Room Temperature Tensile Ductility Research Articles

Computationally guided alloy design and microstructure-property relationships for non-equiatomic Ti–Zr–Nb–Ta–V–Cr alloys with tensile ductility made by laser powder bed fusion

Single-phase body-centered cubic refractory complex concentrated alloys (RCCAs), exhibit strength retention at temperatures above the melting points of conventional Ni-based superalloys. However, their lack of room temperature tensile ductility leads to cracks during processing and/or premature failure under mechanical loading when fabricated by laser-based metals additive manufacturing (AM). We present an alloy design framework for tensile ductility in non-equimolar RCCAs amenable to AM processing within the Ti–V–Cr–Nb–Zr–Ta system. First, density functional theory (DFT) and machine learning informed screening of alloy compositions were used to down-select ∼106 alloys with potential for tensile ductility. Next, Scheil solidification modeling was used to eliminate compositions most at risk for micro-segregation and solidification cracking based on the estimated freezing range of each alloy. This led to the design of two new, representative alloys: Ti0.4Zr0.4Nb0.1Ta0.1 and Ti0.486V0.375Ta0.028Cr0.111. These alloys were evaluated for rapid solidification defect formation using in-situ synchrotron melt pool imaging, fabricated via laser powder bed fusion (L-PBF) AM, and then characterized for processing defects, microstructure, and mechanical properties. Overall, both alloys achieved tensile yield strengths >800 MPa and failure strains >5 %. However, the occurrence of un-melted powders of high melting point elements likely resulted in premature failure during tensile straining. Considerations about mitigating this defect source are discussed and the role of local micro-segregation on ductility are elucidated. Overall, these results highlight the success of our framework, and show potential for laser-based AM-RCCAs with tensile ductility.

A novel NbTaV0.5Hf0.1 complex concentrated alloy with superior room-temperature tensile plasticity and elevated-temperature strength

Refractory complex concentrated alloys (RCCAs) encounter difficulties in balancing room-temperature ductility and high-temperature strength, which restricts their applications. Here, a novel NbTaV0.5Hf0.1 RCCA was prepared through vacuum arc melting for the first time. The as-cast alloy exhibited a stable body-centered cubic phase structure with a small amount of micro-scale dispersed Hf-rich oxides. As a result, the designed alloy exhibits a room-temperature tensile fracture strain of 14.9 %, and superior compressive yield strength values under elevated temperatures (472 MPa under 1200 ℃, 358 MPa under 1400 ℃, respectively). A synergy between room-temperature tensile ductility and exceptional high-temperature strength was achieved in RCCAs.

Substantially improved room-temperature tensile ductility in lightweight refractory Ti-V-Zr-Nb medium entropy alloys by tuning Ti and V content

Lightweight high/medium-entropy alloys (H/MEAs) possess attractive properties such as high strength-to-weight ratios, however, their limited room-temperature tensile ductility hinders their widespread engineering implementation, for instance in aerospace structural components. This work achieved a transformative improvement of room-temperature tensile ductility in Ti-V-Zr-Nb MEAs with densities of 5.4–6.5 g/cm3, via ingenious composition modulation. Through the systematic co-adjustment of Ti and V contents, an intrinsic ductility mechanism was unveiled, manifested by a transition from predominant intergranular brittle fracture to pervasive ductile dimpled rupture. Notably, the modulated deformation mechanisms evolved from solitary slip toward collaborative multiple slip modes, without significantly compromising strength. Compared to equimolar Ti-V-Zr-Nb, a (Ti1.5V)3ZrNb composition demonstrated an impressive 360% improvement in elongation while sustaining a high yield strength of around 800 MPa. Increasing Ti and V not only purified the grain boundaries by reducing detrimental phases, but also tailored the deformation dislocation configurations. These insights expanded the applicability of lightweight HEAs to areas demanding combined high strength and ductility.

Effect of Nitrogen Impurity on the Tensile Ductility and Impact Toughness of Al-Bearing DQP Steels

The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1.1Al (High-Al) with different N contents of 10 and 30 ppm were selected for this study. In addition, two other DQP steels with compositions: Fe-0.3C-1.0Si (High-Si) and Fe-0.3C-0.5Si-0.5Al (Al-Si), both containing about 30 ppm nitrogen, were also included in the study to compare the properties. Detailed metallographic studies using FESEM-EDS, TEM, EPMA and XRD combined with tensile testing and fractographic analysis indicated that already 30 ppm of nitrogen could impair tensile ductility of TMR-DQP processed High-Al steel in comparison to that with 10 ppm nitrogen. Similarly, the effect was adverse also in Al-Si steel (30 ppm N) despite its reduced Al content (0.5 wt.%), but High-Si steel (Al < 0.002 wt.%, N 30 ppm) did not show any such detrimental effect on tensile ductility. Extensive material characterization verified that even 30 ppm of nitrogen could impair ductility of Al-bearing steels, essentially due to the presence of AlN inclusions, despite that TMR-DQP processing enabled stabilization of 6–10% retained austenite (RA) in the steels. The capacity of RA in promoting improved ductility and strain hardening capacity was impaired by the presence of these inclusions. In contrast, impact toughness transition temperature T28J was not clearly affected with Al-Si when compared to low-N High-Al steel, although excessive splitting in Al-Si caused pronounced scatter in the results and increase in upper shelf impact toughness.

Nanoscale structural heterogeneities in metastable refractory high-entropy alloy

Most refractory high-entropy alloys (RHEAs) with the dominant body-centered-cubic (bcc) phase are lack of room temperature tensile ductility, which strictly limits their large-scale applications in industry and aerospace. Recently, it has been found that the strategy of metastability engineering can be utilized to ductilize the TiZrHf-based RHEAs. However, up to now, a nanoscale understanding of the potential structural heterogeneities in this system remains exclusive. Herein, through performing the transmission electronic microscope (TEM) characterization on a prototype TaZrHfTa0.5 RHEA with exceptional tensile properties, we have found that this alloy contains a high density of nanoscale structural heterogeneities that are randomly distributed throughout the bcc matrix, including the hexagonal ω phase forming by the collapse of {111}bcc atomic pairs and the orthorhombic α″ phase forming by coupled shuffle-shear mechanism. It may provide a microstructural basis for manipulating the macroscopic properties of metastable RHEAs.

Thermoplastic Toughening of a Semiaromatic Polyamide with an Amino-telechelic Polyethylene.

Semiaromatic polyamides are used for metal replacement in advanced engineering applications to reduce weight and improve efficiency, but their range of application is limited by their inherent lack of ductility and toughness. Here, we combined semiaromatic polyamide poly(hexamethylene terephthalamide-co-isophthalamide) (PA6TI) with up to 30 wt % amine-terminated polyethylene (PE(NH2)2) by high-temperature melt compounding, which was suggested to lead to the formation of PA-PE block copolymers at the interface between the PE(NH2)2 and the PA6TI. This resulted in PA6TI/PE(NH2)2 blends with smaller, more uniform particle sizes than in PA6TI blended with nonfunctional PE or the commercial impact modifier, maleic anhydride-functionalized styrene-ethylene-butylene-styrene (SEBS) under the same conditions. The PA6TI/PE(NH2)2 blends and the corresponding glass fiber-reinforced composites consequently showed significantly greater increases in room-temperature tensile ductility and fracture energy with respect to unmodified PA6TI, as well as maintained mechanical stability at high temperatures, and only modest decreases in stiffness and strength, even at high PE(NH2)2 contents. These improvements were attributed to the crystallinity of the PE(NH2)2 particles and to improved morphological stabilization and matrix-particle adhesion, consistent with the presence of PA-PE block copolymer at the matrix-particle interfaces.

A novel and sustainable method to develop non-equiatomic CoCrFeNiMox high entropy alloys via spark plasma sintering using commercial commodity powders and evaluation of its mechanical behaviour

A novel approach to developing high entropy alloys (HEAs) using spark plasma sintering (SPS) was explored in this work where a mix of commercial commodity powders like Ni625, CoCrF75, and 316L was used instead of pre-alloyed powders avoiding the expensive pre-alloying steps like mechanical alloying or gas atomizing. Three non-equiatomic HEAs, based on Co, Cr, Fe, Ni, and Mo were designed and developed by blending the powders which were sintered via SPS and resulted in a single FCC phase after homogenization. The HEAs were microstructurally and mechanically characterized with tensile and hot compression tests up to a temperature of 750 °C showing excellent properties. The maximum room temperature tensile strength and ductility demonstrated was 712 MPa and 62% respectively, by the alloy Co23.28Cr28.57Fe25.03Ni21.01Mo2.1. Moreover, the same alloy exhibited a compression strength greater than 640 MPa with a ductility above 45% at a temperature of 750 °C. Also, this study paves the way for a novel fabrication route that offers more flexibility to develop new HEAs cost-effectively and efficiently which is crucial for the discovery of new materials over high-throughput techniques. Using such commodity alloys also opens the possibility of developing ingot casting from recycled scraps avoiding the direct use of critical metals.

Tuning the grain and domain sizes to achieve superior room-temperature tensile ductility in a B2-ordered refractory Al15Nb40Ti40V5 medium-entropy alloy

Herein, we showed that the grain size, rather than the domain size, controlled room-temperature properties of a B2-ordered refractory Al15Nb40Ti40V5 medium-entropy alloy. Grain refinement to 20 μm at any size of B2 domains resulted in elongation of ∼50%, which was superior to all the reported refractory high/medium-entropy alloys.

Temperature-dependent plastic deformation of a refractory Al7.5(NbTiZr)92.5 medium-entropy alloy with a bcc+B2 structure

Non-refractory elements, like Al, can enhance both room-temperature tensile strength and ductility of refractory high/medium alloys (RH/MEAs) due to the formation of ordered nanodomains changing the dislocation glide character. To elucidate their influence at higher temperatures, we reported here the tensile behaviour and microstructure response of a Al7.5(NbTiZr)92.5 (at.%) RMEA having a bcc (body-centred cubic) matrix with embedded B2 (ordered bcc) nanodomains at 298–1073 K, and compared the results obtained with those of a bcc NbTiZr alloy. Detailed analysis showed that profuse B2 nanodomains confined the dislocation motion in localised bands at T < 873 K, intensified the Portevin-Le Chatelier effect at 473 and 673 K, and enhanced the strength by ∼ 20–30 % at T ≤ 873 K, while losing the strengthening ability at 1073 K. These results manifest that ordered nanodomains originating from the non-refractory alloying can improve the medium-to-high temperature mechanical performance of RH/MEAs.

Twin-Related Grain Boundary Engineering and Its Influence on Mechanical Properties of Face-Centered Cubic Metals: A Review

On the basis of reiterating the concept of grain boundary engineering (GBE), the recent progress in the theoretical models and mechanisms of twin-related GBE optimization and its effect on the mechanical properties is systematically summarized in this review. First, several important GBE-quantifying parameters are introduced, e.g., the fraction of special grain boundaries (GBs), the distribution of triple-junctions, and the ratio of twin-related domain size to grain size. Subsequently, some theoretical models for the GBE optimization in face-centered cubic (FCC) metals are sketched, with a focus on the model of “twin cluster growth” by summarizing the in-situ and quasi-in-situ observations on the evolution of grain boundary character distribution during the thermal-mechanical process. Finally, some case studies are presented on the applications of twin-related GBE in improving the various mechanical properties of FCC metals, involving room-temperature tensile ductility, high-temperature strength-ductility match, creep resistance, and fatigue properties. It has been well recognized that the mechanical properties of FCC materials could be obviously improved by a GBE treatment, especially at high temperatures or under high cyclic loads; under these circumstances, the materials are prone to intergranular cracking. In short, GBE has tremendous potential for improving the mechanical properties of FCC metallic materials, and it is a feasible method for designing high-performance metallic materials.

Challenges of Developing Ductile and Lighter Refractory High Entropy Alloys

In the recent past refractory high-entropy alloys (RHEAs) have attracted a lot of attention due to their interesting mechanical properties. These are simple solid solution alloys either with face-centered cubic (FCC) or body-centered cubic (BCC) crystal structures. BCC alloys exhibit relatively higher strengths at high temperatures and therefore, can be considered as structural materials for ultrahigh temperature applications (≥ 1300 ºC). However, the main challenge in developing BCC structured RHEAs for structural applications is balancing their high temperature (HT) strength with their room temperature (RT) tensile ductility and density. Therefore, searching for new and lighter RHEAs with optimized strength and tensile ductility is still a challenge. Improvement of oxidation resistance RHEAs is also an important issue as refractory metals are prone to oxidation in air at high temperatures. This paper describes various theoretical and experimental research works to develop stronger, ductile and lighter RHEAs alloys with novel compositions. The paper also describes various attempts in the development of RHEAs with high creep strength, RT ductility, fracture toughness, lower density, thermo-mechanical fatigue resistance, and host of other physical properties. It will also describe the advance manufacturing techniques in fabricating the components out of these compositions.

Microstructure, mechanical and tribological performances of a directionally solidified γ-TiAl alloy

In the present study, Ti-47Al-2Nb-2Cr-2Mn directionally solidified by electromagnetic cold crucible was mainly composed of alternatively arranged (α 2 + γ) lamellae, and exhibited a room-temperature tensile ductility of 1.55%, and an ultimate strength of 587 MPa. At 800 °C ultimate strength remained high at 512 MPa, with 4.33% ductility. The crack propagated around B2 phase at room temperature, but cut though B2 phase at elevated temperature. Both dislocation slip and twinning governed the room-temperature plastic deformation, and the twin boundary acted as the obstacle to dislocation slip. The friction coefficient of the N-orientation (70°-90°) TiAl sample was the highest, followed by the I-orientation (35°-50°) and P-orientation (0°-15°) samples, while their wear rates presented an opposite trend. The P-orientation sample was more liable to form the tribo-oxide layer. Moreover, the local plastic deformation during dry sliding resulted in pile-ups along the edge of wear tracks. • Cracks bypass (room temperature) and cut through (elevated temperature) B2 phase. • Dislocation- and twinning-controlled deformation mechanisms at room temperature. • Lowest friction coefficient and highest wear rate for P-orientation sample. • Tribo-oxide layer preferentially formed in P-orientation sample.

Design and development of Ti–Zr–Hf–Nb–Ta–Mo high-entropy alloys for metallic biomaterials

Applying empirical alloy parameters (including Mo equivalent), the predicted ground state diagram, and thermodynamic calculations, noble nonequiatomic Ti–Zr–Hf–Nb–Ta–Mo high-entropy alloys for metallic biomaterials (BioHEAs) were designed and newly developed. It is found that the Moeq and valence electron concentration (VEC) parameters are useful for alloy design involving BCC structure formation in bio medium-entropy alloys and BioHEAs. Finally, we find a Ti28.33Zr28.33Hf28.33Nb6.74Ta6.74Mo1.55 (at.%) BioHEA that exhibits biocompatibility comparable to that of CP–Ti, higher mechanical strength than CP–Ti, and an appreciable room-temperature tensile ductility. The current findings pave the way for new Ti–Zr–Hf–Nb–Ta–Mo BioHEAs development and are applicable for another BioHEA alloys system.

Thermomechanical Processing of a Near-α Ti Matrix Composite Reinforced by TiBw.

To further improve the mechanical properties of the as-cast 7.5 vol.% TiBw/Ti–6Al–2.5Sn–4Zr–0.7Mo–0.3Si composite, multi-directional forging (MDF) and subsequent heat treatments were carried out to adjust TiB whiskers (TiBw) and matrix characteristics. The effect of various microstructures on the tensile properties and fracture toughness of the composites was analyzed in this paper. After MDF, the TiBw are broken into short rods with a low aspect ratio and display a random distribution. Moreover, distinct microstructures were obtained after thermomechanical processing and different heat treatments. Both room-temperature and high-temperature tensile strength and ductility are improved after thermomechanical processing. By increasing the solution-treatment temperature, the microstructures transform from equiaxed to fully lamellar. A simultaneous improvement of the room-temperature and high-temperature properties is associated with the microstructural changes. In addition, the fracture toughness exhibits an increasing trend as the volume fraction of equiaxial α phases decreases. The lamellar microstructure demonstrates excellent fracture toughness due to deflection of the crack propagation path.

Extraordinary room temperature tensile ductility of laminated Ti/Al composite: Roles of anisotropy and strain rate sensitivity

Al/Ti/Al laminated metal composites (LMCs), with layer thicknesses ≥800 μm, were fabricated via hot rolling bonding and annealing. The fabricated Al/Ti/Al LMCs displayed improved tensile ductility compared with both the single Ti and single Al layers. Combinational analyses based on digital image correlation and finite element modeling revealed that the width of the interface affected zones (IAZ) could exceed 150 μm, which is more than one order of magnitude larger than previously reported values. The mismatch of plastic anisotropy between the Ti layer and the Al layer may play a crucial role in realizing such a wide IAZ. Slip trace analysis indicated that more pyramidal and basal slips were activated at the Ti/Al interface than in the central part of the Ti layer, which may be attributed to the complex stress/strain state in the IAZ. Although a wide IAZ was observed, the strain hardening capability of the Al/Ti/Al LMCs did not obviously improve. In contrast, the strain rate sensitivity improved, which is assumed to play a key role in the improved ductility of the Al/Ti/Al LMCs. This study can provide a direction for the design of heterostructured materials with high strength and high ductility.

Strain-Rate Sensitivity, Tension-Compression Asymmetry, r-Ratio, Twinning, and Texture Evolution of a Rolled Magnesium Alloy Mg-1.3Zn-0.4Ca-0.4Mn

In this work, the deformation response, texture evolution, and twinning development of a magnesium (Mg) alloy, Mg-1.3Zn-0.4Ca-0.4Mn, for biocompatible applications are investigated. Further, the alloy’s formability, by examining the instantaneous r-ratio and strain-rate sensitivity (SRS) as a function of strain and loading direction, is investigated. It is found that after rolling and peak aging, the alloy has a rolled texture of moderate intensity with the basal planes contained in the rolling plane and with a bimodal, fine-grained microstructure. The alloy shows both high room-temperature tensile strength (300 MPa) and ductility (25 pct) in the rolling direction (RD) and, remarkably, r-ratios saturating close to unity in all three in-plane testing directions. It is also found that the SRS is relatively high and uniform, with averages ranging from 0.015 to 0.025, depending on the in-plane testing directions. These are outstanding properties compared to pure Mg and most of its biocompatible alloys. Typical of Mg alloys, this alloy has a propensity for twinning by multiple twin modes, which leads to rapid texture evolution, anisotropy, and tension-compression (T-C) asymmetry in yield stress, with compression having the weaker response. These mechanical characteristics along with their microstructural origins are presented and discussed in this article.

On the comparative degradation of selected Cr2O3-forming alloys by high-temperature carburization with a case study involving 310 stainless steel

A shroud plate of a heat exchanger made of 310 stainless steel plate about 5 mm in thickness has prematurely failed in a methanol production plant. The function of the heat exchanger is to reduce the temperature of steam reformed gaseous hydrocarbons from 850 °C to 325 °C. It is shown that the plate has been severely damaged as indicated by considerable metal wastage combined with massive precipitation of carbide phases, coke deposition at the inner surface as well as metal dusting associated with pitting. Based upon these observation failure of the plate is correlated with exposure to carbonaceous gases and the relatively low carburization resistance of the Cr2O3-forming 310 stainless steel. Experiment shows that other competitive Cr2O3-forming alloys particularly alloy 800H and alloy 556 grades are more suited for such applications. The carburization resistance of the two alloys in comparison with that of 310 stainless steel is explained in terms of alloy composition particularly the (Ni + Co)/(Cr + Fe) ratio. A correlation is found to exist among this ratio and the activation energy of the carburization reaction, the amount of carbon absorbed by the alloy during exposure to carburizing environment as well as the retained room-temperature tensile ductility after the respective exposure. On the other hand, the results indicate that alloys most resistant to oxidation are also most resistant to carburization. It is concluded that either Incoloy 800H or Haynes alloy 556 would have been a better selection for the shroud plate application.

Influence of manganese on the microstructure and mechanical properties of oxide-dispersion-strengthened steels

A novel high-Mn oxide-dispersion-strengthened (ODS) steel, with high cryogenic temperature impact energy, was developed. The effect of Mn content on the microstructure and mechanical properties of ODS steels were examined. The addition of 25 wt.% Mn to ODS steel resulted in transformation from a full ferrite phase to austenite and εhcp-martensite phases. ODS steel without added Mn contained a high number density of fine Y-Ti-O oxide particles. In contrast, ODS steel with 25 wt.% Mn contained TiMn2O4 and relatively coarser Y2Ti2O7 oxide particles with lower number densities. The TiMn2O4 particles showed a crystallographic orientational relationship with the matrix phase. In addition, a high density of deformation twins were formed in tensile-deformed ODS steel with 25 wt.% Mn, but were absent in tensile-deformed ODS steel without added Mn. The ODS steel containing 25 wt.% Mn exhibited higher room-temperature tensile ductility and higher Charpy impact energy at cryogenic temperature without Mn due to the existence of deformation twins, austenite phase, and εhcp-martensite phase in the former.

Ultra-fine grained Mg-Zn-Ca-Mn alloy with simultaneously improved strength and ductility processed by equal channel angular pressing

The influence of equal channel angular pressing (ECAP) on the microstructure, texture evolution and mechanical properties of extruded Mg-5.0Zn-0.5Ca-0.3Mn (weight percent) alloy has been systematically investigated in the current study. A typical ultra-fine grained (UFG) structure is obtained in the as-ECAPed alloy for 4 passes, due to a completed recrystallization process. With the increasing ECAP pass, the conventional basal texture can be gradually changed to an asymmetric non-basal texture with its basal plane inclined to normal and transverse directions ∼45°, which is related to the typical shear mode, leading to an obvious increase of Schmid factor for the activation of basal slip system. The room-temperature tensile yield strength, microhardness and ductility of extruded alloy are simultaneously improved by successive ECAP processing, which is derived from the grain refinement and texture modification.

The effect of precipitates on voiding, twinning, and fracture behaviors in Mg alloys

The effect of precipitates on fracture behaviors was comparatively investigated among three kinds of Mg alloys with different precipitates, i.e., Mg-Gd alloy with prismatic plate-shaped precipitates, Mg-Zn alloy with [0001]α rod-shaped precipitates, and Mg-Gd-Zn-Zr alloy with basal plate-shaped precipitates. By comparing the fracture behaviors of the alloys before and after aging treatment, it was evident that the presence of precipitates greatly promoted the formation of microvoids that were initiated at primary intermetallic particles. Voiding was the most accelerated in the Mg-Gd alloy, where the precipitate-dislocation interactions are the strongest. While in the Mg-Gd-Zn-Zr alloy with the weakest precipitate hardening, the precipitate-facilitated voiding was the least significant. The precipitate-dependent voiding suppressed the twinning behaviors, causing the volume fraction of deformation twins decreased in turn from Mg-Gd-Zn-Zr to Mg-Zn, and finally to Mg-Gd alloys. The tensile ductility of present Mg alloys approximately scaled with the volume fraction of deformation twins, and was highly sensitive to the precipitates. The fracture scenario of present Mg alloys was proposed that the voiding suppressed twinning and the competition between voiding and twinning can be mediated by precipitates. In the Mg-Gd-Zn-Zr alloy with limited voiding, the twinning dominated the deformation process and concomitantly resulted in a great ductility. The formation of microvoids or the fracture of primary intermetallic particles was quantitatively analyzed by applying a Weibull model, where both the alloy strength and the volume fraction of primary intermetallic particles were considered to rationalize the remarkable difference in fracture behaviors among the present three Mg alloys. Furthermore, the coupling contribution of precipitates and twins to the work hardening was modelled in the aged Mg-Gd-Zn-Zr alloy that displayed a large room temperature tensile ductility of ~ 13.5%.