic planes, collapse of the crystal lattice, and formation of a supercooled metastable melt. Such melt crystallizes in an unloading tail of SW. The mechanical melting ceases after drop in the shear stress giving rise to the shock-induced plastic deformation. The last process triggers a long-term rearrangement of atomic structures in solid. The overall depth of modified layers is limited by SW attenuation to the Hugoniot elastic limit and can reach several micrometers. The obtained results reveal the basic physical mechanisms of surface hardening of metals by ultrashort laser pulses.

Microstructural Hardening Mechanisms and Electrical Property Manipulations of Substantially Undercooled Fe87Cu13 Alloy

Microstructural Hardening Mechanisms and Electrical Property Manipulations of Substantially Undercooled Fe87Cu13 Alloy

Twin-boundary and precipitate interaction in Mg–Al alloy: an MD study

Strengthening of Mg-alloys by precipitation is much less efficient than in other metallic alloys (e.g. Al) as the Mg17Al12 precipitates grow as thin plate or lozenge shaped or long rod shape parallel to the basal plane. Recently atomistic simulations reveal that the dislocation-precipitate interaction is very week to claim for the precipitation hardening mechanism. However, the interaction of twin-boundary with the Mg17Al12 precipitate remains unexplored using atomistic simulation. In the present study we focus on the twin-boundary/precipitate interaction at different temperatures, precipitate sizes and varied applied loads, carried out using classical molecular dynamics methodology. In particular, the activation energies necessary to overcome various precipitates are determined as a function of the temperature, precipitate size and applied load. The velocity profile of the twin is calibrated with these different external conditions. An attractive nature of interaction has been observed while the twin-boundary comes closer to the precipitate and a network of dislocations are observed when the twin-boundary bypass the precipitate, as manifested through our atomistic microstructures. These results provide valuable information about the precipitate hardening mechanisms and suggested new avenues to improve the mechanical properties of Mg–Al alloys.

Mechanism of failure behaviour and analysis of 18650 lithium-ion battery under dynamic loadings

Lithium-ion battery failures, particularly in the case of high-speed collisions in electric vehicles, have become a growing concern. This study investigates the failure mechanism of an 18650 cylindrical battery which is indicated by the occurrence of an inner short circuit at various loading rate. The voltage drop due to an internal short circuit typically occurs shortly before the maximum force is reached in quasi-static loading cases. Whereas, under dynamic loading conditions, the battery exhibits a loading-rate effect, which causes a voltage drop due to short circuits occurring at an earlier displacement. The loading-rate hardening mechanism is primarily attributed to electrolyte flux. A finite element model of an 18650 cylindrical battery is established and calibrated with the in-situ tests results. The failure location inside the jellyroll cross-section is identified with the maximum equivalent plastic strain. Under the dynamic loading, the maximum stress corresponding to the short circuiting is higher than the quasi-static counterpart. The finite element model is used to illustrate the inner short-circuit mechanisms of the batteries under different loading rates, providing a design guide for enhancing the crashworthiness of the battery components.

Strengthening mechanism of Ni–Cu nanotwins with void under different tensile directions based on Molecular Dynamics simulation

Defects such as voids and inclusions are inevitably inserted in the process of improving the mechanical properties by adjusting the microstructure, which results in alteration of deformation mechanism and ductile failure of the material. Although this is a macroscopic phenomenon, it is related to the mechanism at the atomic scale. In this paper, the uniaxial tension processes of Ni–Cu nanotwins containing a void are simulated using the molecular dynamics method, and the effects of the loading direction of perpendicular and parallel to the twin boundary (TB) on the mechanical properties are investigated, and the emphasis is on the hardening behavior after yielding. The results show that the yielding of the material is attributed to the emission of dislocation shear loops on the void surface. The elastic modulus and yield strength are greater under the loading direction of perpendicular to TB. The peak stress is equal to the yield stress under perpendicular loading, while the peak stress is greater than the yield stress under parallel loading. In addition, the hardening behavior is found after yielding under two tensile directions, but the hardening mechanisms are different. For perpendicular loading, the hardening behavior is controlled by the TB and the Lomer-Cottrel lock. While for parallel loading, the hardening behavior is controlled by the stacking faults that distributes uniformly and symmetrically along the TB. The general conclusions obtained may provide new insights into the microscopic properties and deformation mechanisms of nanotwins in depth.

Effect of cyclic loading on microstructure and crack propagation in additively manufactured biomaterial Co–Cr–Mo alloy

Cobalt–Chromium–Molybdenum (Co–Cr–Mo) alloys are commonly used for artificial hip and knee joint metallic implants. These components are subjected to repetitive loads during service. Therefore, materials used for such applications must exhibit a high fatigue crack resistance. In this research, Co–28Cr–6Mo (wt.-%) powder was utilized as a feedstock in a laser powder bed fusion process to produce test coupons. The coupons were then subjected to load-controlled cyclic material tests in the low cycle fatigue regime to study mechanical response and microstructural changes of the material. With the progressing number of cycles, a continuous increase in macroscopic plastic strain was observed. The electron backscattered diffraction analysis revealed that cyclic loading caused deformation-induced face-centered cubic (FCC) → hexagonal close-packed (HCP) phase transformation. In addition, the phase transition generated an accumulation of plastic strain at the FCC/HCP interface giving rise to crack nucleation. The crack propagation path along HCP orientation variants with high mechanical work and strain hardening mechanism is discussed.

Influence of deformation substructure on the strain compatibility of ferrite and bainite in a commercial pipeline steel

The strain instability due to hardening of the soft phase is a key factor in premature failure of pipeline steels after work hardening. In this work, we reveal the hardening mechanism of ferrite is actually the strengthening effect of deformation substructures on the flow stress, where the strengthening contribution by the low angle grain boundaries (LAGBs) is the main stress enhancement way. It is able to subdivide the ferrite into many sub-grain structures through dynamic evolution between geometrically necessary dislocations (GNDs), LAGBs and high angle grain boundaries (HAGBs) during work hardening. These subgrains have independent orientations and can be further subdivided, thus causing the Hall-Petch strengthening effect. Based on the hardening mechanism of ferrite, the representative volume element model with different levels of work hardening is developed. As the level of work hardening increases, the strain compatibility is gradually reduced between ferrite and bainite. The strain localization factor is increased from 0.023 to 0.061, the plastic deformation difference between the two phases is increased from 6.71% to 10.93%, and strain partitioning coefficient is reduced from 7.11 to 3.07. This work will contribute to better understand the substructure strengthening of dual-phase materials and provide a new idea for the protection of work-hardened metals.

Study on Erosion Behavior of Laser Wire Feeding Cladding High-Manganese Steel Coatings.

High-manganese steel (HMnS) coating was prepared using laser wire feeding cladding technology. Erosion damage behavior and erosion rate of both the HMnS coating and the HMnS substrate were investigated at room temperature using an erosion testing machine. SEM/EDS, XRD, EPMA, and microhardness analyses were used to characterize the cross sections of the coating and matrix, as well as the morphology, phase composition, and microhardness of the eroded surface. The phase composition, orientation characteristics, and grain size of the eroded surfaces of both the coating and substrate were examined by using the EBSD technique. The erosion mechanism under different erosion angles was revealed. By analyzing the plastic deformation behavior of the subsurface of the HMnS coating, the impact hardening mechanism of the high-manganese steel coating during the erosion process was investigated. The results demonstrated that the HMnS coating, prepared through laser wire feeding cladding, exhibited excellent metallurgical bonding with the substrate, featuring a dense microstructure without any cracks. The erosion rate of the coatings was lower than that of the substrate at different erosion angles, with the maximum erosion rate occurring at 35° and 50°. The damage to the coating and substrate under low-angle erosion was primarily attributed to the micro-cutting of erosion particles and a minor amount of hammering. At the 90° angle, the dominant factor was hammering. After erosion, the microhardness of both the coating and substrate sublayer increased to 380HV0.3 and 359HV0.3, respectively. Dendrite segregation, refined grains, low-angle grain boundaries, and localized dislocations, generated by laser wire feeding cladding, contributed to the deformation process of HMnS. These factors collectively enhance the hardening behavior of HMnS coatings, thereby providing excellent erosion resistance.

Development of flux-cored wire of Fe – C – Si – Mn – Cr – W – V system with additives of carbon-fluorine-containing material and titanium

The paper considers research of quality of the electric arc coating obtained using flux-cored wire of the Fe – C – Si – Mn – Cr – W – V system with additives of carbon-fluorine-containing material and titanium. The formation of an electric arc coating was carried out using an automatic arc welding machine ASAW-1250 with a new chromium-containing flux-cored wire on plates made of St3 steel. To exclude mixing of the deposited metal with the substrate steel, multilayer surfacing was conducted. The surfacing mode was calculated and refined experimentally. The authors studied the composition and properties of the surface of the electric arc coating after surfacing. As a substitute for amorphous carbon they used a carbon-fluorine-containing material (dust of gas purification of aluminum production). Surfacing was carried out under a flux made from slag produced by silicomanganese with a high content of sulfur. A regression analysis of influence of the deposited layer’s chemical composition on its hardness and wear rate was carried out and mathematical models of the investigated performance characteristics of the electric arc coating were obtained. With an increase in the content of chromium, tungsten, carbon and silicon, hardness of the deposited metal and its resistance to abrasive wear increase. The results of the conducted research make it possible to develop measures ensuring the required level of performance characteristics of the electric arc coating and can be used to make a forecast of hardness of the deposited layer and its wear resistance when the chemical composition of the metal changes, to predict the operational resistance of rolling rolls deposited with wires of the PP-Np-35V9Kh3SF type. Mathematical models of hardness of the deposited layer and its wear resistance help to clarify the mechanism of hardening and formation of protective properties of the surface layers of rolling rolls by means of electric arc coatings deposited with flux-cored wires.

Strong yet ductile self-passivating W-10Cr alloy by laser powder bed fusion

Compared to pure tungsten, self-passivating tungsten alloys (SPTAs) for the future nuclear fusion armour are expected to offer a major safety guarantee in a loss-of-coolant accident (LOCA) event. Here we used laser powder bed fusion (LPBF) to print a self-passivating W-10Cr system. The main challenges for processing W-10Cr alloys are balling phenomena and chromium loss due to their melting point differences. After laser parameters optimizations, W-10Cr blocks with relatively dense and smooth surface were fabricated. As-built specimens exhibited an inhomogeneous microstructure with W-rich regions and Cr-rich ones. In the Cr-rich region, ultrafine equiaxed grains were obtained in view of Spinodal decomposition. Owing to the co-contribution of Spinodal decomposition and fine-grain structure, the compressive strength, fracture strain and Vickers hardness (HV0.005) of W-10Cr alloy reach 1451 MPa, 21.5% and 9.4 GPa, which are obviously superior to pure tungsten counterparts, with the improvement of 41%, 115% and 43%, respectively. Based on the indentation responses, as-measured indentation hardness followed typical indentation size effect (ISE). A higher contribution from elastically surface work in the early part of indentation is responsible for the hardening mechanism of W-10Cr alloy. Additionally, the strengthening and toughening mechanisms associated with improved properties are also discussed. The mechanistic insights into the deformation behavior have extensive implications for the development of additively manufactured SPTAs with exceptional mechanical properties.

The hardening effect of deformation twinning based on visco-plastic self consistent model and a multi-scale grain refinement prediction model during machining of titanium alloy

Deformation twins act as one of the deformation characteristics of hexagonal close packed (HCP) materials and occur during machining of Ti–6Al–4V alloy. To clarify the hardening mechanisms of deformation twins in Ti–6Al–4V machined surface, work-hardening effects induced by grain refinement in meso scale as well as deformation twins in micro scale were investigated. A 2D cutting finite element (FE) model was established to predict grain refinement in multi-scales and its hardening behavior during machining of Ti–6Al–4V. A visco-plastic self consistent model (VPSC) coupled with predominant twin reorientation (PTR) scheme was also used to simulate stress variation with considering the activations of twin systems. The results show that different grain refinement degrees in meso scale are observed with the increasing of cutting speeds (100–500 m/min). {101¯1} compression nano twins generate at higher cutting speeds, and the twin volume fraction is relatively small (20%). The hardening behaviors of grain refinement in meso scale as well as nano twins predicted with FE model and VPSC model are similar, and the generation of nano twins cause inhomogeneous hardening in Ti–6Al–4V machined surface. Nano-indentation tests further prove the accuracy of the prediction results for the hardening effect induced by nano twins in Ti–6Al–4V machined surface. The results provide two explanations and quantitative evaluations for the hardening effects of deformation twins from the macroscopic stress and the microscopic deformation of slip and twinning modes.

Fascinating microstructural evolution during consolidation and remarkable hardening behaviour of micro alloyed Al-Cu-Ni alloys

Ultrafine B2 NiAl and vacancy ordered phase (Al3Ni2 type) (Cu) evolve during consolidation of mechanically alloyed Al-Cu-Ni alloys without and with micro alloying addition of Zr and or Ti and results in high hardness of the alloys. Al-Cu-(Ni15)-Zr alloy consolidated by spark plasma sintering exhibits a remarkable hardness value of 7.2 GPa. Wide range of hardness values of the alloys indicates a significant effect of composition as well as synthesis route. Hardening behaviour through point defect hardening has been demonstrated in terms of constitutional vacancy concentration as well as solute dislocation interaction. Lattice parameter, bulk density, porosity anddislocation density of the alloys have been measured to understand the hardening mechanism further. TEM analysis and wear behaviour are carried out to justify the phase formation and strengthening mechanism of the alloys, respectively.

Substitution solid solution hardening effect of vanadium in Ni–Co–V medium entropy alloys

The single-phase polycrystalline NiCoV medium-entropy alloy (MEA) has shown ∼1 GPa yield strength, surpassing most single face centered cubic alloys. This excellent strength has been theoretically devoted to the special solid solution hardening effect of V. However, the intrinsic reasons for this excellent strengthening effect of V in high-/medium-entropy alloys are still less experimentally explored. To this end, we here investigated the solid solution hardening effect of V in the NiCoV system. The results showed that the solid hardening mechanism changes from lattice distortion to atomic volume mismatch when the V content exceeds 6 at. %. Particularly, the increase in V content increases the Hall–Petch coefficient of NiCoV MEAs significantly. The combination of efficient grain boundary hardening and atomic volume mismatch hardening leads to the excellent yield strength of concentrated NiCoV.

Fabrication and characterization of SiC-ZTA ceramic composites by hot pressing

Al2O3/ZrO2/SiC ceramic composites with different SiC contens have been prepared by hot pressuring. The effect of SiC content on the microstructure and mechanical properties of composites have been studied. The results show that SiC has obvious grain refinement effect on ZTA ceramics and change the fracture mode of the matrix from intergranular fracture to transgranular fracture. Simultaneously, it has been found that the mechanical properties of the material are significantly enhanced in comparison with ZTA matrix. The highest strength is acquired at 10% SiC content, the flexural strength and toughness are obtained when the SiC content is 15 vol%, and the values are 18.86 GPa, 1262 MPa and 6.13 MPa m1/2, respectively. The mechanisms of hardening, strengthening and toughening have been discussed.

Improving the high cycle fatigue property of Ti6Al4V ELI alloy by optimizing the surface integrity through electric pulse combined with ultrasonic surface rolling process

To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy, the electric pulse has been introduced into the ultrasonic surface rolling process (USRP), which is called electric pulse-assisted ultrasonic surface rolling process (EUSRP). With the help of “electroplasticity” of the electric pulse, the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level. However, the surface hardness decreases due to the continuous effect of the pulse current and the “skin effect” during treatment. It is worth noting that the higher the applied pulse current, the more severe the softening. This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens. To break this paradox, the EUSRP treatment is followed by a USRP treatment. The EUSRP-2 (with a pulse current of 200 A) +USRP specimens exhibit excellent surface hardness, a gradient deformation layer thickness of about 400 µm, low surface roughness and high compressive residual compressive stress. Besides, the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis. The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment, the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site. The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.

Unravel hardening mechanism of AlCrNbSiTi high-entropy alloy coatings

Novel high-entropy alloys (HEAs) coatings deposited using physical vapour deposition technique are potential candidates of protective coatings due to their excellent performance. However, the correlation between properties and microstructure of HEAs coatings has not been completely established and the hardening mechanism remains unclear. In this paper, the AlCrNbSiTi HEA coatings were deposited under different bias voltages by radio-frequency magnetron sputtering. The morphology, chemical composition, structure, mechanical properties and thermal stability are discussed in detail. All coatings exhibit smooth surface without obvious elemental segregation and the columnar structure tends to densify with increasing bias. The coatings possess a nanocomposite structure of amorphous-nanocrystalline silicides, in agreement with the results calculated based on three phase prediction parameters (ΔHmix: enthalpy of mixing, δ: atomic size difference and Ω: ratio of entropy of mixing ΔSmix to ΔHmix). The coating deposited at bias of − 50 V presents optimal mechanical properties with nanohardness of ∼15.2 GPa, scratch crack propagation resistance of 1042 N2 and wear rate of 5.4 × 10−8 mm3N−1m−1. Annealing treatment further unravels that the presence of hard silicides is the hardening mechanism of coatings with super-hardness of ∼24 GPa. The results indicate that the AlCrNbSiTi coating is a promising protective coating with broad industrial application prospects.

Microstructures, mechanical and strain hardening properties of squeezed 6016Al matrix composites synthesized under electromagnetic field

Microstructures, mechanical and strain hardening properties of squeezed 6016Al matrix composites synthesized under electromagnetic field

Influence of tool rotational speed on mechanical and corrosion behaviour of friction stir processed AZ31/Al2O3 nanocomposite

Nano-sized reinforcements improved the mechanical characteristics efficiently by promoting more implicit particle hardening mechanisms compared to micron-sized reinforcements. Nano-sized particles lessen the critical particle solidification velocity for swamp and thus offers better dispersal. In the present investigation, the friction stir processing (FSP) is utilized to produce AZ31/Al2O3 nanocomposites at various tool rotation speeds (i.e., 900, 1200, and 1500 rpm) with an optimized 1.5% volume alumina (Al2O3) reinforcement ratio. The mechanical and corrosion behavior of AZ31/Al2O3-developed nanocomposites was investigated and compared with that of the AZ31 base alloy. The AZ31 alloy experienced a comprehensive dynamic recrystallization during FSP, causing substantial grain refinement. Grain-size strengthening is the primary factor contributed to the enhancement in the strength of the fabricated nanocomposite. Tensile strength and yield strength values were lower than those for the base metal matrix, although an upward trend in both values has been observed with an increase in tool rotation speed. An 19.72% increase in hardness along with superior corrosion resistance was achieved compared to the base alloy at a tool rotational speed of 1500 rpm. The corrosion currents (Jcorr) of all samples dropped with increase in the rotational speed, in contrast to the corrosion potentials (Ecorr), which increased. The values of Jcorr of AZ31/Al2O3 were 42.3%, 56.8%, and 65.5% lower than those of AZ31 alloy at the chosen rotating speeds of 900, 1200, and 1500 rpm, respectively. The corrosion behavior of friction stir processed nanocomposites have been addressed in this manuscript which has not been given sufficient attention in the existing literature. Further, this work offers an effective choice for the quality assurance of the FSP process of AZ31/Al2O3 nanocomposites. The obtained results are relevant to the development of lightweight automobile and aerospace structures and components.