Showing papers in "Acta Metallurgica Sinica (english Letters) in 2022"

Strengthening Mechanism and Microstructure of Deformable Ti Particles Reinforced AZ91 Composite

Abstract: In this study, the deformable titanium (Ti) particles reinforced AZ91 composite was successfully prepared by powder metallurgy and subsequent extrusion. The mechanical properties and microstructural evolution of pure AZ91 and 5Ti/AZ91 composite were studied. The yield strength, ultimate tensile strength, and elongation of 5Ti/AZ91 composite are measured to be 212 MPa, 323 MPa, and 10.1%, respectively. Microstructure analysis revealed that Ti particles are elongated along the extrusion direction, forming a discontinuous strip Ti particles, fine precipitated Mg17Al12 phase inhibits dynamic recrystallization (DRX) behavior through Zener pinning effect and hinders the growth of matrix grains, resulting in refiner grains of 5Ti/AZ91 composite. Heterogeneous deformed Ti particles and magnesium (Mg) matrix to generate additional heterogeneous deformation-induced (HDI) strengthening. Heterogeneous deformation-induced strengthening mainly contributed to the increment of yield strength for 5Ti/AZ91 composite.

Development and Property Tuning of Refractory High-Entropy Alloys: A Review

Abstract: In the past decade, multi-principal element high-entropy alloys (referred to as high-entropy alloys, HEAs) are an emerging alloy material, which has been developed rapidly and has become a research hotspot in the field of metal materials. It breaks the alloy design concept of one or two principal elements in traditional alloys. It is composed of five or more principal elements, and the atomic percentage (at.%) of each element is greater than 5% but not more than 35%. The high-entropy effect caused by the increase of alloy principal elements makes the crystals easy form body-centered cubic or face-centered cubic structures, and may be accompanied by intergranular compounds and nanocrystals, to achieve solid solution strengthening, precipitation strengthening, and dispersion strengthening. The optimized design of alloy composition can make HEAs exhibit much better than traditional alloys such as high-strength steel, stainless steel, copper-nickel alloy, and nickel-based superalloy in terms of high strength, high hardness, high-temperature oxidation resistance, and corrosion resistance. At present, refractory high-entropy alloys (RHEAs) containing high-melting refractory metal elements have excellent room temperature and high-temperature properties, and their potential high-temperature application value has attracted widespread attention in the high-temperature field. This article reviews the research status and preparation methods of RHEAs and analyzes the microstructure in each system and then summarizes the various properties of RHEAs, including high strength, wear resistance, high-temperature oxidation resistance, corrosion resistance, etc., and the common property tuning methods of RHEAs are explained, and the existing main strengthening and toughening mechanisms of RHEAs are revealed. This knowledge will help the on-demand design of RHEAs, which is a crucial trend in future development. Finally, the development and application prospects of RHEAs are prospected to guide future research.

Solid–liquid Interdiffusion Bonding of Cu/Sn/Ni Micro-joints with the Assistance of Temperature Gradient

Abstract: A novel solid–liquid interdiffusion (SLID) bonding method with the assistance of temperature gradient (TG) was carried out to bonding Cu and Ni substrates with Sn as interlayer. The element distribution and grain morphology of interfacial intermetallic compound (IMC) in Cu/Sn/Ni micro-joints during both SLID and TG-SLID bonding and in the final Cu/(Cu,Ni)6Sn5/Ni full IMC micro-joints were analyzed. Under the effect of Cu-Ni cross-interaction, interfacial (Cu,Ni)6Sn5 dominated the IMC growth at all the interfaces. The morphology of the (Cu,Ni)6Sn5 grains was closely related to Ni content with three levels of low, medium and high. The full IMC micro-joints consisted of L-(Cu,Ni)6Sn5, M-(Cu,Ni)6Sn5 and H-(Cu,Ni)6Sn5 grains after SLID bonding or TG-SLID bonding with Ni as hot end, while only L-(Cu,Ni)6Sn5 grains after TG-SLID bonding with Cu as hot end, showing that the direction of TG had a remarkably effect on the growth and morphology of the interfacial (Cu,Ni)6Sn5 during TG-SLID bonding. Thermodynamic analysis revealed the key molar latent heat and critical Ni content between fine-rounded-like (Cu,Ni)6Sn5 and block-like (Cu,Ni)6Sn5 were 17,725.4 J and 11.0 at.% at 260 °C, respectively. Moreover, the growth kinetic of the interfacial IMC was analyzed in detail during bonding with and without TG. Under the combination of TG and Cu-Ni cross-interaction, void-free full IMC micro-joints were fast formed by TG-SLID bonding with Cu as hot end. This bonding method may present a feasible solution to solve the problems of low formation efficiency and inevitable Cu3Sn growth of full IMC joints for 3D packaging applications.

A Review of Emerging Metallic System for High-Energy Beam Additive Manufacturing: Al–Co–Cr–Fe–Ni High Entropy Alloys

Abstract: Al–Co–Cr–Fe–Ni high entropy alloy (HEA) system is a newly developed category of metallic materials possessing unique microstructure, mechanical and functional properties, which presents many promising industrial applications. In recent years, additive manufacturing technology has given rise to a great potential for fabricating HEA parts of ultra-fine grains and geometrical complexity, thereby attracting great interest of researchers. Herein, a comprehensive review emphasizes on the recent developments in high-energy beam additive manufacturing of Al–Co–Cr–Fe–Ni HEA, in the aspects of their printing processes, microstructures, properties, defects, and post treatments. The technical characteristics of three typical high-energy beam additive manufacturing technologies for printing HEA, namely, selective laser melting (SLM), selective electron beam melting (SEBM), and directed energy deposition (DED) are systematically summarized. Typical crystal structure, grain, microstructure, as well as corresponding properties of Al–Co–Cr–Fe–Ni HEA manufactured by those technologies are primarily presented and discussed. It also elaborates the formation mechanisms of harmful defects related to the rapid solidification and complex thermal cycle during high-energy beam additive manufacturing. Furthermore, several kinds of post treatments with an aim to improve performance of HEA are illustrated. Finally, future research directions for HEA by additive manufacturing are outlined to tackle current challenges and accelerate their applications in industrial fields.

Mechanical Properties and Corrosion Behavior of Multi-Microalloying Mg Alloys Prepared by Adding AlCoCrFeNi Alloy

Abstract: In this work, a series of multi-microalloying Mg alloys with a high degradation rate and high strength was prepared by adding AlCoCrFeNi HEA particles to the Mg melt followed by hot extrusion. The microstructure evolution and mechanical properties of the alloys were studied, meanwhile, the corrosion properties were evaluated by immersion weight loss and electrochemical tests. Results indicated that HEA particles in the Mg melt were decomposed and formed the Ni-rich phase, which was distributed uniformly in the Mg matrix. Compared with the pure Mg matrix, the Mg-3 HEA alloy exhibited excellent mechanical properties of the ultimate tensile strength ~ 237 MPa and tensile yield strength ~ 181 MPa, an increased rate of ~ 49.1 and ~ 96.7%, respectively, without sacrificing the elongation. And the ultimate compressive strength (UCS) and compressive yield strength increased by ~ 31.5 and ~ 43% to 392 ± 3 and 103 ± 2 MPa, respectively. Based on theoretical analysis, the high YS of the alloys was mainly attributed to fine-grain strengthening and second phase strengthening. Besides, based on the study of corrosion behavior, it was found that with the increase in HEA particle content, the degradation rate of the composites increased because of the promotion of micro-galvanic corrosion, and the Mg-3 HEA alloy showed a maximum degradation rate of ~ 25.2 mg cm−2 h−1.

Microstructures and Mechanical Properties of a New Multi-functional 460 MPa Grade Construction Structural Steel

Abstract: This article reports a new generation of Q460 multi-functional construction structural steel, which has high strength (yield strength larger than 460 MPa), excellent toughness (higher than 110 J/cm2 at − 60 °C), lower yield ratio (lower than 0.8), good fire resistance (yield strength at 600 °C larger than two-thirds of its room-temperature yield strength) and better corrosion resistance. The effects of finish cooling temperature (FCT) on the microstructure and properties were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), emission electron probe micro-analysis (EPMA), electron backscattering diffraction (EBSD), tensile tester, impact tester, periodic immersion cycle acceleration test and electrochemical experiment. The results show that the strength and toughness are simultaneously improved by decreasing the FCT due to more lath-like bainite with large number of dislocations, refined martensite/austenite (M/A) with higher carbon concentration and increased high angle boundaries. In addition, the fire resistance of the newly developed Q460 steel is obviously better than the conventional one, which is mainly due to non-recrystallized lath-like bainite with high dislocation density at elevated temperature. The corrosion resistance of the new Q460 steel is also improved due to the addition of Cu and Cr.

Achieving Superior Strength and Ductility of AlSi10Mg Alloy Fabricated by Selective Laser Melting with Large Laser Power and High Scanning Speed

Abstract: AlSi10Mg alloy was prepared by selected laser melting (SLM) in a high laser power range 300–400 W. The effects of energy density on the relative density, microstructure and mechanical properties of the SLMed AlSi10Mg alloy were studied. The results showed that the SLMed AlSi10Mg alloy fabricated at a laser power of 400 W and a scanning speed of 1800 mm/s had a relative density of 99.4%, a hardness of 147.8 HV, a tensile strength of 471.3 MPa, a yield strength of 307.1 MPa, and an elongation of 9.6%, exhibiting excellent comprehensive mechanical properties. The unique combination of the melt pool structure and microstructure caused by the large laser power and fast scanning was responsible for the excellent performance. The wide and shallow melt pool structure with few defects and proper overlapping between the continuous melt pools were obtained. The growth of columnar crystals was inhibited by a large proportion of equiaxed grains formed at the border of melt pools, and numerous sub-structures were observed within the α-Al grains. This study provided a more efficient process parameters for the preparation of the SLMed AlSi10Mg alloy. The enhanced mechanical property will help to broaden the application of the AlSi10Mg alloy in industry.

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

Abstract: The influence of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of a typical Ni-based superalloy was researched. The optimum parameters of P = 170 W, V = 0.8 m/s were determined, under which the SLMed samples exhibited both the largest relative density of 99.57% and the best mechanical properties, including the microhardness (329.3 ± 3.8 HV), yield strength (726 ± 8.1 MPa), ultimate tensile strength (900 ± 5.9 MPa) and elongation ((31.9 ± 0.24)%). The average grain size ranges of SLMed samples are from 15.2 to 17.4 μm, with a typical mixed grain structure. Owing to the high cooling rate and remelting during SLM process, a large number of low-angle grain boundaries (LAGBs), dislocations and sub-grains were formed, and the fraction of LAGBs reached above 65%. At the same time, the content of low-Σ coincidence site lattice (CSL) boundaries was mostly less than 1%, while there was almost no γ′ phase precipitated in the matrix. The texture of SLMed samples was weak, and there was no obvious preferred growth direction. Combining with the microstructure characterization, both grain refinement strengthening and dislocation strengthening were considered as the main strengthening mechanisms. Moreover, the fracture mechanism of the optimum sample belonged to ductile fracture.

Influence of Ca Content on Microstructure and Mechanical Properties of Extruded Mg–Al–Ca–Mn Alloys

Abstract: Herein, the effects of Ca content on microstructure, texture and mechanical properties of extruded Mg–3Al–0.4Mn–xCa (x = 0.4, 0.8 and 1.2 wt%) rods are systematically investigated. The results reveal that the alloy, with Ca content of 0.8 wt%, exhibits the highest strength and ductility, possessing an ultimate tensile strength of 267.57 MPa and elongation (EL) of 16%. This is mainly due to the gradual transformation of typical fiber texture into a texture with a [10-11] component parallel to the extrusion direction (ED), which increases the Schmid factor of pyramidal slip and enhances the activation rate of pyramidal ⟨c + a⟩ slip. Simultaneously, the as-formed spherical phases and segregation of Ca at grain boundaries render a significant influence on the strength and ductility of the alloy.

Recent Advances on the Corrosion Resistance of Low-Density Steel: A Review

Abstract: This paper reviews the previous literature on the alloy composition design of low-density steel (LDS), focusing on the effect of Al, Mn, Ni, and other alloy elements on the formation of the steel matrix and second phase, and provides classification. The microstructure of LDS after processing includes the matrix structure, к-carbide, and B2 (FeAl, NiAl, or MnAl) phase of ferritic LDS, austenitic LDS, and dual-phase LDS. The influence of alloy elements on the corrosion resistance of LDS is derived from the addition of Al and Mn for metallurgy. Additionally, the influence of Cr and Mo addition on the corrosion resistance improvement was studied. The electrochemical properties of the corrosion process in LDS are discussed. Further, the microstructure of LDS affects the corrosion resistance properties including pitting corrosion, hydrogen embrittlement, and SCC (stress corrosion cracking). Finally, future research directions are proposed.

Microstructure and Mechanical Property of Hot-Rolled Mg–2Ag Alloy Prepared with Multi-pass Rolling

Abstract: A conventional multi-pass rolling is designed to form different microstructures in a Mg–2Ag alloy. The relationship between microstructure and mechanical property is investigated. The result shows that twin-induced nucleation plays a prominent role for the dynamic recrystallization (DRX) behavior of the rolled Mg–2Ag alloys. The DRXed grains distributed around elongated grains have random orientations but gradually turn to the concentrated orientation with strong basal texture when the rolling pass increases. The yield strength and ultimate tensile strength of rolled Mg–2Ag alloy gradually increase with increasing rolling pass. The elongation of rolled sample is gradually improved when the rolling pass increases from one to three, while a significant drop of elongation shows in the four-pass rolling sample. The strong basal texture, refined grains, high-density dislocations, and Ag segregation along grain boundaries are suggested to play a prominent role for enhancing the strength of Mg–Ag alloys, while the low-density dislocations, homogeneously fine-grained microstructure, and weak texture are critical for improving the ductility.

Deformation Behaviors of an Additive-Manufactured Ni32Co30Cr10Fe10Al18 Eutectic High Entropy Alloy at Ambient and Elevated Temperatures

Abstract: Eutectic high-entropy alloys (EHEAs) that have superior formability are attractive for direct laser deposition technology. In this study, a regular-shaped bulk Ni32Co30Cr10Fe10Al18 EHEA without apparent pores and micro-cracks was successfully fabricated by direct laser deposition. The as-deposited alloy showed a high tensile strength of 1.3 GPa with a ductility of 35% at ambient temperature and a tensile strength of 320 MPa at 760 °C. The deformation mechanisms of the as-deposited alloy at ambient and elevated temperatures were investigated by coupling the in-situ tensile test with a scanning electron microscope. It is revealed that the excellent combination of strength and ductility originated from the synergic effects of the FCC and B2 phases in eutectic lamellae. And the generation of cracks along phase boundaries restricted its high-temperature strength above 760 °C.

Solid-State Hydrogen Storage Properties of Ti–V–Nb–Cr High-Entropy Alloys and the Associated Effects of Transitional Metals (M = Mn, Fe, Ni)

Abstract: Abstract Recently, high-entropy alloys (HEAs) designed by the concepts of unique entropy-stabilized mechanisms, started to attract widespread interests for their hydrogen storage properties. HEAs with body-centered cubic (BCC) structures present a high potential for hydrogen storage due to the high hydrogen-to-metal ratio (up to H / M = 2) and vastness of compositions. Although many studies reported rapid absorption kinetics, the investigation of hydrogen desorption is missing, especially in BCC HEAs. We have investigated the crystal structure, microstructure and hydrogen storage performance of a series of HEAs in the Ti–V–Nb–Cr system. Three types of TiVCrNb HEAs (Ti 4 V 3 NbCr 2 , Ti 3 V 3 Nb 2 Cr 2 , Ti 2 V 3 Nb 3 Cr 2 ) with close atomic radii and different valence electron concentrations (VECs) were designed with single BCC phase by CALPHAD method. The three alloys with fast hydrogen absorption kinetics reach the H / M ratio up to 2. Particularly, Ti 4 V 3 NbCr 2 alloy shows the hydrogen storage capacity of 3.7 wt%, higher than other HEAs ever reported. The dehydrogenation activation energy of HEAs’ hydride has been proved to decrease with decreasing VEC, which may be due to the weakening of alloy atom and H atom. Moreover, Ti 4 V 3 NbCr 2 M ( M = Mn, Fe, Ni) alloys were also synthesized to destabilize hydrides. The addition of Mn, Fe and Ni lead to precipitation of Laves phase, however, the kinetics did not improve further because of their own excellent hydrogen absorption. With increasing the content of Laves phase, there appear more pathways for hydrogen desorption so that the hydrides are more easily dissociated, which may provide new insights into how to achieve hydrogen desorption in BCC HEAs at room temperature.

Microstructure and Mechanical Properties of Dissimilar Double-Side Friction Stir Welds Between Medium-Thick 6061-T6 Aluminum and Pure Copper Plates

Abstract: It is difficult to achieve Al/Cu dissimilar welds with good mechanical properties for medium-thick plates due to the inherent high heat generation rate at the shoulder-workpiece contact interface in conventional friction stir welding. Thus, double-side friction stir welding is innovatively applied to join 12-mm medium-thick 6061-T6 aluminum alloy and pure copper dissimilar plates, and the effect of welding speeds on the joint microstructure and mechanical properties of Al/Cu welds is systematically analyzed. It reveals that a sound Al/Cu joint without macroscopic defects can be achieved when the welding speed is lower than 180 mm/min, while a nonuniform relatively thick intermetallic compound (IMC) layer is formed at the Al/Cu interface, resulting in lots of local microcracks within the first-pass weld under the plunging force of the tool during friction stir welding of the second-pass, and seriously deteriorates the mechanical properties of the joint. With the increase of welding speed to more than 300 mm/min void defects appear in the joint, but the joint properties are still better than the welds performed at low welding speed conditions since a continuous uniform thin IMCs layer is formed at the Al/Cu interface. The maximum tensile strength and elongation of Al/Cu weld are, respectively, 135.11 MPa and 6.06%, which is achieved at the welding speed of 400 mm/min. In addition, due to the influence of welding distortion of the first-pass weld, the second-pass weld is more prone to form void defects than the first-pass weld when the same plunge depth is applied on both sides. The double-side friction stir welding is proved to be a good method for dissimilar welding of medium-thick Al/Cu plates.

Enhancement Mechanism of Cerium in 316LN Austenitic Stainless Steel During Creep at 700 °C

Abstract: Effect of cerium (Ce) on creep strength and microstructure of 316LN austenitic stainless steel (316LN steel) at 700 °C/150 MPa was investigated by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and thermodynamic calculations. Addition of 0.032 wt% Ce to 316LN steel results in a prominent increase in creep life from 313 to 556 h. Ce enriches in titanium nitride nanoparticles, increases slightly the activity and diffusion coefficient of Mo, and facilitates the formation of fine and dense intragranular Laves phase precipitates. Thus the creep strength is remarkably enhanced by Ce addition in 316LN steel through the intragranular Laves phase precipitation strengthening. It reveals a new insight into the improvement effect of rare earth (RE) elements such as Ce on creep strength of austenitic stainless steels, which inspired the design of RE-microalloying heat-resistant steels.

Comparative Study on Corrosion Behavior and Mechanism of As-Cast Mg–Zn–Y and Mg–Zn–Gd Alloys

Abstract: Rare earth (RE) elements have large solid solubility in magnesium and are widely used to regulate the microstructure and property of advanced magnesium alloys. However, different kinds of RE elements have different effects on microstructure and property of the alloy. In this study, a Mg–Zn–Y alloy and a Mg–Zn–Gd alloy with alloying elements of the same atomic percentage were designed to clarify the effect of yttrium (Y) and gadolinium (Gd) on the corrosion behavior of as-cast MgZn2Y2.66 and MgZn2Gd2.66 alloys. The results show that the MgZn2Y2.66 alloy is mainly composed of α-Mg phase and long period stacking ordered (LPSO) phase, while MgZn2Gd2.66 alloy is mainly composed of α-Mg phase and (Mg, Gd)3Zn phase (W phase). Generally speaking, the corrosion phenomena of the two alloys in 3.5 wt% NaCl solution are similar. In the early stages of exposure, the alloys underwent uniform corrosion at a relatively low corrosion rate. With prolonged exposure, localized corrosion became dominated and the corrosion rate was greatly increased. However, the corrosion rate of the MgZn2Y2.66 alloy, in terms of the corrosion current density, is about one order of magnitude lower than that of the MgZn2Gd2.66 alloy. The high corrosion resistance of the MgZn2Y2.66 alloy is mainly attributed to the presence of LPSO phase in form of continuous networks and the relatively high corrosion resistance of the corrosion product layer on the alloy.

Temperature Gradient Induced Orientation Change of Bi Grains in Sn–Bi57–Ag0.7 Solder Joint

Abstract: Sn-Bi-X solders are widely used in electronic packaging industry. However, thermomigration (TM) behaviors of Sn-Bi-X solder joints and the orientations change of Bi grains under the temperature gradient are rarely reported. In this study, Sn-Bi57-Ag0.7/Cu solder joints were used to conduct a TM test under a temperature gradient of 625 °C/cm for 400 h, and an isothermal aging test at 85 °C was also conducted for comparison. The microstructural evolution of Sn-Bi-X solder joints after reflow, TM and isothermal aging were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). The results indicated that the Sn/Bi areal ratio after TM did not change significantly whether at the hot end (from 46.78%/52.12% to 50.90%/48.78%) or at the cold end (from 50.25%/49.64% to 48.71%/51.16%) compared with that of as-reflowed samples due to the insufficient thermal energy. The thickness of intermetallic compound (IMC) after TM at hot end (2.49 μm) was very close to that of the IMC at cold end (2.52 μm), which was also close to that of the aged samples. In addition, the preferred orientations of Sn and Bi grains in different Sn–Bi–Ag solder joints resulting from different conditions (reflow, TM and isothermal aging) were characterized by electron backscatter diffraction (EBSD). The obtained results demonstrated that both Sn and Bi grains had no preferred orientation whether after reflow or isothermal aging, while the orientation of Bi grains of the sample after TM changed from random direction to c-axis ([0001] direction) parallel to the heat flow. Ag3Sn could hinder the change of orientation of Bi grains under the temperature gradient, and the corresponding mechanism was also systematically illuminated. This study firstly revealed the orientation change of Bi grains under the temperature gradient, which would have a profound guiding significance for enhancing the reliabilities of Sn–Bi–Ag solder joints.

{112} 〈111〉 Twins or Twinned Variants Induced by Martensitic Transformation?

Abstract: Twinned substructure in lath martensite was induced in the interstitial free (IF) steel via a high pressure thermal cycle (heating up to 1100 ℃ and holding for 30 min, cooling at 10 ℃/s to room temperature under a pressure of 4 GPa). Experimental observations and theoretical simulation confirm that the twinned substructure has the origin related to the twinned variants rather than the bcc {112} 〈111〉 twins, while extra diffraction spots were caused by crystal overlapping rather than any extra phase. The differences in crystallography and electron diffraction behavior between twinned variants and {112} 〈111〉 twins were discussed in detail.

Microstructure and Corrosion Properties of CP-Ti Processed by Laser Powder Bed Fusion under Similar Energy Densities

Abstract: In this work, two types of CP Ti cubes with similar volumetric energy densities (VED) but different process parameters were produced using laser powder bed fusion (LPBF) method. The corrosion behavior of the fabricated specimens was investigated by conducting electrochemical impedance spectroscopy (EIS) and polarization experiments in simulated body fluid (SBF) solution at 37 °C. The results indicated that the microstructure and porosities, which are of great importance for biomedical applications, can be controlled by changing the process parameters even under constant energy densities. The sample produced with a lower laser power (E1) was featured with a higher level of porosity and thinner alpha laths, as compared with the sample fabricated with a higher laser power (E2). Moreover, results obtained from the bioactivity tests revealed that the sample produced with a higher laser power conferred a slight improvement in the bioactivity due to the higher amount of porosity. Lower laser power and hence higher porosity level promoted the formation of bone-like apatite on the surface of the printed specimens. The potentiodynamic polarization tests revealed inferior corrosion resistance for the fabricated sample with higher porosity. Moreover, the EIS results after different immersion times indicated that a stable oxide film was formed on the surface of samples for all immersion times. After 1 and 3 days of immersion, superior passivation behavior was observed for the sample fabricated with lower laser power. However, very similar impedance and phase values were observed for all the samples after 14 days of immersion.