Strong Solid Solution Research Articles

Achieving high tensile ductility in a mechanically-stable lightweight refractory high-entropy alloy via dynamic slip band refinement

In this study, the strain hardening behavior and strengthening mechanisms of a mechanically-stable Ti1.6ZrNbAl0.15 lightweight refractory high-entropy alloy (LRHEA) were investigated. The samples were processed by cold-rolling and subsequent annealing to produce fully recrystallized microstructures consisting of a single-phase body-centered cubic (BCC) structure with different mean grain sizes ranging from ∼25 μm to ∼244 μm. The mechanical tests on these samples revealed that grain size refinement had minimal impact on yield strength but effectively improved ductility. Specifically, a low Hall-Petch coefficient of 45 MPa μm1/2, denoting weak grain boundary strengthening, and a high lattice friction stress of 751 MPa, indicating strong solid solution strengthening, were acquired. These findings underscore the significant role played by lattice distortion in contributing to the strength of this LRHEA. Additionally, in the present LRHEA, dislocation structures demonstrated a planar slip dislocation glide mode. No deformation-induced twinning and phase transformation were observed. As the strain increases, the dynamic slip band spacing refinement serves as the primary mechanism for strain hardening, leading to excellent tensile ductility.

Microstructure-mechanical properties relationships and strain hardening mechanism for medium-Mn steel with low yield and ultra-high tensile strength

Considering the problems of forming ultra-high strength steel, a medium-Mn steel with low yield strength was prepared. The steel exhibited a yield strength of 537 MPa, a tensile strength of 1780 MPa, and an elongation of 17.8%. The study aimed to identify the key factors contributing to the low yield-to-tensile ratio and strain hardening behavior of the steel. To achieve this, interrupted tensile tests, loading-unloading-reloading tensile tests, and microstructure characterization were used, along with theoretical calculations. The results indicated that the low dislocation density in lath ferrite, initial martensite, and retained austenite, as well as the high-volume fraction of retained austenite, were the primary factors contributing to the low yield strength. During tensile deformation, strong solid solution strengthening in strain-induced martensite was the main factor driving strain hardening. Additionally, the rapid multiplication of dislocations in ferrite and martensite, along with the heterogeneous deformation-induced hardening caused by the heterogeneous structure composed of strain-induced martensite and other phases, were also significant factors contributing to strain hardening. When considering grain boundary strengthening, dislocation strengthening, solid solution strengthening, and heterogeneous deformation-induced hardening, the theoretical yield strength and flow stress calculated in this study closely matched the actual measured values.

A revisit to the role of Mo in an MP35N superalloy: An experimental and theoretical study

Molybdenum (Mo) has been recognized as an essential alloying element of the MP35N (Co35.4Cr22.9Ni35.5Mo6.2, at.%) superalloy for enhancing strength and corrosion resistance. However, a full understanding of the addition of Mo on microstructure and mechanical properties of the Mo-free parent alloy is lacking. In this work, we consider five (Co37.7Cr24.4Ni37.9)100-xMox (x = 0, 0.7, 2.0, 3.2, and 6.2) alloys, and reveal that yield/tensile strength and ductility are continuously increased for these alloys with increasing Mo content while a single-phase face-centered cubic structure remains unchanged. It is found that strong solid solution strengthening (SSS) is a main domain to the improved yield strength, whereas grain boundaries are found to soften by the Mo addition. The first-principles calculations demonstrate that a severe local lattice distortion contributes to the enhanced SSS, and the grain boundary softening effect is mostly associated with the decreased shear modulus. Both first-principles calculations and scanning transmission electron microscopy observations reveal that the stacking fault energy (SFE) reduces by the Mo addition. The calculated SFE value decreases from 0.4 mJ/m2 to -11.8 mJ/m2 at 0 K as Mo content increases from 0 at.% to 6.2 at.%, and experimentally measured values of SFE at room temperature for both samples are about 18 mJ/m2 and 9 mJ/m2, respectively. The reduction of SFE promoted the generation of stacking faults and deformation twins, which sustain a high strain hardening rate, thus postponing necking instability and enhancing tensile strength and elongation.

Corrosion behaviour of additively manufactured 316L and CoCrNi

Alloys such as 316L and the medium‐entropy alloy CoCrNi are known for their excellent strength and corrosion properties. In the present study, bulk samples of 316L and CoCrNi (without and with 0.11 wt.% N) alloys fabricated using powder bed fusion laser beam (PBF‐LB) were tested in the as‐printed state for their corrosion behaviour in 0.5 M H2SO4 without and with added 3 wt.% NaCl. The tests were done using potentiodynamic measurements and the results were compared with those of the conventionally manufactured 316L. By means of angle‐resolved X‐ray photoelectron spectroscopy (ARXPS), the passive film characteristics were studied in terms of composition and film thickness. The 316L fabricated using PBF‐LB showed favourable passivation and corrosion behaviour as compared with its conventionally manufactured counterpart. It was observed that all the alloys fabricated using the PBF‐LB showed similar corrosion behaviour, but with CoCrNi and CoCrNi‐N showing better passivation behaviour than 316L alloys in the presence of NaCl. The ARXPS showed the presence of both hydroxide and oxides in all the alloys, with outer hydroxide layer and inner oxide layer. The ARXPS of both 316L alloys showed the expected presence of Cr–Fe oxide on the surface of as‐passivated samples, whereas the presence of sulphide was also depicted for the conventionally manufactured 316L, supposed to be detrimental to its corrosion behaviour. The CoCrNi‐based alloys showed the presence of only Cr2O3 layer in their passivated state, with Co and Ni acting as noble elements in the formation of the passive film. Upon micro‐alloying with the strong solid solution strengthener N, CoCrNi did not show any negative effect on either the corrosion behaviour or the passivation behaviour of the alloy.

Effects of ultra-high pressure solution treatment on microstructure and mechanical property of Zn-Li alloy

Due to the moderate degradation rate and acceptable biocompatibility, zinc and its alloys have become the promising candidates of the biomedical implant materials. Nonetheless, the poor mechanical properties limit their development and applications. Thus, to promote their mechanical properties, the Zn-xLi alloys were treated with ultra-high pressure solid solution (UHPS). The phase content of as-cast (AC) and UHPS Zn-xLi was calculated, and the solid solution line of Zn-xLi under 5 GPa was established according to the lever law. Obviously, the solid solution line was shifted to the left and the solid solution zone expanded. The UHPS samples showed higher solid solubility and compressive yield strength than the AC samples. The optimal strength for Zn-0.3Li and Zn-0.5Li were 537.4 MPa and 568.5 MPa, respectively. The ultra-high pressure solid solution treatment greatly improved the solid solvent of the Zn-Li alloys, and caused a strong solid solution reinforcement, which is the main reason for the high strength of UHPS samples.

Laser-cladded Al-Cr-Ti ternary alloy coatings on Ti-4Al-2V alloy: Specific microstructure and enhanced surface performance

In this study, two Al-Cr-Ti ternary alloy coatings (denoted as AlCr and AlCrTi0.5) were fabricated on a Ti-4Al-2V alloy substrate by using a pulsed laser cladding method. Specific microstructure characteristics of those coatings were thoroughly revealed by using various characterization methods, with their hardnesses and wear rates measured by a Vickers indenter and a tribological tester. The results reveal that both AlCr and AlCrTi0.5 coatings are comprised of a body-centered cubic phase (β-Ti phase with average grain sizes of 12.6 ± 9.1 μm and 14.6 ± 10.0 μm, respectively), which is related to non-equilibrium solidification and sluggish diffusion of alloying elements during the laser processing. Heat-affected zones of both the laser-cladded specimens are mainly composed of submicron martensitic laths (α′-Ti). Average microhardness values of the AlCr and the AlCrTi0.5 coatings are measured to be 526 ± 37 HV and 510 ± 24 HV, respectively, approximately two times higher than the substrate (270 ± 8 HV). Their wear rates are almost the same, ~2.2 × 10−4 mm3 N−1 m−1, only ~1/3 of that of the substrate. Based on dedicated microstructure analyses, the greatly enhanced performance of the laser-cladded coatings could be attributed to joint strengthening from strong solid solution and lattice distortion, as well as Al/Cr-induced phase structure differences from the substrate.

Effect of Silicon Content on Microstructures and Properties of Directionally Solidified Fe-B Alloy.

In order to investigate the effect of Si content on the microstructures and properties of directionally solidified (DS) Fe-B alloy, a scanning electron microscope (SEM) with an energy dispersive spectrum (EDS), and X-ray diffraction have been employed to investigate the as-cast microstructures of DS Fe-B alloy. The results show that Si can strongly refine the columnar microstructures of the DS Fe-B alloy, and the columnar grain thickness of the oriented Fe2B is reduced with the increase of Si addition. In addition, Si is mainly distributed in the ferrite matrix, almost does not dissolve in boride, and seems to segregate in the center of the columnar ferrite to cause a strong solid solution strengthening and refinement effect on the matrix, thus raising the microhardness of the matrix and bulk hardness of the DS Fe-B alloy.

Influence of Nb, Ta and Zr on the Interdiffusion Coefficients and Solid Solution Strengthening of γ-TiAl Single Phase Alloys

The alloying elements Nb, Ta and Zr improve the creep properties of fully lamellar γ/α2 titanium aluminides significantly. Since high temperature deformation mainly occurs in the γ-phase of γ/α2 titanium aluminides, the diffusivity and the solid solution hardening effect of these three elements in the γ-phase is studied by analyzing the concentration gradients of the alloying elements and the resulting hardness across the interdiffusion zone of diffusion couples by energy dispersive X-ray diffraction and nanoindentation. The results reveal that Zr has the highest interdiffusion coefficient but also the largest solid solution hardening coefficient. The mechanical properties of single γ-phase Ti-54Al-5X alloys were investigated by strain rate jump tests. The addition of 5 at.% Nb or Ta lead to an increased strength compared to a binary γ-Ti-54Al alloy. The Zr-containing γ-TiAl alloy reveals the highest strength at 750 °C and 900 °C, which is discussed to be due to the strong solid solution hardening effect of Zr. However, in comparison to the other alloys, Ti-54Al-5Zr shows quite brittle behavior up to 900 °C. The lower diffusivity of Ta compared to Nb leads to a higher strength of the Ta-modified alloy at 900 °C.

A strategy to design eutectic high-entropy alloys based on binary eutectics

• we employed a useful strategy to design EHEAs and quickly locating eutectic compositions in multicomponent HEAs based on binary eutectic compositions. • Nb 0.62 Fe 1.98 Co 1.22 Ni 1.00 exhibits a fully eutectic microstructure confirmed by both SEM and DSC results • Our findings provide a new strategy for a starting point for quickly pinpointing EHEAs. Although eutectic high entropy alloys (EHEAs) display homogeneously fine microstructure, outstanding castability and promising industrial application potential, how to search for the eutectic compositions in multicomponent high-entropy alloys (HEAs) remains to be challenging. Herein, the strategy which was proposed for pinpointing good glass formers in the field of bulk metallic glasses was utilized for quickly locating EHEA compositions in Nb-(Fe, Co, Ni), Zr-(Fe, Co, Ni) and Hf-(Fe, Co, Ni) HEA systems based on the constituent binary eutectics. As a demonstration, the eutectic compositions in the Nb-(Fe, Co, Ni), Zr-(Fe, Co, Ni) and Hf-(Fe, Co, Ni) HEA systems were quickly determined using this new approach. The newly designed EHEAs showed ultrafine eutectic microstructures, good castability and decent compressive mechanical properties. Current findings clearly demonstrated that this proposed approach is effective for searching for eutectic compositions in HEAs, which will expedite alloy design of EHEAs with strong solid solution strengthening effects.

Solid Solution Strengthening of Mo, Re, Ta and W in Ni during High-Temperature Creep

Solid solution strengthening of the unordered γ matrix phase by alloying elements is of great importance during creep of Ni-based superalloys, particularly at high temperatures above 1000 °C. To study the role of different potent solutes, we have conducted creep experiments on binary Ni-2X alloys (X = Mo, Re, Ta, W) at 1000 °C, 1050 °C, and 1100 °C at a constant stress of 20 MPa. Compared to mechanical tests below 800 °C, where the size of the elements mostly determines the solid solution hardening contribution, the strengthening contribution of the different alloying elements above 1000 °C directly correlates with their diffusivity. Therefore, elements such as Ta that lead to strong solid solution hardening at low temperatures become less effective at higher temperatures and are exceeded by slower diffusing elements, such as Re.

Design of refractory compositionally complex alloys with optimal mechanical properties

We use an analytical model to propose candidate compositionally complex alloys of the Mo-Nb-Ta-W family with optimal yield stress. We then introduce a computationally tractable method based on first-principles calculations to model phase equilibria in complex alloys at arbitrary concentrations. We utilize this method to predict the phase diagram at the optimized compositions and observe a tendency towards ordering for some of the proposed alloys. By combining yield stress data and thermodynamic equilibria, we suggest two alloy compositions with optimal mechanical properties and a strong solid solution forming ability for further experimental validation.

High strength NiMnFeCrAlCu multi-principal-element alloys with marine application perspective

Nickel-aluminium bronze (NAB) alloys are traditional marine materials used for decades, but face to the challenge of insufficient strength and corrosion properties. In this work, thanks to the strong solid solution strengthening, corrosion and biofouling resistant ability of multiple components, we developed novel NiMnFeCrAlCu multi-principal-element alloys (MPEAs), which exhibit excellent combination of mechanical and corrosion resistant properties. It was found that NiMnFe0.5Cr0.5Al0.2Cu1.2 MPEA has higher yield and ultimate tensile strength, and lower corrosion current density by one magnitude than those of NBA, and also shows a considerable copper ion release concentration. These merits are attributed to the dual phase of FCC+BCC, and the enrichment of Cu in the FCC matrix and Cr in the BCC grains. This work has implication in the new strategy for the development of advanced marine materials.

Martensitic Transformation of High-Entropy and Medium-Entropy Shape Memory Alloys

As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti45Zr5Pd25Pt20Au5, and Ti45Zr5Pd25Pt20Co5 had the martensitic transformation. The perfect recovery was obtained in Ti45Zr5Pd25Pt20Co5 during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti45Zr5Pd25Pt20Au5 during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti45Zr5Pd25Pt20Co5 is the one possible HT-SMA working between 342 and 450 °C.

Microstructures and Mechanical Properties of Al-Zn-Sn Bearing Alloys for High Performance Applications

The mechanical behavior and indentation creep of Al-20 wt% Zn bearing alloy has been modified with adding 0.2 wt%, 0.5 wt%, 1 wt%, 1.5 wt% Sn. These bearing alloys were prepared by melt spinning technique. The scanning electron microscopy (SEM) was used to study the morphology of the melt spun alloys and x-ray diffractometer (XRD) for the identification of the phases pre- sent in these melt-spun bearing alloys. The results show that the structure of Al80-x-Zn20-SnX (X = 0.2%, 0.5%, 1% and 1.5%) bearing alloys is characterized by the presence of α-Al of FCC structure and SnZn intermetallic compound of anorthic structure. The Al-20Zn-1.5Sn has a smaller crystallite size and grain size as indicated from X-ray and SEM analysis respectively, which leading to the enhancement of the mechanical properties. The mechanical properties and indentation creep of these bearing alloys were studied by tensile test machine and vickers indentation testing at room temperature, respectively. The Al-20Zn-1.5Sn has higher hardness value and creep resistance than other alloys. This was attributed to the strengthen effect of Sn as a strong solid solution element in Al-matrix. The stress exponent values in the range 2.4 - 4.2 indicate that the grain boundary sliding is the possible mechanism during room temperature creep deformation of melt-spun Al-Zn-Sn bearing alloys.

Origin of strong solid solution strengthening in the CrCoNi-W medium entropy alloy

Solid solution strengthening is one of the most conventional strategies for optimizing alloys strength, while the corresponding mechanisms can be more complicated than we traditionally thought specifically as heterogeneity of microstructure is involved. In this work, by comparing the change of chemical distribution, dislocation behaviors and mechanical properties after doping equivalent amount of tungsten (W) atoms in CrCoNi alloy and pure Ni, respectively, it is found that the alloying element W in CrCoNi alloy resulted in much stronger strengthening effect due to the significant increase of heterogeneity in chemical distribution after doping trace amount of W. The large atomic scale concentration fluctuation of all elements in CrCoNi-3W causes dislocation motion via strong nanoscale segment detrapping and severe dislocation pile up which is not the case in Ni-3W. The results revealed the high sensitivity of elements distribution in multi-principle element alloys to composition and the significant consequent influence in tuning the mechanical properties, giving insight for complex alloy design.

Effect of Oxygen Variation on High Cycle Fatigue Behavior of Ti-6Al-4V Titanium Alloy.

The element oxygen is expected to be a low-cost, strengthening element of titanium alloys due to its strong solid solution strengthening effect. High cycle fatigue behaviors of Ti-6Al-4V alloys with different oxygen contents (0.17%, 0.20%, 0.23% wt.%) were investigated in this paper. The results illustrated that Ti-6Al-4V-0.20O alloy possesses the highest fatigue strength and the lowest fatigue crack propagation rate. The fatigue fracture morphology verified that the fatigue cracks propagated transgranularly in both Ti-6Al-4V-0.17O and Ti-6Al-4V-0.20O alloys, and the fatigue cracks tended to extend intergranularly in the Ti-6Al-4V-0.23O alloy. The maximum nano-hardness varied from the <0001> direction to the and directions with the increasing oxygen content, which suggested that the dominant slip system varied from prismatic slip to pyramidal slip. The number of the type dislocations increased with the oxygen content, which indicated that the number of the first-order pyramidal and the second-order pyramidal slip systems increased. The oxygen can significantly change the fatigue fracture mechanism of Ti-6Al-4V alloy: From transgranular fracture to intergranular fracture. These results are expected to provide valuable reference for the optimization of the composition and mechanical properties of titanium alloys.

Differential effects of Zn and Co solutes on the properties of Cu–Ni–Sn alloys

In order to solve the segregation and discontinuous precipitation of Cu–Ni–Sn alloys and explore the influence mechanism of solid soluble microelements on the microstructure and properties. Herein, based on the cluster-plus-glue-atom model, Zn or Co, with different solubilities in Cu, was selected as the fourth element in the way of replacing a small portion of Sn for the composition design of Cu–Ni–Sn–Zn (Co) series alloys. The solid solution forms and positions of Zn and Co solutes were revealed according to EPMA composition analysis and variation of the lattice parameter of the Cu matrix in solution treated alloys, and form atomic site occupation point of view, the influence of Zn or Co on solidus-liquidus temperature range were discussed. Furthermore, phase transformation and resistivity thermal stability with temperature rising were investigated and discussed utilizing results of variable temperature resistance. These results show that Zn solutes dissolve in the form of clusters, resulting in a strong solid solution strengthening, existing less effect on conductivity, and increasing comprehensive properties after long-term aging, which indicates that Zn can be a good candidate for the multi-component design of Cu–Ni–Sn alloys. These findings may offer new insights on composition optimization of Cu–Ni–Sn alloy.

Unraveling the dislocation–precipitate interactions in high-entropy alloys

The precipitates play a significant role in not only enhancing the strength, but also maintaining the high toughness in alloys. However, the interactions of the nanoscale precipitates with dislocations in the high entropy alloys (HEAs) are difficult to observe directly by in-situ TEM experiments due to the limits of the resolution and time. Here, using atomic simulations we report the synergistic strengthening of the coherent precipitate and atomic-scale lattice distortion in the HEAs at cryogenic/elevated temperatures. The effects of temperature, chemical disorder, precipitate spacing, precipitate size, elemental segregation, and dislocation-cutting number on the critical stress for the dislocation to overcome a row of precipitates are studied. A random stacking fault energy landscape along the slip plane, the lattice distortion at different temperatures, and the interface/surface energy at various crystallographic orientations are obtained. Compared with the traditional metals and alloys, HEAs have the severe atomic-scale lattice distortions to generate the local high tensile/compressive stress fields. This complex stress causes the dislocation line to bend, and thus improves the dislocation slip resistance, resulting in the strong solid-solution strengthening. The stacking fault strengthening induced by the obvious difference of the stacking fault energies between the HEA matrix and precipitate (within the inner of the HEA matrix), and the formation of the antiphase domain boundary contribute to the high strength. The precipitate embedded by the solute atoms produces the strong lattice distortion to enhance the dislocation slip resistance at high temperatures. Hence, the current results provide the mechanistic insight into the phenomenon that the coherent precipitate combined with the severe atomic-scale lattice distortion can enhance the strength at cryogenic/elevated temperatures to further broaden the scope of applications of advanced HEAs.

Effects of Ni and Cr addition on the wear performance of NiTi alloy

Development of hard materials for structural and bearing applications is a challenging task due to complexities in materials processing. Nickel-based intermetallics are a choice for the wear resistance applications. In this study, in order to examine the effects of Ni and Cr addition on the wear performance of Ni-rich alloys, 60Ni, 55Ni-5Cr, 50Ni-5Cr, and 50Ni-10Cr (at.%) alloys were developed through arc melting technique. The aim was to conduct a microstructural and phase analysis study and their effects on hardness and wear behavior. Samples were solution treated at 900 °C for 5 h under vacuum followed by equilibrium cooling. Microstructural and phase analysis revealed the presence of B2 cubic NiTi austenite along with Ni-rich hexagonal Ni3Ti and Cr-rich tetragonal Cr3Ni2 phases throughout the microstructure of alloys. The Cr developed phase was not present in binary composition because of Cr absence. Ternary compositions possessed higher hardness and wear resistance than the binary alloy. Hardness and wear resistance of 50Ni-5Cr and 50Ni-10Cr were comparable with tool steels and conventional 440C martensitic stainless steel bearing material; this high hardness and wear resistance was attributed to the strong solid solution strengthening and precipitation hardening effects of Cr along with the combined effect of Ni-rich Ni3Ti and NiTi austenite phases. Improvement in hardness and wear resistance were correlated with the Cr addition in the developed alloys.

Correlation between crystal structure and mechanical performance of Cr-implanted 300M high-strength steel using X-ray diffraction method

In order to study the influence of crystal structure change due to implantation dose on the hardness and wear performance of 300M high-strength steel, samples were surface modified by Cr implantation with dosages of 5.0 × 1016, 1.5 × 1017 and 3.0 × 1017 ions/cm2. X-ray diffraction method, which was already applied in studies on the microstructure of deformed and heat-treated materials, was used to study the crystal structure of the implanted steel, and the results were corrected with the hardness and wear performance. The solid solution strengthening effect and microstructure vary with increase in implantation dose. Owing to strong solid solution hardening of Cr, small average crystallite size and high dislocation density, the hardness and wear resistance of implanted steel with dose of 5.0 × 1016 ions/cm2 were found to be the highest compared with other samples. Moreover, although the crystallite size of the implanted sample with dose of 3 × 1017 ions/cm2 was similar to that of substrate and the dislocation density was lower than that of the substrate, its higher hardness and lower specific wear rate were due to the solid solution hardening and perhaps Cr clusters reinforcement.