Particle-stimulated Nucleation Research Articles

Using multiscale second phases to fabricate high-performance Mg-3Al-3Nd-0.1Mn alloys with bimodal grain structures

The quest for magnesium alloys that combine high strength and high ductility drives significant research effort aimed at broadening their use in various sectors. In this research, a low-alloy Mg-3Al-3Nd-0.1Mn (AEM330) was successfully prepared through Hard Plate Rolling (HPR), achieving an ideal combination of high strength and ductility via carefully modifying grain structure at lower rolling temperature. The optimized processing conditions led to an alloy with an ultimate tensile strength of 427 MPa and an notable elongation of 11.4 %. The AEM330 processed by HPR displayed unique bimodal grain structure with coarse grains (CGs) sizes ∼13.66 µm and fine grains (FGs) sizes ∼2.36 µm, and their strength and ductility were found to be directly related to the microstructural differences between CGs and FGs. A crucial discovery of this study is the essential role of the Al2Nd phase (larger than 1 µm) and the Al11Nd3 phase (smaller than 1 µm) in forming bimodal grain structure. The former phase plays a significant role in recrystallization by particle-stimulated nucleation (PSN), and the latter restricts grain size via Zener pinning. Furthermore, it was found that the ratio of CGs to FGs could be controlled by modifying the rolling temperature. Importantly, the simple composition and direct shaping process of the AEM330 alloy not only improve its cost-effectiveness but also streamline the manufacturing process; thus, enhancing its potential for broad industrial use.

Developing a novel Mg[sbnd]2Zn[sbnd]3Li[sbnd]1Gd alloy sheet with high room-temperature formability by introducing an elliptical texture distribution

In recent years, modification of texture distribution has been considered a valid approach to improve the room-temperature (RT) formability of magnesium (Mg) alloys. In this study, a novel Mg2Zn3Li1Gd alloy sheet with weak elliptical-texture was fabricated by cold rolling and subsequent annealing, and it showed an excellent Erichsen (IE) value near 7.1 mm. Both quasi-in-situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analysis indicate that considerable basal and pyramidal dislocations can be activated in the cold rolling process. During annealing, these dislocations can induce nucleation and then cause preferential misorientation relationships around 〈uvt0〉 concerning the nuclei and parent grains, which can facilitate the formation of elliptical texture. Furthermore, the particle-stimulated nucleation (PSN) mechanism and the co-segregation of Zn and Gd at grain boundaries (GB) further weak texture intensity. Finally, the mechanical properties of the Mg2Zn3Li1Gd alloy sheet are significantly improved.

Microstructure, mechanical and corrosion study of friction stir processed Inconel 625 additive layers deposited via wire arc direct energy deposition

This study explores the application of friction stir processing (FSP) as an effective post-deposition surface treatment technique for improving the mechanical and corrosion properties of wire arc direct energy deposited (WA-DED) Inconel 625 additive layers. The coarse and cube-oriented microstructure observed in as-built Inconel 625 components results in anisotropic mechanical properties. The non-equilibrium solidification in WA-DED Inconel 625 leads to severe micro-segregation of alloying elements, forming large Nb and Mo-rich Laves and carbide phases in the inter-dendritic areas. This causes a heterogeneous distribution of vital alloying elements within the γ-Ni matrix of the material, adversely affecting its mechanical properties and corrosion resistance, thereby restricting the utility of this manufacturing technique in critical applications. The incorporation of FSP has been observed to successfully eradicate the presence of typical coarse and highly oriented microstructure through grain refinement, achieving a more uniform microstructure. Through severe plastic deformation (SPD), coarse eutectic phases are broken down and redistributed within the γ-Ni matrix, enhancing the uniformity of material properties. The research thoroughly investigates the changes in microstructure, mechanical performance, and corrosion resistance of both WA-DED and FSP-enhanced WA-DED Inconel 625 samples. A Thermo-Calc simulation using the classical Scheil model was conducted to understand the solidification pathway and the formation mechanism of the Laves phase in the WA-DED sample. The grain size was significantly reduced by 98.23%, from 138 µm in the WA-DED sample to 2.44 µm in the stir zone (SZ) of the FSP-treated area, due to dynamic recrystallization (DRX), particle-stimulated nucleation (PSN), and the Zener pinning effect. This grain refinement led to remarkable enhancements in the yield strength (YS) by approximately 113%, ultimate tensile strength (UTS) by about 37% and micro-hardness by around 60% compared to the WA-DED sample. In a 3.5 wt% NaCl solution, the FSP-treated sample demonstrated superior corrosion resistance by forming a stable, thicker passive film than the as-built sample. The results of this study will offer valuable insights into utilizing the FSP technique as post-weld surface treatment of WA-DED deposited IN625 superalloy.

Effect of Al–5Ti–1B Addition on Solidification Microstructure and Hot Deformation Behavior of DC-Cast Al–Zn–Mg–Cu Alloy

This study investigated the effect of adding Al–5Ti–1B grain refiner on the solidification microstructure and hot deformation behavior of direct-chill (DC) cast Al–Zn–Mg–Cu alloys. The grain refiner significantly decreased the grain size and modified the morphology. Fine-grained (FG) alloys with grain refiners exhibit coarse secondary phases with a reduced number density compared to coarse-grained (CG) alloys without grain refiners. Dynamic recrystallization (DRX) was enhanced at higher compression temperatures and lower strain rates in the CG and FG alloys. Both particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) are enhanced in the FG alloys, resulting in decreased peak stress values (indicating DRX onset) at 450°C. The peak stress of the FG alloys was higher at 300-400°C than that of the CG alloys because of grain refinement hardening over softening by enhanced DRX.

The Influence of Precipitated Particles on the Grain Size in Cold-Rolled Al-Mn Alloy Foils upon Annealing at 100-550 °C.

The Al-Mn alloy heat exchanger fin production process includes a brazing treatment at s high temperature of 600 °C, in which coarse grains are preferred for their high resistance to deformation at elevated temperatures by decreasing the grain boundary sliding. In this study, Al-1.57Mn-1.57Zn-0.58Si-0.17Fe alloy foils cold rolled by 81.7% (1.1 mm in thickness) and 96.5% (0.21 mm in thickness) were annealed at 100-550 °C for 1 h to investigate their recrystallization behavior, grain sizes, and precipitates by increasing the annealing temperature, using micro-hardness measurement, electron back-scattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The micro-hardness results showed that the recrystallization finishing temperatures for the two samples were almost the same, 323 ± 2 °C. The EBSD results showed that when the annealing temperature decreased from 550 to 400 °C, the recrystallized grain sizes of the two samples were nearly identical-both increased slightly. Further decreasing the annealing temperature from 400 to 330 °C caused the grain sizes to increase more, with the thinner foil sample having a more significant increase. The SEM and TEM observations showed that the micron-sized primary-phase remained unchanged during the annealing process. The nano-sized secondary phase precipitates formed during the hot-rolling process experienced a coarsening and dissolving process upon annealing. The particle size of the secondary phase increased from 32 nm to 44 nm and the area fraction decreased from 4.2% to 3.8%. The nucleation analysis confirmed that the large primary-phase could act as a nucleation site through particle stimulated nucleation (PSN) mode. The relatively dense secondary phase precipitates with small sizes at lower temperatures could provide higher Zener drag to the grain boundaries, leading to fewer nuclei and thereafter coarser grains. The coarsening of the recrystallized grains in the foils could be implemented through thickness reduction and/or precipitation processes to form densely distributed nano-sized precipitates.

Dynamic precipitation and twinning-induced dynamic recrystallization in Mg–6Al–3Sn–1Zn alloy during multi-pass high-speed rolling

Ultrafine-grained microstructure was successfully obtained in Mg–6Al–3Sn–1Zn (ATZ631) alloy by multi-pass high-speed rolling (HSR) at a rolling speed of 1100 m/min and a rolling temperature of 400 °C. The grain refinement mechanism of ATZ631 alloy during HSR was investigated, which is attributed to mechanically fragmented grains and twinning-induced dynamic recrystallization (TDRX). During the initial stages of rolling, the initial coarse grains are refined by the intersection of twins. With increasing strain, the grain refinement mechanism is the typical dynamic recovery and TDRX. In general, twins are filled with a high density of dislocations and nano-sized precipitates at the dislocations, which can accelerate DRX by increasing the driving force or particle-stimulated nucleation (PSN) within the twins. Subsequently, the coarse grains are refined by forming fine DRXed grains via dislocation entanglement, dislocation rearrangement, and dislocation absorption. As the process proceeds, grain boundaries (GBs) migration is suppressed by the precipitates at the GBs of new DRXed grains. Thus, the growth of the DRXed grains can be inhibited. Finally, ultrafine-grained microstructure with an average size of ∼116 nm is obtained. Moreover, the new grains formed by TDRX exhibit random orientation during this process, indicating that the basal texture gets effectively weakened by increasing the fraction of DRXed grains. These findings bring forward the microstructural evolution during dynamic precipitation and DRX, thus providing a novel notion for improving microstructures and grain refinement in Mg alloys.

Effect of CNTs on microstructure, mechanical and wear properties of the AZ91–3B4C–3SiC–xCNTs composites

This article presents a study on the effect of adding carbon nanotubes (CNTs) (0, 0.5, 1 and 2 wt-%) and hot extrusion on the microstructures, mechanical properties, and wear behaviour of AZ91–3B4C–3SiC hybrid composites. The matrix alloy is AZ91 Mg alloy which primarily consists of 9 wt-% Al and 1 wt-% Zn. A mechanical-induction stir casting process was used to add CNTs to the composite. XRD patterns and EDX analysis showed that all composites have a microstructure of α-Mg, Mg17Al12, SiC and B4C. The outcomes demonstrated that the Mg17Al12 networks can be significantly thinned by the additional CNTs. The hot extrusion also dispersed the B4C and SiC particles in the structure, improving the particle-stimulated nucleation (PSN) plus twinning mechanism, but the presence of micron particles caused stress concentration, which did not affect the composites’ strength and elongation. The wear behaviour of the as-cast and extruded composites was studied by applying 5, 10 and 20 N forces at room temperature. The results showed that the mass loss and the coefficient of friction values are decreased with the addition of CNTs, which is attributed to the increased microhardness and self-lubricating properties of the CNTs.

Improving strength-ductility of Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy due to bimodal LPSO and dislocations

Rare-earth (RE) magnesium alloys have attracted lots of attention due to their excellent mechanical properties. In this work, the microstructure and mechanical properties of as-extruded 8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy under different solution treatment were examined with the optical microscope (OM), scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), electron back-scattered diffraction (EBSD) and Instron testing machine. The results show that the ES12 alloy (solution treatment for 12 h at 520 °C) has the highest ultimate tensile strength (UTS) of 390 MPa with a fracture elongation of 24.5% at the cost of a minor drop in yielding strength (YS) compared to the as-extruded alloy. During solution treatment, the block-shaped long period stacking ordered (LPSO) in as-extruded alloy evolves into plate-shaped LPSO, which disperses at grain boundaries (GBs), and lamellar LPSO, which distributes in grains. The coexistence of plate-shaped and lamellar LPSO, which impedes the dislocations movement, and the activated <c + a> dislocations are regarded as the primary reasons for mechanical properties improvement. Furthermore, the (11–21) <1–100> texture in as-extruded alloy transforms into the (11–20) <0001> texture in ES12 alloy. The average grain size increases from 3.45 μm in as-extruded alloy to 18.70 μm in ES12 alloy. The Schmid factors of {0001} <11–20>, {10-10} <11–20>, {10–11} <11–20>, and {11–22} <11–23> increase, which indicate that slip systems are more easily activated in plastic deformation. The dynamic recrystallization (DRX) grains fraction increase to 92.8% for ES12 alloy due to the particle-stimulated nucleation (PSN) mechanism triggered by block-shaped and plate-shaped LPSO. The freshly DRXed grains further weaken the texture, and reduce the dislocation density. All of these factors increase elongation of RE magnesium alloy.

Elucidating the dual effect of vanadium microalloying on hot deformation behavior of high nitrogen martensitic stainless steel

Precipitates always have a complex effect on the hot work hardening and recrystallization softening of materials. In this work, the dual effect of V-microalloying on hot deformation behavior and microstructure evolution of high nitrogen martensitic stainless steel (HNMSS) was investigated by hot compression experiments, modeling study and microstructure characterization. The results showed that V-microalloying enhanced the flow stresses and activation energy of 0.2V steel due to the fine grain strengthening effect and the blocking effect of dynamic precipitation on dislocation movement and grain boundary migration. On the other hand, the recrystallization model and deformed microstructure indicated the more intense dynamic recrystallization (DRX) softening effect in 0.2V steel, inducing the faster decrease of flow stress after the peak stress. V-microalloying promoted the dynamic precipitation of HNMSS during hot deformation, which reduced the size and increased the number of precipitates in 0.2V steel. The fine precipitates hindered dislocation movement, promoted dislocation accumulation and entanglement, formed a particle deformation zone (PDZ) with high dislocation density and large orientation gradient, and then induced the occurrence of particle-stimulated nucleation (PSN). In addition, the dislocation cells evolved from dislocation entanglement were also favorable nucleation sites for DRX.

Dynamic Recrystallization, Texture Evolution, and Improved Mechanical Properties of Mg-Y-Zn-V Alloy during Forging and Subsequent Extruding Deformation

In this paper, the electron backscatter diffraction (EBSD) technique was used to analyze the dynamic recrystallization (DRX), twinning, slip behavior, and texture evolution during forging and subsequent extruding deformation. The results show that, as the degree of strain increased (forging to extruding), the degree of DRX increased, and the DRX mechanism changed from discontinuous DRX (DDRX) during forging to DDRX and continuous DRX (CDRX) during extruding. Particle stimulation nucleation (PSN) promoting DRX occurred during deformation. The deformation process mainly produced {10–12} twins (TTW) and played a role in coordinating the deformation. The slip behavior also changed according to an analysis of in-grain misorientation axes (IGMA) results, changing from slip-dominated with a basal &lt;a&gt; slip to co-dominated with multiple slip modes, with the activation of mainly prismatic &lt;a&gt; and pyramidal &lt;c+a&gt; slip. Meanwhile, the strong basal texture at the beginning of the deformation also changed, and the texture strength decreased from 24.81 to 15.56. The weakening of the texture was mainly due to the formation of DRX grains and twins, as the newly formed DRX and twins reoriented. In the later stages of deformation, the activation of prismatic &lt;a&gt; slip and pyramidal &lt;c+a&gt; slip changed the basal texture component. Based on microstructural analysis, the improvement in mechanical properties was due to fine-grain strengthening and load-transfer strengthening. The ultimate tensile strength (UTS) was 370.5 MPa, the yield strength (YS) was 340.1 MPa, and the elongation (EL) was 15.6%.

Intermetallic and dispersoid structures in AA3104 aluminium alloy during two-step homogenisation

During homogenisation of the AA3104 cast ingot, a phase transformation of intermetallic particles from β-Al6(Fe,Mn) orthorhombic phase to harder α-Alx(Fe,Mn)3Si2 cubic phase occurs. The large constituent intermetallic particles, regardless of phase, assist in the recrystallisation nucleation process through particle stimulated nucleation (PSN). Ultimately, this helps to refine grain size. The sub-micron dispersoids act to impede grain boundary migration through a Zener drag mechanism. For this reason, the dispersoids that form during homogenisation are critical to the recrystallisation kinetics during subsequent rolling, with smaller dispersoids being better suited to instances where the minimisation of recrystallisation is required during hot rolling. This work simulates an industrial two-step homogenisation practice with variations in the peak temperature of the first step between 560 °C and 580 °C. The effect of this temperature variation on the intermetallic particle phase evolution is investigated. The aim is to identify the ideal intermetallic phase balance and the dispersoid structure that are best suited for the minimisation of recrystallisation during hot rolling through maximising Zener drag and maintaining galling resistance. The results indicate a trend where an increase in homogenisation temperature from 560 °C to 580 °C yields, firstly, an increase in the volume fraction of the α-phase particles to greater than 50% of the total volume fraction at both the edge and the centre of the ingot and, secondly, it yields an increased dispersoid size. Thus, a lower temperature homogenisation practice produces a near-ideal combination of intermetallic particle phase distribution, as well as dispersoid size, which is critical for Zener drag and the minimization of recrystallisation during the hot rolling processes.

Recrystallization behavior and strengthening mechanism of extruded Mg−Gd−Y−Zn−Zr alloy with different pre-aged states

Recrystallization behavior and strengthening mechanism of extruded Mg−Gd−Y−Zn−Zr alloy with different pre-aged states

Microstructural regulation and mechanical behavior of asymmetrically extruded high-content TC4p reinforced AZ31 composite

Asymmetric extrusion (AE) is frequently used to weaken the texture of magnesium (Mg) alloys to improve their mechanical properties. In this paper, to explore the influence of asymmetric extrusion on the microstructure and mechanical properties of Mg matrix composites (MMCs), we synthesized TC4/AZ31 composite sheets via ball milling coupled with sintering and then various extrusions (including conventional extrusion (CE), transverse-directional AE (TDE), and normal-directional AE (NDE)). The differences in microstructure and performance among the three processed samples were compared and analyzed using SEM, XRD, EBSD characterization and tensile testing. The results showed that the asymmetrically extruded samples achieved better mechanical properties compared to their CE counterparts. It found that the grain refinement was relatively significant and texture evolution was dispersed in AE samples compared to the CE ones, which was attributed to the TC4 particle (TC4p) disturbance and extra asymmetric shear of matrix flow. Three different macro-textures were obtained in various extruded processes, and the micro-texture of each sample changed in diverse regions. Based on grain orientation spread (GOS) analysis, the introduction of TC4p resulted in dynamic recrystallization (DRX) and particle-stimulated nucleation (PSN) effects during the AE process at 350 °C. The mechanisms of the combination of ductility and strength in the AE-processed sheets were related to the finer grains, high Schmid factor (SF), evenly distributed dislocations and scattered texture. This work can also provide the feasibility and prospects of applying asymmetric extrusion to MMCs.

Deformation behavior of LPSO phases, grain refinement mechanism, and texture evolution of a Mg-Gd-Y-Zn-Zr alloy processed by forward extrusion combined with dual-directional angular deformation

In this study, a Mg-Gd-Y-Zn-Zr alloy was processed by forward extrusion combined with dual-directional angular extrusion (FE-DDAE), and the corresponding microstructure evolution, grain refinement mechanism, and texture were investigated systematically. The deformed materials were subjected to a combination of three-way compressive stress and shear stress, and five different areas were selected to study the typical deformation behaviors. The results show that microstructures are heterogeneous in different areas, and the grain size is refined from 138.70 μm to 37.18 μm through continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). The block-shaped and lamellar long period stacking ordered (LPSO) phases can coordinate severe deformation through dissolution, tearing, breaking for the former, and bending, severe kinking, refining into small pieces for the latter. Moreover, the broken block-shaped LPSO can promote DRX process through particle-stimulated nucleation (PSN) mechanism, and the lamellar LPSO phases can hinder the dislocation motion to restrict DRX. A strong basal texture is observed during the initial stage of FE-DDAE process, and the basal texture intensity gradually weakens to 4.9 MRD due to the random orientations of high volume fraction of DRX grains. A mixed texture component is formed with basal planes inclined 50° from ED to TD, showing a trend of spreading.

A study on microstructural, mechanical properties and optimization of wear behavior of friction stir processed AlCrCoFeNi High Entropy Alloy reinforced SS410 using response surface methodology

The equimolar High Entropy Alloy (HEA) is incorporated on the surface of SS410 steel to enhance the mechanical properties for the current industrial scenario. The objective of the present work is to make a first attempt at surface modification of SS410 steel with gas atomization synthesized AlCrCoFeNi HEA powder through Friction Stir Processing (FSP). The microhardness and ultimate tensile strength of the FSP-HEA sample are increased by 41.3 % and 39.1 % respectively due to the high degree of refined grains with 2.84 μm and evenly distributed HEA particles. The wear rate of FSP-HEA samples is optimized by response surface methodology with process parameters including applied load, sliding distance, and sliding velocity. The most influential factor and regression model are derived from experimental results that predict the wear rate by the analysis of variance technique. The worn surface of FSP-HEA samples is evaluated by morphological analysis with corresponding induced wear mechanisms. The minimum wear rate is achieved by optimum process parameters along with higher hardness through particle-stimulated nucleation mechanism, Hall-Petch relation, and dynamic recrystallization. The grain refinement, barrier effect, and grain growth hindrance of HEA particles lead to enhancement in the strength of processed HEA samples.

Restoration kinetics and microstructural evolution of moderately warm-rolled yttria dispersion-strengthened tungsten plate during annealing at high temperatures

The thermal stability of an yttria particle dispersion strengthening tungsten (W-2vol%Y2O3) plate with 67 % rolling thickness reduction (WY67) fabricated by powder metallurgy and warm rolling is investigated through isochronous annealing and isothermal annealing. Hardness testing of annealed specimens allows tracking the degradation of the mechanical properties and electron backscatter diffraction (EBSD) was used to characterize the corresponding microstructure. Isochronous annealing for 1 hour between 1150 °C and 1450 °C indicates that the recrystallization temperature of WY67 is about 1400 °C. Herein, the recovery kinetics, recrystallization kinetics, and the evolution of microstructure and texture of WY67 during isothermal annealing at temperatures ranging from 1300 ℃ to 1450 ℃ were investigated in detail. The effect of both the heterogeneity of the deformed microstructure and the yttria particles on the recovery and recrystallization was further studied. The results show that the stored energy of the deformed grains of yttria dispersion strengthened tungsten is texture-dependent and the grain with γ-fiber (normal direction // <111>) texture has the largest stored energy of 546 KJ/m3. The grains with higher stored energy are conducive to recrystallized nucleus/grain growth rather than nucleation during subsequent annealing. The recrystallized texture inherits the deformed texture and is mainly composed of γ-fiber texture. There are more pre-existing potential recrystallization nuclei in the elongated grains with larger deformation in the as-rolled microstructure. Micron-sized yttria particles can stimulate the formation of recrystallization nuclei through particle-stimulated nucleation mechanism, nevertheless, this is not the dominant nucleation mode for WY67 during recrystallization in this study. Fine (submicron) yttria particles hinder the recovery and recrystallization processes through the Smith–Zener pinning effect.

Long-period stacking ordered phase kinking, twinning behavior and dynamic recrystallization of Mg-Gd-Y-Zn-Zr alloy under severe shear deformation

A novel severe shear specimen with a maximum shear strain of 3 was designed to further clarify kinking and fragment behavior of long period stacking ordered (LPSO) phases, as well as their interaction with twinning behavior and dynamic recrystallization (DRX) behavior under severe shear deformation (SSD). The results indicate that promoted kinking and fragment behavior for LPSO phases is obseved for hihger activation of basal <a> dislocations induced by shear deformation. Besides the known crack causd by LPSO kinking, a new fragment process without kink bands formation is found for block-shaped LPSO phases under SSD, which follows three stages of micro-crack at the interface of LPSO/α-Mg matrix, crack and crush. Moreover, the actvition of tension twinning detected at kinked boundaries is reveled to be controlled by the combined effect of LPSO kinking and shear stress state. Specifically, crystal rotation related to LPSO kinking reorient grains to be possbile for twinning, and then high effective Schmid factor (ESF) under shear stress furter prioritise the twining acivaiton. Additionally, orientation dependence of DRX behaviors was weakened due to the particle-stimulated nucleation (PSN) mechanism caused by interdendritic block-shaped LPSO phases, which can accelerate the formation of low angle grain boundaries and DRX grains by activated pyramidal <c+a> slip systems even at basal grains.

Effect of Sn microalloying on microstructure and mechanical properties of Mg-11Gd-1Zn-0.5Zr alloy

The effect of Sn microalloying on the microstructure and mechanical properties of the Mg-Gd-Zn-Zr alloy has been systematically investigated. The grain size of the cast alloy was found to be coarse. The two alloys were mainly composed of α-Mg and (Mg, Zn)3Gd phases, and the addition of 0.5 % Sn also caused a small amount of Sn3Gd5 phases to appear. In the subsequent homogenization heat treatment at 500 °C, adding Sn promoted the formation of many lamellar 14H-LPSO phases. After undergoing three rounds of hot rolling, the Sn-containing alloy exhibited a refined grain size and a significant reduction in the volume fraction of twins, mainly due to the particle-stimulated nucleation (PSN) mechanism and the inhibiting effect of LPSO on twins. In addition, the ultimate tensile strength (UTS) of the alloy (380 ± 1 MPa) was significantly increased by the addition of Sn, although the age hardening effect was prolonged. The high strength was attributed to the dispersion distribution of fine chain-like β′ structure, which was conductive to the movement of pinning basal dislocations.

The coupling effect of particle and electric current on the grain refinement of Al–Cu–Li alloy by an improved thermo-mechanical treatment with electrically-assisted annealing

An improved thermo-mechanical treatment with electrically-assistant recrystallization annealing (EARA) was proposed for Al–Cu–Li alloy with the fine and equiaxed grains. The Al–Cu–Li alloy strips were subjected to the different overaging times, asymmetric rolling and EARA. The size and number of particles had a significant effect on the average grain size. The critical particle size for particle-stimulated nucleation (PSN) was determined. The introduction of electric current did not change the critical size, however, the efficiency of PSN triggered by coarse particles was improved by 3.6 times, compared with the conventional recrystallization annealing (CRA). By the optimized overaging time, the grains were refined and the texture was weakened. The recrystallization mechanism for the specimens containing coarse particles was the discontinuous static recrystallization (DSRX) by PSN and the continuous static recrystallization (CSRX). At the constant current density and annealing time, the critical temperature for rapid recrystallization for EARA decreased by 40 °C, compared with CRA. The results were due to the local Joule heating effect, the activation energy reduction and atomic diffusion rate increase by the athermal effect of electric current. The improved RI-ITMT with EARA would be an ideal candidate for rapid grain refinement in Al–Cu–Li alloy alloys.

Influence of tool plunge depth and multiple passes on the microstructure, texture evolution and hardness of age-hardenable aluminium 7075 alloy

In this study, the surface properties of AA7075 were modified through the solid-state friction stir processing technique using a hexagonal-shaped H13 pin. Single pass friction stir processing was conducted at two different shoulder plunge depths at constant tool rotation and linear speeds with the resulting cross-sections being examined using optical macroscopy. Six friction stir processing passes were then carried out intermittently at optimal shoulder plunge depth and the corresponding microstructural evolution along the nugget was investigated using advanced microstructural characterization techniques such as electron backscatter diffraction and high-resolution transmission electron microscopy. Bulk texture examination was conducted using the neutron diffraction method, while hardness measurements were taken using Vicker's hardness testing machine. Single pass friction stir processing conducted at a shoulder plunge depth of 0.25 mm produced defect-free material consolidation with a refined grain structure along the nugget. Minimum grain size averaging 3.78 μm was observed at the top, while slightly increased grain sizes were noticed at the middle and bottom regions of the nugget. After 6 friction stir processing passes, grain size was reduced significantly averaging 1.84 μm at the top, while size at the pin-driven zone exhibited an increasing trend due to heat input and strain rate variations. Both, single-pass and 6 pass specimens exhibited a weaker texture along the nugget due to randomness in grain orientation and the effect of particle-stimulated nucleation mechanism. In the single-pass specimen, hardness decreased along the nugget, with a maximum of 85.2 HV at the top, while 6-pass specimen exhibited an improved hardness of 133.5 HV due to the strengthening caused by grain refinement, precipitates and dislocations.