Higher Ultimate Tensile Strength Research Articles

Achieving synergy of strength and ductility in powder metallurgy commercially pure titanium by a unique oxygen scavenger

Excessive interstitial oxygen (O) contamination, usually causing a dramatic loss of ductility, remains a longstanding challenge for titanium (Ti) and its alloys. Here, we propose to solve this critical problem by adding a minor CaC2 oxygen-scavenger via a simple powder metallurgy (PM) pressureless sintering approach. It is found that the surface oxide layer of hydride-dehydride (HDH) Ti powder begins to dissolve into the Ti matrix between 700 °C and 800 °C during the PM sintering process. The incorporation of CaC2 can react with the surface oxide layer (617-676 °C) prior to its active dissolution and form micrometer-sized TiC and nano-sized CaTiO3 particles, significantly refining the α-Ti grains and creating clean and well-bonded interface with Ti matrix. Therefore, the unique oxygen-scavenging effect by CaC2 produces high strength and superior ductility for Ti alloy. Even with an initially high oxygen content (∼4000 ppm), the Ti-0.4 wt.%CaC2 sample still exhibits a high ultimate tensile strength of 621±25 MPa and superior elongation of 29.3±2.6%, respectively. These values correspond to an increase of 17.6% and 301.4% compared to the as-sintered commercially pure titanium (CP-Ti) properties and far exceed the ASTM standard B381 for Grade 4 wrought Ti alloy (550 MPa and 15%) with the same O content. This work offers a novel method to develop high-strength and superior-ductility Ti materials from much more affordable Ti powder.

Significant enhancement in high temperature performance of TiAl matrix composites by a novel configuration design

Ti5Si3/TiAl composites were successfully prepared by pressure infiltration and subsequent reactive annealing. Final microstructure of Ti5Si3/TiAl composites was dependent on the reactive annealing temperature, which varied from 1350 °C to 1420 °C, two typical microstructures could be achieved: (i) An unique core-shell structure (Core: fully lamellar α2-Ti3Al/γ-TiAl with in-situ synthesized Ti5Si3 particles predominantly distributed at the interfaces of α2/γ lamellar colonies; Shell: equiaxed γ-TiAl with Ti5Si3 particles distributed in γ grain boundaries); And (ii) A novel quasi-network structure composed of fully lamellar α2/γ and a majority of Ti5Si3 particles with a quasi-network distribution. The formation of the two microstructures could be attributed to the chemical composition heterogeneity which was caused by the difference in interdiffusion rate of Ti and Al elements in different annealing temperatures. The comparative investigations on high temperature tensile properties and creep behavior showed that the core-shell Ti5Si3/TiAl composites exhibited a better specific strength (165 MPa⋅g−1⋅cm3) and higher ultimate tensile strength (633 MPa) at 800 °C, which was 12 % higher than that of the quasi-network Ti5Si3/TiAl composites. While the creep behavior analysis showed that in compassion with equiaxed γ phase, the slip distance of dislocations in fully lamellar α2/γ phase was shorter, impeding the motion of dislocations more effectively. And core-shell Ti5Si3/TiAl composites possessed a large number of grain boundaries concentrating in the shell which became weaker in the high temperature, facilitating the formation of creep pores in the shell and the final premature fracture. Hence, the quasi-network Ti5Si3/TiAl composites containing a higher content of fully lamellar α2/γ exhibited a better creep resistance than that of core-shell Ti5Si3/TiAl composites. More importantly, the steady state creep rate at 750 °C under the fixed stress of 250 MPa of quasi-network Ti5Si3/TiAl composites was relatively low, only 3.305 × 10−8 s−1, comparable to that of third-generation TiAl alloys.

Process development for laser powder bed fusion of GRCop-42 using a 515 nm laser source

Copper is widely used in high heat flux and electrical applications because of its excellent electrical and thermal conductivity properties. Alloying elements such as chromium or nickel are added to strengthen the material, especially for higher temperatures. Cu4Cr2Nb, also known as GRCop-42, is a dispersion-strengthened copper-chromium-niobium alloy developed by NASA for high-temperature applications with high thermal and mechanical stresses such as rocket engines. Additive manufacturing (AM) enables applications with complex functionalized geometries and is particularly promising in the aerospace industry. In this contribution, a parametric study was performed for GRCop-42 and the AM process laser powder bed fusion (PBF-LB/M) using a green laser source for two-layer thicknesses of 30 and 60 μm. Density, electrical conductivity, hardness, microstructure, and static mechanical properties were analyzed. Various heat treatments ranging from 400 to 1000 °C and 30 min to 4 h were tested to increase the electrical conductivity and hardness. For both layer thicknesses, dense parameter sets could be obtained with resulting relative densities above 99.8%. Hardness and electrical conductivity could be tailored in the range of 103–219 HV2 and 24%–88% International Annealed Copper Standard (IACS) depending on the heat treatment. The highest ultimate tensile strength (UTS) obtained was 493 MPa. An aging temperature of 700 °C for 30 min showed the best combination of room temperature properties such as electrical conductivity of 83.76%IACS, UTS of 481 MPa, elongation at break (A) at 24%, and hardness of 125 HV2.

Influence of bimodal copper grain size distribution on electrical resistivity and tensile strength of silver - copper composite wires

In order to obtain the best compromise between high strength and low resistivity it was decided to introduce a controlled content of copper (Cu) large grains in copper-silver (Ag-Cu) composite wires. Composite powders are prepared by mixing of 1 vol% Ag nanowires and bimodal Cu powder. The so-obtained composites powders are consolidated into cylinders (8 mm in diameter and 30 mm long) by Spark Plasma Sintering (SPS) at only 450 °C. The cylinders served as starting materials for room temperature wire-drawing (WD) for the preparation of fine wires (1–0.2 mm diameter). The microstructure of the cylinders and the wires was investigated by scanning electron microscopy imaging and electron backscattered diffraction analysis. The electrical resistivity and the tensile strength were measured at 293 K and 77 K. The electrical resistivity of the wires is the lowest when the large Cu grain content is the highest. Cu large grains greatly limit the electronic scattering, forming areas with few grain boundaries and no Ag/Cu interfaces and therefore can be considered as fast conduction channels. Conversely, the addition of Cu large grains leads to a moderate decrease in mechanical strength. By presenting a 12 % lower electrical resistivity and an equivalent ultimate tensile strength (UTS) wires with bimodal Cu matrix show a better compromise between low electrical resistivity and high UTS (0.45 μΩ cm, 1082 MPa at 77 K) compared to wires with a fine-grained Cu matrix (0.51 μΩ cm, 1138 MPa at 77 K).

Combined strengthening from nanoprecipitates, stacking faults and nano-twins enables a strong and ductile medium-entropy alloy

Single-phase face-centered cubic (fcc) alloys typically exhibit remarkable strain-hardening capability and large ductility but possess low yield strength. Conventional strengthening strategies inevitably lead to the loss of ductility. Here, an Nb alloyed CrCoNi-based medium-entropy alloy with a superb yield strength of ∼1.82 GPa, an ultimate tensile strength of ∼1.94 GPa, and good ductility of ∼24 % at 93 K is designed through the combined strengthening from nanoprecipitates, stacking faults, and nano-twins. The cold-rolling is employed to introduce high-density stacking faults and nano-twins, while subsequent low-temperature annealing generates numerous Nb-rich γ″ nanoprecipitates in the fcc matrix. The combined effects of precipitation strengthening, stacking fault- and nanotwin-induced strengthening contribute to an increase in flow stress to surpass the critical stress for the formation of stacking faults and deformation twinning. Moreover, the precipitation of high-density Nb-rich γ″ nanoprecipitates reduces the stacking fault energy of the fcc matrix. Therefore, the precipitation of γ″ nanoprecipitates produces more stacking faults and nano-twins during tensile deformation. The formation of stacking faults and nano-twins in turn, by dynamically introducing high-density interfaces for reducing the mean free path of dislocations, generates a high strain-hardening rate, thus high ultimate tensile strength and large ductility. More importantly, the coherent interfaces between γ′′ nanoprecipitates and the fcc matrix facilitate the transmission of stacking faults and nano-twins across the γ′′ nanoprecipitates. This mechanism could relieve the stress/strain concentrations at interfaces that could otherwise lead to premature crack initiation. This kind of combined strengthening from nanoprecipitates, stacking faults, and nano-twins provides a new avenue to further enhance the strength of nano-twinned materials or precipitation-strengthened materials while maintaining good ductility.

Effect of solid solution temperature on microstructure and mechanical properties of Sc-containing 7085 alloy

The microstructure, mechanical properties, and electrical conductivity of 7085 alloy sheet without and with Sc (0.25 wt%) at different solid solution temperatures were investigated by using OM, SEM, TEM, EBSD, XRD, and the strengthening mechanism of the alloys were analyzed. The results show that the micrometer-scale second phases in the 7085 alloy are mainly the T(AlZnMgCu) phase and the Al7Cu2Fe phase, while Al3(Sc, Zr) particle and the AlZnMgCuSc phase are also formed in the 7085-Sc alloy. The Mg, Zn, and Cu elements in the soluble second phase of both alloys are mainly dissolved as the solid solution temperature increases from 460 °C to 510 °C. The 7085-Sc alloy shows a smaller grain size, higher ultimate tensile strength (UTS, 624 MPa) and yield strength (YS, 580 MPa) after treatment of 480 °C/1 h + 120 °C/24 h. The high strength of 7085-Sc alloy could be attributed to fine-grained strengthening & dislocation strengthening of Al3(Sc, Zr) particles and the precipitation strengthening of MgZn2 phases. The optimum solid solution temperature of 7085 alloy should be controlled in the range of 470–480 °C and that of 7085-Sc alloy is higher, should be 480–490 °C.

Simultaneously healing cracks and strengthening additively manufactured Co34Cr32Ni27Al4Ti3 high-entropy alloy by utilizing Fe-based metallic glasses as a glue

Solidification cracking issues during additive manufacturing (AM) severely prevent the rapid development and broad application of this method. In this work, a representative Co34Cr32Ni27Al4Ti3 high-entropy alloy (HEA) susceptible to crack formation was fabricated by selective laser melting (SLM). As expected, many macroscopic cracks appeared. The crack issues were successfully solved after introducing a certain amount of Fe-based metallic glass (MG) powder as a glue during SLM. The effect of MG addition on the formation and distribution of defects in the SLM-processed HEA was quantitatively investigated. With an increasing mass fraction of the MG, the dominant defects transformed from cracks to lack of fusion (LOF) defects and finally disappeared. Intriguingly, the MG preferred to be segregated to the boundaries of the molten pool. Moreover, the coarse columnar crystals gradually transformed into equiaxed crystals in the molten pool and fine-equiaxed crystals at the edge of the molten pool, inhibiting the initiation of cracks and providing extra grain boundary strengthening. Furthermore, multiple precipitates are formed at the boundaries of cellular structures, which contribute significantly to strengthening. Compared to the brittle SLM-processed Co34Cr32Ni27Al4Ti3 HEA, the SLM-processed HEA composite exhibited a high ultimate tensile strength greater than 1.4 Ga and enhanced elongation. This work demonstrates that adding Fe-based MG powders as glues into SLM-processed HEAs may be an attractive method to heal cracks and simultaneously enhance the mechanical properties of additively manufactured products.

Process for achieving 2000 MPa H13 steel by eliminating temper embrittlement

A toughening-tempering process is devised to eliminate the second type of temper embrittlement, which is the strength-ductility trade-off during the tempering heat treatment process. The proposed methodology is implemented on H13 steel, with initial tempering at 500 °C for 1 h followed by subsequent tempering at 560 °C for 1 h. The result demonstrates an exceptionally high ultimate tensile strength of 2012.6 MPa, surpassing even that of the sample in a state of temper embrittlement. More importantly, the elongation and impact energy of the treated sample achieve as high as 8.2 % and 13.5 J, respectively, which is remarkably enhanced comparing with the embrittlement sample's figures of 1.4 % and 6.9 J. The microstructure analysis indicates that the toughening-tempering can retain the high-density dislocation in the temperature of temper embrittlement to improve the strength. On the other hand, recrystallization induced by high-angle grain boundaries improves the plastic deformation ability and obtain excellent ductility.

Improving the strength-ductility balance of medium-Mn Q&P steel by controlling cold-worked ferrite microstructure

The phase transformations, microstructure and properties of two Medium-Mn processed via quenching and partitioning steels were compared in this contribution and a new strategy for controlling mechanical properties by introducing and controlling cold-worked ferrite prior to heat treatment is proposed. It was found that during heating, the recovery and recrystallization of cold-worked ferrite compete with austenitization, thereby inhibiting the coarsening of austenite. The cold-worked ferrite interface will significantly delay the austenitization kinetics during the partitioning local equilibrium stage compared to martensite. These results lead to a diverse parent austenite, as well as a refined martensite substructure. As a result, the randomly distributed variants increase the number of effective grain boundaries, thus enhancing yield strength. The intercritical annealing process at a temperature of 860 °C resulted in the formation of fresh martensite-retained austenite (M/RA) constituents exhibiting a remarkably fine (<2 μm) and uniform grain morphology. Such microstructure yielded substantial improvement in both the strength and ductility of the steel. The proposed treatment led to excellent elongation (24%) at fracture, combined with very high ultimate tensile strength and yield strength of 1345 MPa and 1163 MPa, respectively, of the steel, resulting in a product of strength and elongation that exceed 32 GPa%.

Significant enhancement in comprehensive mechanical properties of Cr-Fe-Ni-Al-Si-Ti-Cu high-entropy alloy in as-cast state via C microalloying

An as-cast CrFeNiAl0.28Si0.08Ti0.02Cu0.01C0.01 high-entropy alloy (HEA) was designed via minor C substitution for Si in a high-strength CrFeNiAl0.28Si0.09Ti0.02Cu0.01 HEA with FCC + BCC dual-phase heterostructure. Compared to its C-free counterpart with fine sideplate structure, this HEA has higher proportion of FCC phase with relatively coarser island-like structure, and exhibits remarkable increase in production of ultimate tensile strength (UTS) and plasticity with ∼ 25 %, while maintaining its UTS to > 1GPa, which has excellent comprehensive mechanical properties in as-cast heterostructure HEAs. High dislocation storage ability is possibly the main reason for high UTS and plasticity of this HEA. This work provides a novel insight for the optimization of microstructure and mechanical properties of heterostructure HEAs in as-cast state.

Bending deformation behavior of a 1000 MPa grade dual-phase steel with superior bending toughness

Poor bending toughness is still a critical drawback of high-strength ferrite-martensite (F-M) dual-phase (DP) steel, which is unfavorable to their collision safety. In this study, a 1000 MPa grade F-M DP steel with excellent bending toughness was fabricated by reducing the strength difference between ferrite and martensite via introducing solid solution strengthening (Si), precipitation strengthening (VC) and reasonable tailoring high martensite content. The obtained DP steel exhibits a high ultimate tensile strength (UTS) of 1123 MPa, together with a good total elongation (TE) of 16.0 % and an excellent maximum bending angle (αf) of 98°. The αf of the obtained DP steel is obviously higher than that of 62–80° for reported DP steels with similar UTS levels. The related bending deformation and fracture behavior are emphasized and indicate that high bending toughness in obtained DP steel can be attributed to the excellent coordinate deformation between ferrite and martensite, improved martensite deformability and crack blocking by the fibrous deformed martensite.

Significance of FSW process parameters and tialite particle reinforcement at the weld zone of AA6082 alloy

In the current work, tialite (Al2TiO5) ceramic particles are added at the weldment during the friction stir welding of AA6082-T6 aluminium alloy. Three different values of tool rotational speed (TRS) (1000, 1200 and 1400 rpm) and welding speed (WS) (35, 40 and 45 mm/min) are taken into consideration in fabricating joints. Totally nine joints are fabricated by adding tialite particles and examinations like microstructural analysis and mechanical properties are carried out to find the quality of the joints. Defect-free welds are obtained for optimal process parameters which is a result of good mixing of base materials with uniform particle distribution. The highest ultimate tensile strength (UTS) (273 MPa) is noticed for specimen fabricated using TRS 1200 rpm and WS 40 mm/min. The peaks from the X-Ray diffraction (XRD) analysis have confirmed the presence of tialite particles in the welded samples. According to the micro hardness profile, the stir zone (SZ) (136.4 HV) and heat affected zone (HAZ) (79.8 HV) have the highest and lowest micro hardness values, respectively.

Structure and Properties of High-Strength Cu-7.7Nb Composite Wires under Various Steps of Strain and Annealing Modes

Microstructure and mechanical properties of in situ Cu-7.7Nb microcomposite (MC) wires manufactured by cold drawing with intermediate heat treatment (HT) have been studied. The evolution of Nb filaments morphology under various steps of deformation and modes of intermediate HT have been studied by the SEM and TEM methods. According to X-ray analysis, internal microstresses accumulate in the niobium filaments of the drawn MC, leading to a decrease in ductility. After heat treatment, the ductility of the wire increases significantly, since the microstresses in the niobium decrease even at the lowest HT temperature. The strength of the composite decreases under the HT because of negative changes in morphology and interface density of Nb filaments. The &lt;110&gt;Nb texture is stable under the HT up to 800 °C. The Nb filaments morphology and semi-coherent boundaries at Cu/Nb interfaces are restored under the post-HT cold drawing, leading to a sharp increase in the strength of the MC wire. Reducing the niobium concentration to 7.7%Nb relative to the traditional MC with 16–20%Nb and the recovery of the wire ductility under the HT makes it possible to obtain long-scale high-strength microwires with an extremely small diameter of 0.05 mm and high ultimate tensile strength of 1227 MPa.

Laser powder bed fusion (LPBF) of commercially pure titanium and alloy development for the LPBF process.

Laser powder bed fusion (LPBF) of titanium or titanium alloys allows fabrication of geometrically more complex and, possibly, individualized implants or osteosynthesis products and could thus improve the outcome of medical treatments considerably. However, insufficient LPBF process parameters can result in substantial porosity, decreasing mechanical properties and requiring post-treatment. Furthermore, texturized parts with anisotropic properties are usually obtained after LPBF processing, limiting their usage in medical applications. The present study addresses both: first, a design of experiments is used in order to establish a set of optimized process parameters and a process window for LPBF printing of small commercially pure (CP) titanium parts with minimized volume porosity. Afterward, the first results on the development of a biocompatible titanium alloy designed for LPBF processing of medical implants with improved solidification and more isotropic properties are presented on the basis of conventionally melted alloys. This development was performed on the basis of Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au, a near-α alloy presented by the authors for medical applications and conventional manufacturing, with yttrium and boron additions as additional growth restriction solutes. In terms of LPBF processing of CP titanium grade 1 powder, a high relative density of approximately 99.9% was obtained in the as-printed state of the volume of a small cubical sample by using optimized laser power, scanning speed, and hatch distance in combination with a rotating scanning pattern. Moreover, tensile specimens processed with these volume settings and tested in the as-printed milled state exhibited a high average yield and ultimate tensile strength of approximately 663 and 747N/mm2, respectively, combined with a high average ductility of approximately 24%. X-ray diffraction results suggest anisotropic mechanical properties, which are, however, less pronounced in terms of the tested specimens. Regarding alloy development, the results show that yttrium additions lead to a considerable microstructure refinement but have to be limited due to the occurrence of a large amount of precipitations and a supposed higher propensity for the formation of long columnar prior β-grains. However, phase/texture and microstructure analyses indicate that Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au-0.1B-0.1Y is a promising candidate to achieve lower anisotropy during LPBF processing, but further investigations on LPBF printing and Y2O3 formation are necessary.

Achieving high strength-ductility synergy in Mg-6Sn-3Zn-0.3Zr (wt.%) alloy via a combination of casting, pre-treatment and hot extrusion

Achieving the strength-ductility synergy in Mg alloys is a gigantic challenge, especially in rare-earth-free Mg alloys. In this study, a new Mg-Sn-Zn-Zr alloy with high ultimate tensile strength (∼284–326 MPa) without sacrificing elongation-to-failure (∼22.1–27.6%) was developed by utilizing casting, pre-treatment and hot extrusion. Strong random rather than basal texture is observed both in as-cast and pre-treatment samples. Subsequently, the strong texture is effectively weakened via hot extrusion whilst remaining random. More importantly, after hot extrusion, the grain sizes of as-cast and pre-treatment samples were significantly refined down to about 10 µm. Examination of as-extruded microstructures of the alloy reveals that the grain refinement is highly associated with the particle stimulated nucleation (PSN) and continuous/discontinuous dynamic recrystallization (C/DDRX) mechanisms. Moreover, the results suggest that the combination of pre-treatment and hot extrusion not only promotes multiplication of geometrically-necessary dislocations (GNDs) but enhances dynamic precipitation, which boosts the formation of fine and homogenous precipitates. Based on the results of X-ray diffraction (XRD), transmission electron microscope (TEM) and selected area electron diffraction (SAED), the precipitates are Mg2Sn phases. Furthermore, the main orientation relationship identified by high resolution TEM (HRTEM) between Mg2Sn phases and α-Mg matrix could be described as (111)Mg2Sn or (220)Mg2Sn ∥ (0001)Mg with a coherent interface. The refined grains size, ultra-fine precipitates and high density of GNDs would substantially contribute to the enhancement of the strength and the corresponding contributions are calculated to be ∼183–185 MPa, ∼30.9–38 MPa and ∼14.2–31.7 MPa, respectively. Besides, texture weakening or randomizing, grain refinement and coherent interfaces are mainly responsible for the high ductility. The current study can provide beneficial insights into the development of high-performance rare-earth-free Mg alloys with favorable microstructure via a combination of casting, pre-treatment and hot extrusion processing.

THE SIGNIFICANCE OF RETAINED AUSTENITE IN THE HIGH STRENGTH AND PLASTICITY OF MEDIUM CARBON Q&amp;amp;P STEEL

The Fe–0.44%C–1.8%Si–1.3%Mn–0.82%Cr–0.28%Mo steel treated by the quenching-partitioning process showed a product of strength and elongation of 30 GPa·% with yield stress of 1350 MPa. Such combination of high ultimate tensile strength and good ductility is attributed to a high portion of retained austenite (≥20%) transforming to martensite under tension. The high yield stress is provided by carbon supersaturation of austenite and a high dislocation density in this phase.

Evaluation of Strengthening Mechanisms in Novel Fully Ferritic Advanced High‐Strength Steels

New steel alloying concepts are designed in order to produce a fully ferritic, low‐alloy steel with high (1 GPa) ultimate tensile strength (TS). A simulated hot‐deformation process of the Ti–Mo–V–Nb and Ti–Mo–V steels is designed for that purpose, and the strengthening mechanisms of the steels are evaluated after the isothermal dwell at three different temperatures (590, 630, and 680 °C). The TS and the yield strength (YS) of the test alloys are estimated via hardness measurements. Results show that the estimated TS of over 1000 MPa and YS of over 900 MPa can be achieved in both steels, although the contribution of different strengthening mechanisms to the YS varies between the steels. The effect of the dislocation strengthening can especially compensate the reduced effect of the precipitation strengthening at all tested coiling temperatures (CTs). Based on the results, a CT range of 590–630 °C with the 1800 s dwell time seems to be a potential process window for the studied steels after the present thermomechanically controlled processing (TMCP) route.

Exceptional strength-ductility synergy of Ni and Co–Mg–La ferrite nanoparticles reinforced Sn-1Ag-0.5Cu matrix composite

The use of innovative Co–Mg–La ferrite nanoparticles (ferrite) facilitates the production of superior Pb-free solder alloy materials that suffer from the strength-ductility trade-off (SDT). The current study introduces an original Pb-free Sn-1Ag-0.5Cu (SAC105) electrical connection alloy. It is fortified with cutting-edge ferrite and nickel providing a novel approach for improving SDT by avoiding early necking throughout different temperatures. The heterogeneous structures in Sn-1Ag-0.5Cu (SAC105) alloy are assembled by controlling a fraction of reinforcing ferrite and Ni, processed under cold-drawn and partial recrystallization. The optimal ferrite concentration in SAC105 composites is about 0.3 wt%, which possesses a significant strain hardening potential and the highest strength-ductility synergy. The excessive degree of heterogeneity of SAC105-0.3ferrite induces numerous strengthening mechanisms and dislocation accumulation, resulting in attaining high ultimate tensile strength (UTS) of 40.7 MPa with significant ductility of ∼47.7%, with ∼34.3% and 39% increasing when compared to SAC105. Once SAC105 solder was doped with 0.3 wt% ferrite or 0.05Ni with 0.3 wt% ferrite, the activation energy transferred from pipe diffusion to lattice diffusion. The solitary Ni addition, on the other hand, results in the SDT of plain SAC105.

Achieving high strength near 400 MPa in an ultra-fine grained Mg–Bi–Ca ternary alloy via low-temperature extrusion

Significantly enhancing the mechanical properties of Mg–Bi base series alloys is of great significance for their wider application prospects. Here, a new Mg–3Bi–1Ca (BX31, wt.%) alloy showing high tensile yield strength of ∼397.1 MPa, high ultimate tensile strength of ∼416.2 MPa and acceptable elongation of ∼8% was fabricated by low-temperature extrusion. The analysis of microstructure showed that the enhanced yield strength was attributed to the ultra-fine grain structure (∼0.84 μm), nano-scale second phases including Mg2Bi2Ca and Mg2Ca, and Ca segregation at grain boundaries. In addition, a weak basal fiber texture to some extent provided more basal slip activities, guaranteeing the suitable ductility. The dynamic recrystallization and dislocation behavior of the BX31 billet during extrusion were also systematically observed and discussed to reveal the formation mechanisms of the ultra-fine grain structure with the weak basal fiber texture in the as-extruded BX31 alloy. As mentioned above, this work offers a new insight to obtain high-performance Mg–Bi base series alloys.

Effect of friction stir welding parameters on microstructure and mechanical properties of the dissimilar alloys of AZ91D and AA7075

Friction stir welding is an efficient joining process for light metal alloys, offering benefits such as high joint efficiency and reduced grain sizes. This work is to investigate the process parameters that can improve the welding quality and reduce the brittle intermetallic compounds, including Al3Mg2 (β) and Al12Mg17 (γ) in the welded region. It was perceived that the process parameters influenced the morphological characteristics and input heat, thereby affecting the materialization of intermetallic compounds. The primary diffraction peaks of both Al and Mg were present in all the joints. Two specific intermetallic phases, Al3Mg2 and Al12Mg17, were formed as a result of the diffusion of Al and Mg atoms during welding. These intermetallic compounds were found in all the joints. The Al12Mg17 phase, when mixed with the Mg phase, tended to migrate towards the weld surface where the AZ91D plate was positioned. This migration might lead to constitutional liquation, which refers to the melting of specific phases during solidification, resulting in the materialization of eutectic microstructures. The highest ultimate tensile strength (UTS) value of 116.64 MPa was achieved with a rotational tool speed of 800 r/min and a traverse speed of 20 mm/min. On the other hand, the lowest UTS value of 68.32 MPa was observed with a rotational tool speed of 400 r/min and a traverse speed (TS) of 20 mm/min. It was noted that lower UTS values were associated with the materialization of dense layers of intermetallic compounds (Al12Mg17 and Mg + Al12Mg17) in the stir zone when using lower rotational tool speed and TS. It suggests that improper processing parameters can lead to the materialization of intermetallic compound in the stir zone, which subsequently decreases the mechanical properties of the weldments.