Hardness Inhomogeneity Research Articles

Regulating hardness homogeneity and corrosion resistance of Al-Zn-Mg-Cu alloy via ECAP combined with inter-pass aging

The novel processing strategies of Equal Channel Angular Pressing (ECAP) combined with inter-pass aging was designed, and XRD, SEM, TEM, electrochemical testing, and hardness testing were used to evaluate the relationship between microstructure and performance of alloys in this work. The results indicate that the hardness uniformity of the circular cross-section perpendicular to the extrusion direction was improved by combining ECAP with inter-pass aging, resulting in a hardness inhomogeneity factor (∼0.030) that was lower than that achieved by peak aging and double-stage aging alloys. Polarization curves and electrochemical impedance spectra show that a more negative corrosion potential and a smaller corrosion current density were possessed by the alloy, with the maximum intergranular corrosion depth (IGC) being reduced to 38 μm. Microstructure observation indicates that the improvement in hardness uniformity is due to the uniformity of grain size and the improved distribution and size of the η’ phase, which is attributed to the interaction between the pinning and shear effects of precipitates and dislocations during deformation. The enhanced corrosion resistance was attributed to an orderly combination of deformation and aging that increases the grain boundary volume and disrupts the η phase continuity within grain boundaries.

An Improvement in Constrained Studded Pressing for Producing Ultra-Fine-Grained Copper Sheet

In this study, constrained studded pressing (CSP) was modified to produce ultra-fine-grained copper sheets and is called modified-CSP. In modified-CSP, due to the selection of asymmetric semicircle studs, the maximum groove depth can increase up to three times the sheet thickness. In the CSP method, the groove depth is selected to be equal to the sheet thickness. To investigate the effective plastic strain, a finite element model (FEM) of modified-CSP was established. For this purpose, DEFORM-3D commercial software was used. A simulation showed that the modified-CSP process was capable of using higher strain, about 0.8 in each pass than in CSP. Copper sheets were deformed up to 10 passes by modified-CSP. The microstructure of the produced samples was analyzed. The results show that the grain size decreases in the first pass. In addition, with increasing plastic strain, the structure of the twins bands was observed. Mechanical properties, including tensile properties and Vickers microhardness, of the samples during the process were investigated. The strain inhomogeneity factor (SIF) and the hardness inhomogeneity factor (HIF) were used to quantitatively express the uniformity of distribution of effective plastic strain and Vickers microhardness, respectively. Compared with CSP and constrained groove pressing (CGP), the results showed that although the ultimate tensile strength (UTS) has significantly increased, the ductility values have remained almost constant. Moreover, in modified-CSP, the load-die stroke diagram increases almost evenly due to the removal of the stud interface area and progressive engagement. Therefore, modified-CSP copper sheets showed superior tensile properties such as good toughness.

Microstructures and mechanical properties of Zr-based metallic glass ablated by nanosecond pulsed laser in various gas atmospheres

The surface with micro-convex prepared by laser ablation is beneficial to improving the functional properties of metallic glasses (MGs). However, accompanied by the formation of micro-convex, surface hardness of the ablated area generally decreases due to the softening effect caused by laser thermal shock. Previous studies have shown that the mechanical properties of laser-ablated MG surface are significantly different when using different gas atmospheres. In this study, a comparative investigation was performed to analyze the influence of gas atmosphere (i.e. argon, nitrogen, and air) on the formation of micro-convex as well as its surface characteristics and mechanical properties. Experimental results showed that the atmosphere type did not affect the formation of micro-convex, but significantly affected the surface morphology and element composition of the laser-ablated area. The element analysis and Raman spectral measurements indicated that laser ablation in nitrogen or air atmosphere resulted in local nitridation or oxidation. The results of nanoindentation tests demonstrated that laser ablation in nitrogen or air atmosphere also resulted in surface hardness inhomogeneity, i.e., softening and hardening effects coexist in the ablated area, which could be attributed to the combined influence of laser thermal shock, the introduction of secondary phase as well as laser ablation induced loose structure.

Experimental studies and optimization of process variables in multi-angular twist extrusion (MATE) of Cu-Cr-Zr alloy

Researchers have recently interested in devising integrated severe plastic deformation (SPD) techniques that can create ultra-fine grained (UFG) metal in one pass. This paper proposes a novel integrated severe plastic technique called multi-angular twist channel extrusion (MATE). This technique was created the aim of retaining strain uniformity while applying a significant strain in a single pass. The input parameters such as annealing temperature, type of lubricant and ram speed, were optimized in this study in relation to the Vickers hardness, hardness inhomogeneity and ultimate tensile strength of the extruded material. Experiments were carried out based on full factorial design, and the TOPSIS approach was used to find the best combination of input parameters. The analysis of variance is used to determine the amount of contribution of input parameters on the mechanical properties. Finite element analysis was performed with the identified optimum parameter of annealing temperature = 550 °C, type of lubricant = MoS2 and ram speed = 3 mm/sec. The average effective strain of 2.72 is obtained by in the MATE process. Experimental studies revealed that the strength and hardness of copper alloy was increased by 69.8 % and 80.5 % respectively. Scanning electron microscope image revealed the grain refinement by MATE process in the copper alloy.

Coexistence of two distinct fatigue failure mechanisms in Super304H welded joint at elevated temperatures

Super304H steel is a promising candidate for boiler tube materials in thermal power plants operating under ultra-supercritical conditions, and the fatigue properties of its welded joint play a key role in the structural reliability of the thermal power plants. Herein, we report that there is a transition in the fatigue failure location (i.e., fatigue failure mechanism) from the base metal at a high strain amplitude (≥~0.4%) to the weld metal at a low strain amplitude (≤~0.3%) at a constant temperature in the operating range of 500–700 °C, and this leads to a significant reduction in the fatigue resistance of the welded joint as compared to that of the base metal. We found that a longer fatigue test time at a low strain amplitude induces a thermal aging effect that promotes different microstructural evolutions in the base metal and weld metal, deepening material inhomogeneity in the welded joint, and thereby triggering strain localization in the weld metal. With increasing fatigue test time (i.e., thermal aging time), Cr23C6 carbides precipitated in the interdendritic region of the weld metal and at the austenitic grain boundary of the base metal, and Nb (C, N) phases precipitated in the interior of austenitic grain in the base metal, which increased local hardness, whereas there was no significant microstructural change in the dendrite core of the weld metal, retaining its initial hardness. The intensified local hardness inhomogeneity caused the softest zone in the welded joint, wherein strain localization occurred, serving as a crack nucleation site and propagation path, to shift from the base metal to the weld metal (dendrite core). This induced a shift in the fatigue failure location from the base metal to the weld metal with decreasing strain amplitude.

Interface engineering of additively manufactured maraging steel-H13 bimetallic structures

This paper presents a study that aims to modify the interfacial characteristics of maraging steel-H13 bimetal structures through heat treatment procedures. Laser powder bed fusion (LPBF) was used to deposit maraging steel powder (MS1) on H13 hot work tool steel blocks. Subsequently, printed samples went through two different treatment cycles: (i) solution treatment, and (ii) aging treatment recommended for maraging steel. The microstructure, grain morphology, and mechanical response of MS1-H13 bimetals were assessed and compared with the as-built condition. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) techniques were used to analyze the interfacial modification upon heat treatment. Using uniaxial tensile testing and microhardness measurements along with 2D nanoindentation mapping, the mechanical properties of bimetal structures were correlated to the microstructural evolution of the samples during different heat treatments. Although the aging treatment was not effective to induce tangible microstructure changes, the solution treatment succeeded in the modification of MS1-H13 interface. A fully lath martensitic morphology was promoted across the interface, eliminating microstructure and hardness inhomogeneity as well as interface distinction between both sides. Finally, the solution treatment enhanced the mechanical properties of MS1-H13 bimetal structure by tripling the ultimate tensile strength.

Effect of pre-deformation on quench-induced inhomogeneity of microstructure and hardness in 7050 aluminum alloy

7xxx aluminum alloys usually have high quenching sensitivity, which results in the inhomogeneous microstructure and mechanical properties of large-size components. The effect of pre-deformation on the quench-induced inhomogeneity of the microstructure and properties in a high strength 7050 aluminum alloy was investigated by means of the end-quenching technique, transmission electron microscopy (TEM) and three-dimensional electron tomography (3DET). The size of the η′ precipitates and quench-induced precipitates (QP) and the width of the grain boundary (GB) precipitate-free zones (PFZ) increased with the decrease of the quenching rate, leading to a decrease in hardness along the long axial direction of the end-quenching sample. However, after introducing 20% pre-deformation, the PFZ around the GB and QP in the slow-cooled end of the aged samples was significantly reduced, and dispersive η′ precipitates were formed around the GB and QP instead, which greatly improved the strength of the slow-cooled end. Compared with the sample without pre-deformation, the introduction of 20% pre-deformation increased the hardness of the slow-cooled end by 30% without obviously reducing the hardness of the water-quenched end, which means that the 20% pre-deformation effectively reduced the hardness inhomogeneity of the 7050 aluminum alloy. This work provides a new method for reducing the quench-induced inhomogeneity of aluminum alloys.

Development of microstructural inhomogeneity in multi-pass flow forming of TA15 alloy cylindrical parts

Revealing the development of microstructural inhomogeneity in the multi-pass flow forming of titanium alloy components is of great significance to the microstructure control and property tailoring. To this end, the microstructural inhomogeneity of TA15 alloy spun cylindrical parts was analyzed based on the deformation history. The results indicate that the material underwent significant compressive strain in the normal direction (ND), tension strain in the rolling and circumferential directions (RD and CD), while tension strain in the CD is slightly small due to the limited material flow in this direction. These strain characteristics make the microstructure, especially the primary α (αp), present different morphologies in the different planes of the part. Meanwhile, the combined effects of inhomogeneous deformation and temperature distribution in the ND also cause the inhomogeneity of microstructure morphology and parameters in this direction. Quantitative analyses show that with the forming pass increasing, the aspect ratio of αp increases most in the normal-rolling plane, then in the normal-circumferential plane and least in the circumferential-rolling plane, whereas αp content decreases in an opposite trend. Along the ND, the aspect ratio and content of αp is relatively high in the outer and inner surface areas but lowest in the central area, and these inhomogeneous characteristics can be gradually diminished with the forming pass increasing. Furthermore, the variation of hardness inhomogeneity factor indicates that a four-pass forming with the total reduction ratio of 63% could obtain a homogenous microstructure along the ND of the TA15 alloy spun cylindrical part.

Deformation Behaviors of Flat Rolled Wire in Twinning-Induced Plasticity Steel

The influence of room temperature flat rolling on microstructure, mechanical properties, and shape change in twinning-induced plasticity (TWIP) steel wire has been investigated to understand the deformation behaviors of flat rolled wire in TWIP steel and to apply TWIP steels to flat rolled wire products. Numerical simulation, hardness test, and EBSD techniques were used to analyze the distribution of strain, mechanical properties, and microstructure of flat rolled TWIP steel wire. The shape of flat rolled TWIP steel wire was also evaluated and compared with plain carbon steels having low strain hardening rate. A very different behavior of hardness, strain, twinning, and KAM value was observed with area of flat rolled wire due to the different stress state and strain with area of wire. The center area had the maximum twin density, KAM value, effective strain, and hardness; whereas free surface area had the minimum values. The hardness inhomogeneity factor (HIF) along the horizontal direction was much higher in comparison with that of the vertical direction. The maximum HIF value occurred at the specific reduction in height, i.e., 27%. This means HIF value gradually increased and then decreased with reduction in height, which is inconsistent with the results of plain carbon steel and Cu wire. The lateral spread and width of contact area of flat rolled TWIP steel wire were lower than those of plain carbon steels, indicating that material properties such as strain hardening exponent are crucial parameters that influence the shape of flat rolled wire products.

Failure modes of soft materials with sharp-hard inhomogeneity under heat flux

This article investigates the failure modes of the structure composed of a soft matrix and a hard line inhomogeneity under uniform remote thermo-mechanical loading. The inhomogeneity is assumed to be thermally permeable and the thermal expansion-induced stretch of the inhomogeneity is incorporated. For arbitrary uniform remote stresses and heat flux, the explicit solution to the full stress field in the matrix is derived by solving the corresponding Riemann-Hilbert problem, and the critical condition for the failure modes of the structure is obtained. The effects of the direction of the remote heat flux and those of the mismatch of thermal expansions on the failure modes of the structure are illustrated via numerical examples. It is shown that the heat-induced failure modes of the structure are distinct at the two tips of the inhomogeneity and are either interface puncture or interface separation depending on the direction of the remote heat flux and the disparity between the thermal expansion coefficients of the inhomogeneity and matrix. In addition, the heat flux-induced stress intensity factors at the inhomogeneity tips are found to be proportional to where a is the half-length of the inhomogeneity, indicating that the failure of the structure may be more sensitive to thermal loading other than mechanical loading.

Microstructure and mechanical properties variations of pure aluminum subjected to one pass of ECAP-Conform process

Aluminum rod (AA1100) is severely plastic-deformed by one pass of the ECAP-Conform method. Optical and electron backscatter diffraction (EBSD) methods are used for microstructural characterizations. EBSD examinations of in process and processed samples reveal that grain sizes are not changed appreciably and they are mostly elongated by the plastic deformation. The EBSD data shows an increase in the number of subgrains and inhomogeneous dislocation distributions. Estimation of the geometrically necessary dislocation (GND) densities is done by using EBSD data. Estimated average GND density of the annealed material is as low as 5 × 107 m−2. For severely deformed Al specimen, the estimated GNDs and SSDs densities are 2.9 × 1014 m−2 and 4.6 × 1014 m−2 respectively with an averaged total dislocation density of 7.5 × 1014 m−2. Mechanical properties of samples at different stages of deformation are examined through hardness and tensile tests. Microhardness results show the inhomogeneous distribution of hardness at these sections due to the complex and varied loading conditions. Despite hardness inhomogeneity, macro tensile test results show that ECAP-Conform process is very effective in increasing the strength of the material at least by the factor of three. This increase in strength can be well interpreted by an increase in dislocation densities and formation of subgrains.

Influence of inhomogeneity on several length scales on the local mechanical properties in V-alloyed all-weld metal

Recently, a new, vanadium alloyed welding consumable with a minimum yield strength of 1100 MPa was developed. The mechanical properties of welding consumables for gas metal arc welding are usually classified by producing and testing all-weld metal samples, which are typically a multipass weld. Chemical and microstructural fluctuations of a vanadium alloyed all-weld metal sample on a macro- and microscale and their influence on the local mechanical properties were investigated. On a macroscale, hardness mappings show a pattern of hard and soft zones which can differ up to 60 HV. Despite the existence of these fluctuations, undersized Charpy V-notch tests revealed no significant difference between the last weld bead and the underlying ones. It is explained how vanadium and its tendency to form precipitates affect both the hardness inhomogeneity and the toughness homogeneity. On a microscale, segregations of several alloying elements and significant grain size fluctuations were found. Their influence on fluctuations of the mechanical properties is discussed as well.

On the interaction of solute atoms with circular inhomogeneity and edge dislocation

A two-dimensional elastic solution for the stress field of a line dilatation near a circular inhomogeneity is obtained. A line dilatation is used to represent a row of solute atoms, referred to as a solute rod. The image effect is investigated with an especial attention. It is shown that a soft inhomogeneity attracts solute rods and a hard inhomogeneity repels solute rods. Based on this, the interactions of solute atoms with inhomogeneity-edge dislocation are studied by inserting more solute atoms into the matrix one-by-one, with an especial focus on the distribution of solute atoms (rods) around inhomogeneity and edge dislocation. As two typical applications of the present method, two simple cases are analyzed in details, with the inhomogeneity considered as a nano-void and a hard nano-fiber, respectively. It is shown that the equilibrium concentration distribution of solutes near an inhomogeneity is non-local, which not only depends on the local hydrostatic stress but also on the hydrostatic stress in the adjacent region. Besides, the shear stress on an edge dislocation, generated by hydrogen atmospheres between a circular nano-void/fiber and the dislocation, is calculated numerically. The results show that the hydrogen atmosphere around dislocations can shield the long-range elastic interaction between dislocations and other internal stress sources, supporting the so-called hydrogen enhanced localized plasticity.

Crack interaction with nanoscale twinning near a second-phase particle in fine-grained magnesium alloy

A theoretical model is established to address the effect of nanoscale twinning near a second-phase particle on crack growth in fine-grained magnesium alloys. The numerical solutions of singular integral equations are obtained by the considering complex variable method of Muskhelishvili, the superposition principle of elasticity, and the distributed dislocation technique. The expressions of stress intensity factors near the left crack tip are derived, and the energy release rate (ERR) characterizing the condition for crack propagation is also calculated. The influences of relevant parameters such as the location of nanoscale twin, the size of particle, and the relative distance between the inhomogeneity and the left crack tip on the ERR are examined in detail. The results indicate that the ERR is strongly influenced by the nanoscale twinning and the second-phase particle. The hard inhomogeneity decreases the ERR while the soft inhomogeneity increases the ERR when nanoscale twinning is not taken into account. At the same time, there is competition between the effects of the hard particle and nanoscale twinning on the ERR, and the nanoscale twin band with an optimum size displays the best toughening effect.

Effects of caliber rolling on microstructure and mechanical properties in twinning-induced plasticity (TWIP) steel

The effect of caliber rolling on microstructure and mechanical properties of Fe-Mn-Al-C twinning-induced plasticity (TWIP) steel has been investigated to find alternative methods of wire drawing process using the numerical simulation, electron backscatter diffraction (EBSD) techniques, transmission electron microscopy (TEM), and hardness test. Behavior of twinning, texture, and effective strain was different with areas of rolled wire due to the difference in stress state and strain. The center area had maximum twin density, low angle boundary (LAB), effective strain, and hardness; whereas the surface area had minimum values. In comparison with wire drawing process, caliber rolling process imposed higher stain with slightly uniform distribution on wire, indicating that caliber rolling can manufacture high strength wires more effectively. For instance, after the area reduction of 22% by caliber rolling, the tensile strength was 47% higher and hardness inhomogeneity factor was 15% lower as compared to samples processed by wire drawing. In other words, caliber rolling process was suitable to make high strength materials deformed and hardened by twinning mechanism such as TWIP steels due to the characteristics of imposing severe strain with multi-direction, multi-pass with different shape at each pass, and alternating the loading direction between passes. Therefore, caliber rolling process can be a strong candidate in replacement of wire drawing process, especially TWIP steels.

Severe plastic deformation driven nanostructure and phase evolution in a Al0.5CoCrFeMnNi dual phase high entropy alloy

The effect of severe plastic deformation on microstructure and phase evolution was investigated in a dual phase Al0.5CoCrFeMnNi high entropy alloy (HEA). For this purpose, the as-cast HEA was subjected to initial thermo-mechanical processing by warm-rolling and annealing. The annealed dual phase alloy showed FCC and B2 phases. The B2 phase was enriched with Ni and Al, while the converse held good for the FCC phase. These annealed HEA specimens were severely deformed by high pressure torsion (HPT) up to five complete rotations (R). The severely deformed HEA revealed nanostructured FCC grains containing nano-twinned regions and coarser B2 phase. The nanostructure formation in the softer FCC phase was attributed to greater strain partitioning and propensity for the formation of nano-twins. Although with increasing rotations, the hardness difference between the edge and centre region was reduced, the 5R HPT processed specimens showed inhomogeneity featured by intermittent hardness spikes. Upon annealing, recrystallized dual phase microstructure was confirmed in the 5R HPT processed specimen. Microstructural differences between centre and edge regions were revealed by way of large B2 clusters (5 μm-10 μm) at the centre region. Remarkably, annealing resulted in the formation of a (Fe,Cr) rich σ-phase. The formation of σ-phase resulted in much greater hardness inhomogeneity in the annealed material as compared to the 5R HPT processed material.

Interaction of a Sliding Wedge with a Metallic Substrate Containing a Single Inhomogeneity

Surface folding is a recently discovered unsteady plastic flow mode in metal sliding, caused by microstructure-related spatial heterogeneity. In this work, we simulate a wedge sliding against a metallic specimen with a single, near-surface inhomogeneity or pseudograin, representative of unit asperity–grain interactions. The inhomogeneity is either plastically softer or harder than the surrounding substrate and is perfectly bonded to it. Remarkably, this simple model is able to reproduce numerous experimentally observed aspects of unsteady sliding, including the development of bumps and depressions on the prow, surface self-contact (fold) formation and the development of crack-like damage features on the residual surface. It is found that a hard inhomogeneity causes surface depressions, while a soft inhomogeneity causes surface bumps. Both features develop into folds, and subsequently, crack-like damage features. The model also reproduces the effects of sliding incidence angle, friction and size of inhomogeneity on the sliding response. The propensity to bump and fold formation decreases on increasing the depth at which the inhomogeneity is embedded. Analysis reveals that plastic buckling is a plausible mechanism for the development of perturbations on the surface of the prow. The present model differs from the classical triboplastic models of Oxley and Torrance mainly by way of the added inhomogeneity and is a minimal model to produce folding and associated damage in a single sliding pass. Implications for wear and deformation processing are discussed.

Microstructure and hardness inhomogeneity of fine-grained AM60 magnesium alloy subjected to cyclic expansion extrusion (CEE)

In the present paper, the effect of cyclic expansion extrusion (CEE) process on the microstructure inhomogeneity and microhardness distribution of AM60 magnesium alloy is investigated. The results demonstrated that after the first pass of CEE at 300°C, ultrafine grains nucleated along initial grain boundaries which caused a bimodal structure. The area fraction of the newly formed grains and Mg17Al12 dissolution was higher in the outer region rather than the central areas which is related to the different contribution of shear and normal strains. By increasing the number of passes to three, the level of inhomogeneity decreased and the average grain size of samples reduced from ∼185μm to ∼6μm. After the second pass of CEE, an excellent combination of ultimate strength (∼367MPa) and elongation to fracture (∼20%) was improved with the initial values of 261MPa and 13.8%, respectively. To show the variations of hardness, Vickers microhardness measurements were performed over the cross-section. After the first pass, heterogeneous microhardness distribution was seen. However, the microhardness inhomogeneity decreased by increasing the number of passes. Harness results were in good agreement with the microstructure results.

Hardness Inhomogeneity and Microstructure Stability in Aluminum-copper Alloys Processed by Equal Channel Angular Pressing

Three alloys; Al- 2Cu, Al- 2Cu- 0.5Ag and Al-2Cu- 0.5Mg (composition in wt.%) were casted from high purity metals into rods of 14 mm diameter and 80 mm long, using chill casting. Samples of 11 mm diameter and 70 mm long were machined, solution treated and overaged at 350℃ and then subjected to equal channel angular pressing (ECAP) processing, for one pass and two passes (route A) at room temperature. The microhardness of ECAP processed specimens was measured along a diametral direction from the top to bottom surface at room temperature (RT). The results indicate homogeneous deformation with hardness values fluctuating around mean value with standard error of ± 5 HV. The thermal stability and recovery process of ECAP processed samples, were investigated by isochronal aging in the temperature range of 200-375℃ for half an hour. Hardness values were compared and showed gradual decrease with increasing aging temperature for one-pass ECAP processed samples. For two-pass processed samples hardness tends to increase, after initial decrease, with increasing annealing temperature above 300℃. Mechanical testing showed high increase in tensile properties with number of passes as compared to overaged samples. Two-pass samples exhibited high tensile strength, high strain hardening and high ductility.

Procedure of the multiple indentations for determination of the hardness parameters of structurally heterogeneous materials

A procedure of multiple indentations has been proposed as a promising method of studying surface properties of structurally heterogeneous bodies. It has been shown that this procedure is applicable to a wide spectrum of multiphase (including superhard) materials, ceramics, and metals and makes it possible to assess mean properties of separate phases and a composite as a whole as well as a degree of the sample hardness inhomogeneity at various scale levels. Such information is important in prediction of the wear and machinability of structurally heterogeneous materials. Experimental and model results that verify the efficiency of the procedure have been presented.