Second-phase Particles Research Articles

Attributes of intergranular corrosion in AA6061-AA7075 double sided friction stir weld

The present study aims to observe the distinguishable progression of intergranular corrosion (IGC) in dissimilar AA6061-AA7075 double-sided friction stir welds (DSFSW) with and without employing secondary heating. Transverse weld specimen sectioned along the weld line, to separate advancing and retreating side, are electrochemically corroded. Secondary heating helps in impeding the most accelerated corrosion rate dominated over the AA7075 region by approximately 50%. The double stirred region remains least susceptible to corrosion due to the dissolution and break down of precipitates. The degree of homogenization during stirring of dissimilar alloy controls the degree of material depletion during corrosion, making intercalated structure unfavourable in terms of corrosion. IGC is found more aggressive in AA7075 structurally intercalated with AA6061 in stir zone. The coarse grain, wider precipitate free zones and continuity of the precipitates along grain boundary (GB) are interpreted as most influential factors. Additional/secondary heating resulted into retardation of IGC within the AA7075 but resulted continuous IGC along the GBs of AA6061. Low angle GBs appears more resistive against IGC as they allow the formation of lean precipitates on contrary to high angle GBs. Secondary heating boosts breakdown and dissolution of corrosion sensitive secondary phase particles reducing the intergranular corrosion tendency.

Microstructural effects on the spall failure of 7085 aluminum alloy

Designing aluminum alloys for spall resistance requires an understanding of the active failure mechanisms under dynamic loading. However, it is time-consuming and expensive to obtain sufficient data to investigate these mechanisms from conventional plate impact spall experiments. Here we use a high-throughput laser-driven micro-flyer plate impact technique to connect the spall failure of aluminum alloy Al7085-T711 to its microstructure. By conducting tests at four impact velocities, we observe the full range of behaviors from incipient spall to complete spall failure. The spall strength of Al7085-T711 increases with both increasing strain rate and peak shock stress, as is typically the case in aluminum alloys. Examination of recovered samples indicates that incipient spall voids initiate primarily at Al7Cu2Fe second-phase particles. To further explore the effect of microstructure on spall failure, we annealed some specimens at 500°C to increase the aluminum grain size while retaining the Al7Cu2Fe particles, which had only a minor effect on spall strength. Solutionizing at 600°C to eliminate the Al7Cu2Fe second-phase particles, on the other hand, increases the spall strength significantly. Our results suggest that spall failure of Al7085-T711 is dominated by the presence Al7Cu2Fe second-phase particles, and that eliminating these particles could result in improved spall resistance of commercial alloys.

Fracture Behavior of AlMg4.5Mn Weld Metal at Different Temperatures under Impact Loading

This paper deals with a three-component aluminum alloy AlMg4.5Mn that was welded using a GTAW process in the shielded atmosphere of Ar+70%He+0.015%N2. The weld-metal toughness was evaluated at three different temperatures using instrumental Charpy pendulum impact testing to measure not only the total energy, but also the crack initiation energy and the crack growth energy. Fractographic analysis of the fracture surfaces and EDS analysis of large second-phase particles on fractured surfaces at each temperature were also carried out. Fractographic analysis at different temperatures indicated a clearly distinguishable fracture mechanism. It was inferred that the absorbed energy was closely correlated with the fracturing of surfaces. Moreover, it was concluded that with decrease in the amount of microscopic voids and dimples, the total energy absorbed also decreased.

The corrosion behavior and mechanical properties of 5083 Al-Mg alloy manufactured by additive friction stir deposition

Corrosion behavior and mechanical property of 5083 Al-Mg alloy manufactured by additive friction stir deposition (AFSD) were investigated compared with 5083-H112 alloy. AFSD is beneficial to reduce the pitting corrosion sensitivity of 5083 alloy due to refinement and re-dissolution of Mn rich secondary phase particles. But the intergranular corrosion (IGC) susceptibility of AFSD alloy is more significant due to the exfoliation of fine equiaxed grains after sensitization. This can be ascribed to that the grain boundaries of AFSD alloy after sensitization are dotted with β-phase which tends to be attacked by acid.

Unexpected {[formula omitted]} pyramidal stacking faults in Zr(Fe, Cr)2 secondary phase particle induced by the C15→C14 Laves phase transformation in Zircaloy-4

The formation mechanism of (011¯1)C14 pyramidal stacking faults (SFs) with a 8.6° deflection to the SFs on (111¯)C15 planes of Zr(Fe, Cr)2 secondary phase particle (SPP) in Zircaloy-4 under cyclic deformation were investigated by using transmission electron microscopy. Results showed that local C14 domain structures can be induced into the C15 SPP when the SFs formed on (111¯)C15 close-packed planes by the emission of 16[112]C15 Shockley partial dislocations under cyclic stress effects. Moreover, when the shear stresses along [112¯]C15 direction were large enough to initiate the 16[112¯]C15 partials on every other (111)C15 planes, the C15→C14 Laves phase transformation would be accomplished, resulting in a significant reconstruction of atomic configurations of the SFs on original (111¯)C15 planes. Thus, the formation of (011¯1)C14 pyramidal SFs during C15→C14 phase transformation in SPP were presented.

Stimulating the inverse transformation of amorphous second-phase particles in irradiated Zr alloy to modulate the nanostructure

Amorphization of the second phase particles in irradiated zirconium alloys tends to lead to defect formation, radiation growth and corrosion resistance reduction, while conventional annealing requires higher temperature to promote amorphous crystallization transformation, which may reduce the performance of materials. Based on the difference of electrical properties between the second phase particles and the matrix, the pulsed electric current can induce the crystallization of amorphous second phase particles at a lower temperature by introducing additional electrical free energy, so as to achieve the microregion regulation of materials. The pulsed electric current effectively reduces the recrystallization activation energy of the second phase particles from 96 kJ/mol to 63.68 kJ/mol, thus lowering the recrystallization barrier to accelerate its process. The proposed pulsed electric current can rapidly reverse the crystal structure of amorphous second phase particles at lower temperatures, which provides a novel idea for adjusting the microstructure of metal materials.

Inhomogeneous Microstructure Evolution of 6061 Aluminum Alloyat High Rotating Speed Submerged Friction Stir Processing.

An inhomogeneous microstructure induced by high rotating speed submerged friction stir processing (HRS-SFSP) on 6061 aluminum alloy was researched in detail.The microstructures of the aluminum alloy processing zone were characterized by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM) qualitatively and quantitatively.The results show that the recrystallization proportion in the inhomogeneous structure of the processing zone is 14.3%, 37.8% and 35.9%, respectively. Different degrees of grain deformation can affect the dislocation and lead to the formation of a plastic-elastic interface. At the same time, the second-phase particles in the processing zone were inhomogeneity and relatively, which further promotes the plastic-elastic interface effect. The plastic-elastic interface can significantly improve the strength of aluminum alloy, whileat the same time, rely on recrystallized grains to provide enough plasticity. When the rotation speed was 3600 r/min, the strength and ductility of the aluminum alloy after HRS-SFSP were increased by 48.7% and 10.2% respectively compared with that of BM. In all, the plastic-elastic interface can be formed by using high rotating speed submerged friction stir processing, and the strength-ductility synergy of aluminum alloy can be realized at the plastic-elastic interface.

Interface Microstructure and Properties of Vacuum-Hot-Rolled 55#/316L Clad Rebars

The existing process for the preparation of cladded rebars is too complicated for large-scale industrial production. Therefore, this paper proposes a 55#/316L rebar preparation method based on vacuum hot rolling. The microstructure and mechanical properties of the composite interface of the rebar, along with the connecting technique, were studied using transmission electron microscopy, X-ray diffraction, and Vickers hardness testing. The obtained results showed that the minimum thickness of the 55#/316L rebar cladding was 0.25 mm, which was twice that of the M 329M/M 329-11 design standard used in the United States of America. Due to the diffusion of carbon, large numbers of second-phase particles were precipitated on the stainless-steel side, which resulted in intergranular chromium depletion. After multi-pass hot rolling, the minimum bonding strength of the composite interface reached 316.58 MPa, which was considerably higher than the specified value of 210 MPa. In addition, we designed three different types of rebar connection joints: sleeve, groove-welded, and bar-welded. According to the tensile test, the bar-welded joint had higher yield strength (385 MPa) and tensile strength (665 MPa) than the base rebar (376.6 MPa and 655 MPa), as well as a very high corrosion resistance.

Microstructure evolution and tensile behaviour of fine-grained 6082 Al wire with high ultimate strength and high work hardening by friction stir extrusion of bulk Al sheet

Driven by the highly enhanced demand for metallic materials with excellent strength-ductility synergy, fine-grained alloy fabricated by severe plastic deformation (SPD) is a long-lasting research interest. In this study, a fine-grained aluminium (Al) alloy wire with high ultimate tensile strength (316.8 MPa) and high work hardening capability (n = 0.34) was developed via a novel bulk-consolidation friction stir extrusion (FSE) enabled by CoreFlow™ process based on FSE processed 6082-T6 Al sheet. Grain refinement of 200 times occurred in CoreFlowed Al alloy, which is rarely observed in FSEed Al alloy counterpart based on milling metal chips. The undesirable feature of coarse and preferred secondary phase particles in the initial 6082-T6 Al billet was well tailored during CoreFlow™ process, which led to an attractive characteristic of dispersed and refined particle phases in CoreFlowed Al alloy. Texture components of S (V = 8.24%) and Goss (V = 6.86%) were observed, however, the overall micro-texture intensity of CoreFlowed Al wire was significantly weakened. Due to the cooperative deformation modes consisting of dislocation slip and grain boundary sliding, as well as the positive compatibility effect of fine-grains on deformation, CoreFlowed Al specimens demonstrated an excellent ductility (EI = 19.3%) and acceptable yield strength (YS = 182 MPa) compared to initial 6082-T6 Al sheet (EL = 10.4%, and YS = 225 MPa). Benefiting from the higher work hardening capability, the final ultimate tensile strength (UTS) of CoreFlowed Al (316.8 MPa) reached almost the same level as the 6082-T6 Al sheet (324.6 MPa). Furthermore, after one simple post-processing one-step heat treatment (175 °C for 10 h), the wire's yield strength was improved to 250 MPa, although only maintaining 9% elongation. This low elongation could be attributed to micro-cracks induced by coarsening of the second phases, while high yield strength resulted from fine grain size and precipitate strengthening. The wide range of variations in the mechanical properties of CoreFlowed Al wire under different conditions provides significant freedom in tailoring the mechanical properties of alloy wire in applications.

Microstructure and mechanical properties of Mg–Ni–Gd alloy synthesised by powder metallurgy

ABSTRACT The hexagonal close-packed structure renders magnesium weak at room and high temperatures for structural applications despite its low density. Inducing thermally stable and coherent second phases would enhance/retain the strength of Mg-based alloys even at high temperatures. This paper aims to develop a high-strength Mg-based nanocomposite. A master alloy composed of Ni and Gd was cast and the composition of Mg97.56Ni1.22Gd1.22 (at.-%) was prepared using ball milling for 150 h. XRD plots of the as-milled powder having nano-size crystallites confirm the partial dissolution of the master alloy. Consolidation through sintering with 5, 7 and 9 h of exposure at 550°C and extrusion at 500°C resulted in the formation of Mg5Gd, Mg2Ni, Gd2O3 and MgO phases. The extruded samples possessed a high strength of 804 MPa, which can be attributed to ultra-fine grains and dispersoid strengthening by homogeneously distributed second-phase particles in the 100–200 nm range.

Recent Studies of Corrosion - Fatigue Crack Growth on the 7081-T73511 Aluminium Alloy

This work aims to evaluate the effect of localized corrosion on the fatigue crack growth (FCG) resistance of the 7081-T73511 aluminium alloy. The understanding of this complex combined process was explained by the dissolution of adjacent 7081-T73511 aluminium matrix promoted by the Al7Cu2Fe second-phase particle and the crack tip interaction time with the aggressive medium. Fracture surfaces produced in air and in sea water fog at the testing conditions applied here, basically present the same appearance, with a transgranular, cleavage-like cracking mode for 8 <DK< 30 values.

Зависимость процесса коррозии биорезорбируемого сплава ZX10 от структурных факторов и локального уровня pH

Magnesium biodegradable alloys are a promising material for self-dissolving surgical implants. Magnesium is known to be sensitive to electrochemical corrosion due to the galvanic effect between the matrix and particles of secondary phases and inclusions. Another important factor is the pH level. The behavior of certain chemical reactions depends on the pH level, so one can assume that the pH level of a corrosive medium at the material surface is a factor determining what chemical reactions can occur there. Finally, there is evidence that variability of the crystallographic orientation of the grains may be a cause of anisotropy of corrosion properties. The purpose of this work is to reveal the influence of the electrode potential of the microstructural elements, the crystallographic orientation of the grains, and the pH level of the near-surface volume of the corrosion solution on the corrosion process. In the study, sections of 2×1.5 mm were marked on the ZX10 alloy samples, for which maps of the distribution of crystallographic orientations and chemical composition were drawn. To assess the influence of the electrode potential of the particles, the authors carried out a Kelvin probe mapping in the 90×90 µm area. Next, corrosion tests were carried out with video filming of the surface on the marked area. To determine the pH level influence, the solution circulation in the cell was varied. Upon completion of the tests, corrosion products and corrosion damage were examined in detail. According to the results, the pH level in the liquid near-surface micro-volumes has a greater influence than the electrode potential of the particles as it provokes the formation of corrosion products of a different composition, which leads to passivation of the surface areas around the particles. The authors identified two different types of filiform corrosion. For filiform corrosion, a correlation between the corrosion direction and the crystallographic orientation of the grains was established.

CREEP RESISTANCE AND STRUCTURE OF 10% CR-3% СО-2% W-0.29% CU-0.17% RE STEEL WITH LOW NITROGEN AND HIGH BORON CONTENTS USED IN UNIT COMPONENTS FOR COAL POWER PLANTS

The creep resistance and structure of 10% Cr-3% Co-2% W-0.29% Cu-0.17% Re steel with 0.1% carbon, low nitrogen content, and high boron content were investigated in a long-term fracture strength test at 650°C under loads from 200 to 100 MPa applied in increments of 20 MPa. For comparison, 9% Cr steel with 0.1% carbon, 0.05% nitrogen, and 0.005% boron was studied. The steels were subjected to preliminary heat treatment including normalization at 1050°C for 1 hour, tempering at 750-770°C for 3 hours, and cooling in air. The structures of both heat-treated steels exhibited martensite laths with boundaries pinned by М23С6 carbides, and the rearrangement of dislocations was retarded by MX particles. A significant difference between 10% Cr steel and 9% Cr steel was the presence of fine М23С6 carbide particles characterized by orientational relationships with the ferrite matrix and MX carbonitrides, whose volume fraction was 6 times lower. Short-term tensile tests at room temperature showed no differences between the steels, while the long-term tensile strength of 10% Cr steel was 13% higher compared to 9% Cr steel. The creep deformation mechanism of the steels was also different. Structural analysis of 10% Cr steel after creep tests revealed no substantial changes in its lath structure: the lath width increased by only 58% and the dislocation density was reduced by a factor of 2. Comparison with 9% Cr steel showed that the good structural stability of 10% Cr steel during creep is caused by the high coarsening resistance of second-phase particles, whose coarsening rate is 1-2 orders of magnitude lower than that in 9% Cr steel.

Oxide formation mechanism of a corrosion-resistant CZ1 zirconium alloy

Tailoring microstructure and microchemistry by altering elemental compositions and thermomechanical treatment parameters enables superior corrosion performance in zirconium alloys for nuclear applications. However, our understanding of the relationship between various defects and the corrosion process remains limited in the newly developed zirconium alloys. Here we report the oxide formation mechanism of a CZ1 zirconium alloy with corrosion resistance surpassing many other zirconium alloy systems, such as Zircaloy-4 and Zr-1Nb-1Sn alloys. Autoclave experiments of CZ1 alloy and Zr-1Nb-1Sn model alloy were performed in 360 °C water for up to 820 d. We quantitively determined oxide phases by transmission Kikuchi diffraction (TKD) and examined lateral cracks, nano-porosity, and second-phase particles in oxide scales by transmission electron microscopy (TEM). Compared to the Zr-1Nb-1Sn model alloy, CZ1 alloy with lower Nb and Sn concentrations has shown smaller and lower-density lateral cracks but slightly larger oxide grains, reducing the diffusion route for oxidating species. Using analytical scanning and transmission electron microscopy, we demonstrate that due to the lower content of Sn (∼0.9 wt.%), there is less tetragonal ZrO2 phase formed in the oxide, and the level of tetragonal to the monoclinic phase transition is reduced. Although the Nb content (0.1 wt.%–0.3 wt.%) is lower than the solid solution limit of Nb in Zr, by introducing minor elements such as Fe, Cr, and Cu, there are still a reasonable number of second-phase particles to relieve the high stress associated with the metal-to-oxide transformation. These mechanisms have substantially changed the density and distribution of lateral cracks in the oxide, thus reducing the corrosion rate of zirconium alloys.

Tempering Behavior of Novel Low-Alloy High-Strength Steel

The effect of tempering on the mechanical properties, structure, and dispersion of secondary phase particles is studied in 0.4%C-2%Si-1%Cr-1%Mo-VNb steel. This steel austenitized at 900 °C with subsequent water quenching exhibits a yield stress of 1445 MPa and a lath martensite structure with MX particles of ~40 nm located in matrix and boundary M6C carbides of ~210 nm. Tempering in the temperature interval of 200–400 °C provides a yield stress of 1625 MPa due to the precipitation of ε-carbide and cementite within laths. The yield stress decreases to 1415 and 1310 MPa after tempering at 500 and 650 °C, respectively, due to the replacement of matrix carbides by boundary M23C6 carbide. A Charpy V-notch impact energy of ~12 J/cm2 is almost independent from tempering temperatures of up to 400 °C and increases up to ~33 J/cm2 after tempering at 650 °C due to decreased yield stresses and increased plasticity.

The Effect of Rotary-Die Equal-Channel Angular Pressing Process on the Microstructure, the Mechanical and Friction Properties of GW103 Alloy.

In this study, the effect of rotary-die equal-channel angular pressing (RD-ECAP) on the microstructure and texture evolution of GW103 alloy is studied. RD-ECAP processes were carried out for 1, 4 and 12 passes at 450 °C. The mechanical properties and friction behavior of RD-ECAP-processed Mg-10Gd-3Y (wt%) alloy (GW103) are discussed. The results reveal that the size of dynamic recrystallized grains and second-phase particles are significantly refined to about 1.3 μm and 1 μm, respectively. The texture evolution of the processed samples is studied by X-ray diffraction and electron backscattered diffraction techniques. The multiple texture components formed are not observed after the conventional ECAP process. Moreover, different dynamic recrystallization (DRX) mechanisms are systemically analyzed and discussed in view of the texture evolution of ECAP processed samples. The final textures obtained after 12 passes are identified as two types: The C-texture type induced by continuous and discontinuous DRX, and the random texture components induced by reorientation of the initial <101¯0> fiber. Based on the grain refinement, precipitate strengthening and texture weakening mechanisms, a high-performance ternary alloy of Mg-Gd-Y was firstly obtained through 12 passes RD-ECAP processing, with a combination of high yield strength of 312 MPa and a high ductility of 22%. In addition, the friction behaviors are also studied. The multi-pass-processed samples exhibit a relatively lower friction coefficient under a load of 10 N at room temperature.

Mechanisms of abnormal grain growth in friction-stir-welded aluminum alloy 6061-T6

The relationship between the temperature conditions during friction-stir welding (FSW), the stir-zone microstructure, and the thermal stability of welded aluminum-alloy 6061 joints was established. To facilitate interpretation of the microstructural data, temperature distributions generated during FSW were quantified using a finite-element-modeling (FEM) approach. In all cases, the stir-zone microstructure was found to be unstable against abnormal grain growth (AGG) during post-weld solution annealing. Moreover, it was found that AGG always developed very rapidly, being nearly complete during heating of the welded material from ambient condition to the solution temperature. In all welded conditions, annealing behavior followed Humphrey's cellular-growth model. In particular, for low-heat-input conditions, a combination of a fine-grain structure and a low content of secondary particles gave rise to a competition between normal grain growth and AGG. As a result, the final grain size was relatively small. By contrast, high-heat-input conditions gave rise to a combination of relatively-coarse grains and a high fraction of particles in the weld nugget, which provided microstructural stability. However, a fine-grain surface layer associated with the tool shoulder triggered catastrophic grain coarsening that eventually consumed the entire weld zone and resulted in millimeter-scale grains. Remarkably, AGG led to a 40 o 〈111〉 rotation of the crystallographic texture in the stir zone. It was thus concluded that the AGG was governed by at least two mechanisms, viz., the pinning effect exerted by second-phase particles and the enhanced mobility of 40 o < 111 > boundaries. • Annealing behavior of FSW joints is critically influenced by the weld heat input. • Abnormal grain growth develops at the early stage of the post-weld heat treatment. • Abnormal grain growth is governed by two different mechanisms.

Dependency of recrystallization kinetics on the solidification microstructure of 316L stainless steel processed by laser powder bed fusion (LPBF)

In this study, the recrystallization kinetics of two 316 L stainless steels produced by Laser Powder-Bed Fusion (LPBF) were compared. The as-built microstructures of the studied materials differed by their grain size, their grain boundary character distributions, and their populations of nanoprecipitates. Strong differences in their recrystallization kinetics were observed; they were attributed to a delayed nucleation in the steel exhibiting the finer grains, the lower density of low-angle boundaries (LAB), and the higher volume fraction of precipitates. This work shows that a high density of LABs stemming from the solidification stage in the LPBF process promotes recrystallization by locally increasing the stored energy close to grain boundaries, and hence the formation of recrystallization nuclei. The design of low-LAB microstructures could thus lead to higher stability at high temperatures. • Recrystallization kinetics are heavily influenced by the as-built microstructure. • Kinetics are not affected by a second-phase particles volume fraction increase. • Low angle boundaries promotes the formation of recrystallization nuclei.

The effect of multi-pass friction stir processing on microstructure, mechanical properties, and corrosion behavior of WE43-nHA bio-composite

The influence of multiple passes during friction stir processing (FSP) on the evolution of microstructure, hardness, compressive strength, and corrosion behavior of a Mg-based bio-composite has been investigated. WE43 Mg alloy was used as the matrix of the composite and nano-sized hydroxyapatite (nHA) was added as the second material to form the composite. After fabrication of the composite by one pass of FSP, the process was repeated six times. Significant grain refinement and fragmentation of secondary phase particles were observed during FSP. Increasing the number of passes of FSP was found to be associated with pronounced grain refinement coupled with the uniformity in distribution of nHAs. Corrosion resistance is enhanced with an increasing number of FSP passes, which is attributed to the fragmentation and redistribution of secondary phase particles in addition to a homogeneous distribution of nHA.

Phase-Field Simulation on the Effect of Second-Phase Particles on Abnormal Growth of Goss Grains in Fe-3%Si Steels.

A phase-field model was revised to study the abnormal growth of Goss grains during the annealing process in Fe-3%Si steels, in which the interaction between the second-phase particles and Goss grain boundaries (GBs) was considered. The results indicate that the abnormal growth of Goss grains occurs due to the different dissolvability of the particles at Goss GBs compared with the other GBs. Moreover, the degree of abnormal growth increases first and then decreases with an increasing particle content. Meanwhile, the size advantage of Goss grain can further promote the degree of abnormal growth. Two types of island grains were found according to the simulated results, which is consistent with the experimental observations. A proper GB dissolvability of particles is the key factor for the formation of isolated island grains, and a higher local particle density at GBs is the main reason for the appearance of serial island grains. These findings can provide guidance for the desired texture control in silicon steels.