Excellent Strengthening Effect Research Articles

Design of D022 superlattice with superior strengthening effect in high entropy alloys

Precipitation strengthening is one of the most promising mechanisms to develop high-performance high entropy alloys (HEAs). However, the design of a reinforcing phase with an excellent strengthening effect is still one of the most pivotal challenges. In the present study, a design strategy based on overall valence electron concentration (OVEC) is developed, and a coherent D022 superlattice (noted as γ″ phase) with superior strengthening effect is designed. The newly developed γ″ phase is systematically characterized using transmission electron microscope and atom probe tomography. Differentiating from the traditional Ni3Nb γ″ phase, the present high-entropy γ″ phase contains ∼7.7% Co and follows the (Ni,Co,Cr,Fe)3(Nb,Fe) stoichiometry. Three γ″ phase variants are observed with crystallographic orientation relationships of [001]γ″//<001>γ and (001)γ″//{100}γ. The lenticular γ″ particles with small volume fraction (7%) causes a significant yield strength increase (670 MPa) and ductility retention (40%), resulting in excellent yield strength-ductility combination. The excellent strengthening effect of the γ″ phase is attributed to both ordering strengthening and coherency strengthening. The present study proposes a new design strategy of precipitates and develops a superior reinforcing phase for HEAs. These findings will not only promote the development of precipitation-hardened HEAs but deepen the fundamentals of precipitates design for other complex concentrated alloys as well.

Enhanced corrosion resistance in metal matrix composites assembled from graphene encapsulated copper nanoflakes

Corrosion protection of metals is of great importance due to their widespread use. Graphene (Gr) has shown promising corrosion resistance as a coating for metals, however, its excellent strengthening effect that has been widely shown is suppressed in such coating applications. Here, inspired by positive anti-corrosion role of graphene coating for metal macro-foils, graphene encapsulated Cu (Cu@Gr) micro-/nano-flakes are designed and fabricated, and then used as building blocks for assembling bulk Gr/Cu composites. Thanks to its protective role and strengthening effect of uniformly dispersed graphene in Cu matrix, as compared to bare copper, corrosion rate of the as-fabricated bulk Gr/Cu composites is reduced by 50% in NaCl solution and its yield strength is increased by ∼180% (2.5 vol% Gr) simultaneously, without deterioration on electrical conductivity. The anti-corrosion mechanisms are understood by studying etching behavior of the Cu@Gr flakes in FeCl3 solution, and surface morphology evolution in the samples subjected to salt spray corrosion, and electrochemical corrosion tests. Graphene's anti-corrosion effect is also reflected by an anisotropic corrosion behavior of the Gr/Cu composite because of a “brick-and-mortar” microstructure. The results presented here shed light on expanding metal matrix composite's applications to a wider range and more complex situation by incorporating graphene.

Design of D0 &lt;sub&gt;22&lt;/sub&gt; Superlattice with Superior Strengthening Effect in High Entropy Alloys

Precipitation strengthening is one of the most promising mechanisms to develop high-performance high entropy alloys (HEAs). However, the design of a reinforcing phase with an excellent strengthening effect is still one of the most pivotal challenges. In the present study, a design strategy based on overall valence electron concentration (OVEC) is developed, and a coherent γ″ phase with superior strengthening effect is designed. The newly developed γ″ phase is systematically characterized using transmission electron microscope and atom probe tomography. Differentiating from the traditional Ni3Nb γ″ phase, the present high-entropy γ″ phase contains ~7.7% Co and follows the (Ni,Co,Cr,Fe)3(Nb,Fe) stoichiometry. Three γ″ phase variants are observed with crystallographic orientation relationships of [001]γ″// γ and (001)γ″//{100}γ. The lenticular γ″ particles with small volume fraction (7%) causes a significant yield strength increase (670 MPa) and ductility retention (40%), resulting in excellent yield strength-ductility combination. The excellent strengthening effect of the γ″ phase is attributed to both ordering strengthening and coherency strengthening. The present study proposes a new design strategy of precipitates and develops a superior reinforcing phase for HEAs. These findings not only promote the development of precipitation-strengthened HEAs but deepen the understanding of precipitates design in HEAs as well.

Influence of Ca-Ce/La synergistic alloying on the microstructure and mechanical properties of extruded Mg–Zn alloy

Compared with the sole Ca or Ce/La element, the Ca-Ce/La synergistic microalloying represents an excellent strengthening effect in extruded Mg–Zn alloy, which is mainly attributed to the grain refinement during the extrusion process. Ca-Ce/La addition results in the refinement of the secondary phases and precipitates, promoting the pinning effect for dynamic recrystallized grain growth and dispersion strengthening effect, which also contributes to the increased yield stress. Furthermore, Ca-Ce/La combined addition leads to the more uniform distributions of fine secondary phases, which can improve the ductility and promote the particle stimulated nucleation effect, resulting in a weak basal texture in extruded Mg–Zn alloy.

Effect of coherent L12 nanoprecipitates on the tensile behavior of a fcc-based high-entropy alloy

Effect of coherent L12 nanoprecipitates on the tensile behavior of a novel-designed Al0.2CrFeCoNi2Cu0.2 high-entropy alloy is investigated in this study. Different heat treatments after cold rolling were used to achieve the alloys with two different microstructures. One is composed of a fcc matrix and coherent L12 (Ni,Cu)3Al nanoprecipitates after annealing at 700°C for 20h; the other owns a fcc single-phase structure after annealing at 800°C for 1h as reference. Furthermore, the two alloys have nearly identical average grain sizes (~4.5µm), and thus the exclusive effect of L12 nanoprecipitates could be identified by directly comparing their tensile behaviors. It is shown that the presence of L12 nanoprecipitates results in increases of 259MPa and 316MPa in the yield strength and ultimate tensile strength, respectively, accompanied by the maintenance of a high elongation (30.4%). This excellent strengthening effect is explained by the interaction between dislocations and L12 nanoprecipitates during the tensile straining process. Dislocation shearing of the L12 nanoprecipitates, which is evidently observed and also verified by the yield strength increment calculation, leads to predominant planar dislocation glide and the formation of crystallographically aligned slip bands as the main deformation mechanism, eventually resulting in a considerable work-hardening capacity enhancement with almost none harm to the elongation.

Effects of solute size on solid-solution hardening in vanadium alloys: A first-principles calculation

We performed first-principles calculations to quantify the effect of 26 transition metals on the mechanical properties of vanadium (V) alloys. The generalized stacking-fault (GSF) energies of the solutes were evaluated in three slip systems. Solutes with similar atomic radii to that of V present high GSF energies, which increase the energy barrier for double-kink nucleation and result in excellent strengthening effect. This size effect indicates that noble metals can significantly improve the mechanical strength of V alloys.

Early precipitation of Ni2(Cr,Mo) phase

The early precipitation behavior of long-range-ordered (LRO) Ni2(Cr,Mo) strengthening phase in a Ni–21Cr–17Mo (wt%) superalloy that was subjected to a patent two-step heat treatment (705°C/16h/furnace cooling – 605°C/32h/air cooling) was revealed by means of transmission electron microscopy (TEM) and electron diffraction pattern (EDP) analysis. The coherent interfacial structure between the long-range-ordered Ni2(Cr,Mo) precipitate and the Ni-based matrix leads to the excellent strengthening effect of the alloy. High-spatial-resolution energy-disperse X-ray spectrum (EDX) analysis reveals that the Ni2(Cr,Mo) precipitates were slightly rich in Mo. The minor microstructure change after the key first-step heat treatment was especially focused on. It was found that weak streak fringes were present along 〈311〉Ni-base⁎ directions in the EDP along the [112] zone axis and the ordered nano-regions parallel to {311}Ni-base planes were distributed homogeneously in the matrix. Combined with the detailed microstructural characteristic, it is concluded that Mo atoms might enrich at every two {311}Ni-base planes during the first step of aging at high temperature (705°C), which could increase the transition temperature of Ni2Cr resulting in its formation when aging at the second aging step of 605°C. Meanwhile, Ni2(Cr,Mo) phase precipitates by the substitution of Mo atoms for Cr atoms every three {311}Ni-base planes in Ni2Cr phase with the continuous long-range diffusion of Mo atoms.

In Situ Synthesized Al4C3 Nanorods with Excellent Strengthening Effect in Aluminum Matrix Composites

In Situ Synthesized Al₄C₃ Nanorods with Excellent Strengthening Effect in Aluminum Matrix Composites

Melt Compounding and Mechanical Properties of Polyphenylene Sulfide/Low Melting-temperature Glass Composites

Polyphenylene sulfide( PPS) with the melt temperature about 280 ℃ and low-melting temperature glasses( LMTG) with the soft-temperature( T s) about 325 ℃ were used to prepare PPS/LMTG organicinorganic hybrids by melt compounding above both characteristic temperatures. The surfaces of PPS / LMTG prepared at different processing conditions were observed by scanning electron microscope( SEM).Meanwhile,the mechanical properties for the composites marked as PPS / LMTG300 and PPS / LMTG400prepared below and above T s of LMTG respectively were investigated. The results indicate that liquid-liquid melt compounding is feasible in a wide temperature range from 325 ℃ to 450 ℃ for the selected composites.The SEM images show that dispersed phase in situ morphologies are influenced significantly by processing temperature,shear time and the mixer rotation speed. During liquid-liquid compounding,the dispersed phase,i. e. LMTG drops,goes through a series of process of deformation,break-up,coalescence and recovery in shearing field,similar to the evolution of dispersed polymer phase in immiscible blends with sea-island structure. Compared with PPS / LMTG300,the size of in situ morphology for LMTG phase in PPS / LMTG400obtained by liquid-liquid melt compounding is much smaller,leading to weaker interfacial interaction between the filler and matrix. In addition,LMTG particles still have an excellent strengthening effect on PPS /LMTG400 containing 66 wt% inorganic filler since particle dispersion is improved by liquid-liquid compounding.

The precipitation strengthening behavior of Cu-rich phase in Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel for USC power plant application

Copper has been used as a strengthening element in newly developed Fe–Cr–Ni type austenitic heat resistant steel for inducing Cu-rich phase precipitation to meet high temperature strength requirement for 600°C Ultra Super-Critical (USC) coal fired power plants for many years. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in these advanced austenitic heat resistant steels is still unclear. In order to understand the precipitation strengthening behavior of Cu-rich phase and to promote high strength austenitic heat resistant steel development, 18 Cr9 NiCuNb steel which is a Cu-added Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel has been selected for this study to be aged at 650°C till to 10,000h. Micro-hardness and room temperature tensile test were conducted after long-time aging. SEM, TEM, HRTEM and three dimensional atom probe (3DAP) technology accompanying with thermodynamic calculation have been used to investigate the Cu-rich phase precipitation behavior during 650°C aging. The experimental results showed that Cu atoms can quickly concentrate in clusters at very early precipitation stage to form the fine nano-size Cu-rich “segregation areas” within less than 1h at 650°C. With increasing aging time at 650°C Cu atoms continuously concentrate to Cu-rich segregation areas (clusters) and simultaneously other kinds of atoms such as Fe, Cr and Ni diffuse away from Cu-rich segregation areas to austenitic matrix, and finally to complete the transformation from Cu-rich segregation areas to Cu-rich phase. However, there is only Cu atoms concentration but not crystallographic transformation from early stage of Cu-rich clusters forming to the final Cu-rich phase formation. Even the Cu atom becomes the main composed element after 500h aging at 650°C the Cu-rich phase still keeps coherent relationship with austenitic matrix. According the experimental results in this study, Cu-rich phase precipitation sequence which starts from the Cu atom segregation followed by the Cu diffusing from matrix to segregation areas and Fe, Cr and Ni atoms diffuse out from Cu-rich areas to matrix without crystallographic transformation is proposed. The Cu-rich phase is the most dispersed phase and contributes the most important strengthening effect among all precipitated phases (M23C6, MX and Cu-rich phase). It has been found that Cu-rich phase is very stable and still keeps in nano-size even for 10,000h aging at 650°C. The unique precipitation strengthening of Cu-rich phase in combination with nano-size Nb-rich MX phase and grain-boundary M23C6 carbide contributes excellent strengthening effect to 18 Cr9 NiCuNb austenitic heat resistant steel.

Contribution of Ti addition to characteristics of extruded Cu40Zn brass alloy prepared by powder metallurgy

The aim of this paper was to investigate the properties of Cu40ZnTi for the purpose of developing a new high-strength, lead-free brass by powder metallurgy. The effect of Ti addition on precipitation hardening behavior of Cu40Zn (denoted as BS40) brass was studied with respect to mechanical properties and microstructures. BS40 and Cu40Zn − 1.0 wt.%Ti (denoted as BS40-A) brass powders were prepared by water atomization process, and β phase was retained in the raw powders predominately. The BS40 powder and Ti powder were elementally mixed to prepare Cu40Zn + 0.5 wt.%Ti (denoted as BS40-B) and Cu40Zn + 1.0 wt.%Ti (denoted as BS40-C) premixed powders. The alloy powders and premixed powders were solidified at 1053 K for 600 s by spark plasma sintering (SPS) and extruded subsequently. It was observed that Cu 2ZnTi intermetallic compound (IMC) and CuZnTi metastable phase resulted from the reaction between Ti and CuZn showed distinct grain refinement effect on extruded Cu40Zn brass. Thus, the excellent strengthening effect processes by precipitation hardening and deform working was obtained, which responding to an yield strength of 345 MPa, and a ultimate tensile strength of 597 MPa, showed 65.9% and 30.4% higher than that of extruded Cu40Zn brass, respectively.