Elevated Temperature Mechanical Properties Research Articles

Effect of external magnetic field on microstructures and mechanical properties of Ti-2Al-1.5Mn titanium alloy resistance spot welds

The excellent specific strength and elevated-temperature mechanical properties make titanium alloys essential structural materials in aerospace. Resistance spot welding (RSW) has high efficiency and cost-effective and is widely used in titanium joining. However, the exceeded welding penetration and coarse martensite α′ limit the mechanical properties of titanium welds. This paper implemented an external magnetic field (EMF) in the RSW process of Ti-2Al-1.5Mn titanium alloy, with detailed investigations into microstructure and mechanical properties. It was found that the penetration rate was reduced, and the nugget diameter was increased under the EMF. Moreover, the coarser columnar prior β grains (PBGs) and martensite α′ formed in traditional RSW transformed into the equiaxed PBGs and widmannstatten structure with α colonies. Due to the significantly improved microstructure and larger nugget diameter, the lap-shear properties were also improved.

Improved elevated-temperature strength and thermal stability of additive manufactured Al–Ni–Sc–Zr alloys reinforced by cellular structures

Laser powder bed fusion (LPBF) processed AlNi alloys typically exhibit cellular structures, contributing to superior strength at ambient temperature as the cell walls can effectively impede dislocation motion during plastic deformation. However, it is still unclear whether cellular structures can lead to excellent elevated-temperature performance. In this study, we conducted a systematic investigation into the elevated-temperature mechanical properties and thermal stability of an LPBF Al–Ni–Sc–Zr alloy with cellular structures. The as-built alloy exhibits a yield strength of 368MPa at 25℃ and 332MPa at 250℃, with an exceptional strength retention rate of ~90% at 250℃. Moreover, the alloy maintains a hardness of ~146 HV following exposure at 250℃ for 100h and shows no apparent decrease despite increasing the exposure time to 500h. The excellent elevated-temperature performance can be attributed to the cellular structures. The cell walls effectively confine dislocations at elevated temperatures, suppressing the dislocation annihilation and leading to exceptional strength retention. On the other hand, even if the cell walls decompose into nanoparticles after prolonged thermal exposure, the reduction in strength due to the decomposition can be compensated by the strengthening of the nanoparticles, resulting in excellent thermal stability. By studying the influence of the cellular structures on elevated-temperature performance, this study could provide valuable insights into developing high-performance Al alloys for elevated-temperature applications by additive manufacturing.

Microstructure Characteristics and Elevated Temperature Mechanical Properties of a B Contained β-solidified γ-TiAl Alloy

Microstructure Characteristics and Elevated Temperature Mechanical Properties of a B Contained β-solidified γ-TiAl Alloy

Elevated-temperature mechanical property and constitutive model of galvanized parallel wire strands

High-temperature conditions lead to severe degradation of the mechanical properties of steel cables. This study conducted uniaxial tensile tests on 19-wire galvanized parallel wire strands (GPWSs) at elevated temperatures. Using a non-contact measurement system, the failure modes under high-temperature conditions were observed, and stress-strain curves were obtained for 12 temperature levels ranging from 20 °C to 700 °C. The yield strength corresponding to a strain level of 1.5% was determined as the nominal yield strength, which ensures both material strength and sufficient safety. By analyzing the stress-strain curves, reduction factors for the proportional limit, elastic modulus, nominal yield strength, and ultimate strength at various temperatures were obtained for the GPWS. Additionally, a Boltzmann curve was used to fit the reduction factors of the mechanical properties of the GPWS. A constitutive model for the entire process of the GPWSs was established through the segmented fitting of the elastic, plastic hardening, and necking stages. High temperatures severely affect the tensile strength of the GPWS. When the temperature reaches 400 °C, the ultimate tensile strength of the GPWS is only about 50% of that at room temperature. At 700 °C, the ultimate tensile strength of the GPWS is reduced to only 2.5%.

Role of Austenite Stability in Elevated Temperature Mechanical Properties of Gas Metal Arc-Directed Energy Deposition Austenitic Stainless Steels

A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.

A Zr-45Ti-54Al-3V alloy with excellent elevated temperature mechanical properties: Millimeter-scale coarse grains with initial subgrains

For improved elevated temperature mechanical properties, a Zr-45Ti-54Al-3V alloy with initial subgrains microstructure and millimeter-scale coarse grains is prepared herein via an electric field-assisted high-temperature compression process at temperatures of 1200 °C and 1400 °C. After the treatment at 1400 °C, the tensile strength and elongation of the alloy at 500 °C are increased by 25.2% and 21.7%, respectively, compared with those of the as-forged 47Zr. Both treatment temperatures lead to a compressive strength of 1400 MPa, along with good elevated temperature compression deformation ability. The rapid grain growth is attributed to the promotion of atomic diffusion by the pulsed current, which also results in a uniform distribution of each solute element. The dynamic recovery and continuous dynamic recrystallization during hot deformation are the main reason of elevated temperature plasticity improvement of EFHC sample. The elevated temperature strength of EFHC sample is higher than that of as-forged sample because it has more α phase and LAGB than as-forged sample

Site preference and elastic properties of L21-Ni2TiAl doped with refractory metal elements from first principles

Refractory high entropy alloys (RHEAs) are attracting wide attention due to their excellent high-temperature strength. L21-Ni2TiAl was found as a strengthening phase in high entropy alloys (HEAs) which significantly improves room and elevated-temperature mechanical properties. To explore the potential application in RHEAs, we address the calculations of 7 refractory elements (i.e., V, Cr, Zr, Nb, Mo, Hf, and Ta) doping from first principles on the site preference, elastic properties and bonding effect of Ni2TiAl. The results show that the antisite defects is more stable than the vacancy defects. Elements Cr and Mo prefer the Ni-sites, while V, Zr, Nb, Hf, and Ta prefer the Ti-sites. Moreover, all the doping elements lead to a lattice shrinkage, among which V cause the largest distortion. For the normalized shear modulus (G/G0), all the elements increase except Nb, of which Hf rank the top. While for the normalized bulk modulus (B/B0), all the elements slightly increase except Hf. Furthermore, Nb is the only one to improve the toughness but has little impact on hardness (Hv), then Hf improves hardness the most. Additionally, all the doping elements decrease the anisotropy of Young's modulus for Ni2TiAl, of which Nb has the largest anisotropy, leading to a weak resistance to deformation. Notably, the density of states (DOS) shows that V and Cr can effectively improve the bonding stability and strength of Ni2TiAl. Overall, the alloying of Nb, V, and Cr may synergistically improve the stability and toughness of Ni2TiAl, which is expected to develop novel RHEAs.

Study on anti-sliding performance of cable clamps under and after elevated temperature

Cable clamps are key joints in building cable structures and are vulnerable positions in fire accidents. Currently, there is little research on the elevated-temperature mechanical properties of cable clamps. Therefore, the high-temperature mechanical properties of cable clamps were investigated in this study. First, anti-sliding tests at room temperature, elevated temperatures, and after elevated temperatures were conducted for the cable clamps, and the pushing force-sliding displacement curves of the cable clamps were obtained. The relaxation law of the bolt tightening force and the anti-sliding performance of the cable clamps were studied. The results indicated that when the temperature exceeded 300 °C, the anti-sliding performance of cable clamps started to decline. When the temperature reached 500 °C, the anti-sliding bearing capacity of cable clamp was approximately 27 % of that at room temperature. For cable clamps after high-temperature treatment, when the heat treatment temperature exceeded 300 °C, the anti-sliding bearing capacity was approximately 50 % of that at room temperature. Therefore, based on the ABAQUS finite element software, simplified and refined models of the cable clamps are proposed to simulate the sliding process of locked coil wire rope clamps. The effects of material properties and non-uniform expansion on the anti-sliding performance of cable clamps were studied, which provides a theoretical basis for the fire resistance design of building cable structures.

Improving mechanical performances at room and elevated temperatures of 3D printed polyether-ether-ketone composites by combining optimal short carbon fiber content and annealing treatment

Development of high performance polyether-ether-ketone (PEEK)) composites is of great significance for aerospace engineering application. The fused filament fabrication (FFF)-based 3D printing process is promising for manufacturing high performance short carbon fiber (SCF) reinforced PEEK composites. However, a comprehensive investigation of the combined effects of SCF content and annealing condition has not been conducted on the mechanical performances of SCF/PEEK composites; also, elevated temperature mechanical properties that are required for aerospace engineering have not been reported for 3D printed SCF/PEEK composites. This work develops a creative strategy that combines the optimal short carbon fiber (SCF) content with annealing treatment to achieve significant improvements in the mechanical properties both at room and elevated temperatures of FFF-3D printed SCF/PEEK composites. First, the influences of SCF content on the mechanical performances of 3D printed SCF/PEEK composites are studied to determine the optimal fiber content (5 wt%) for the best mechanical performances of non-annealed PEEK composites. Then, the effects of SCF content and annealing on room temperature and elevated temperature mechanical performances are examined for both PEEK and the 5 wt% SCF/PEEK composite. It is elucidated that the increase of interface strength and crystalline region as well as the reduction of residual stress by annealing are all responsible for the observed improvements in mechanical performances. This work presents a comprehensive study of 3D printed short fiber reinforced thermoplastics and serves as a valuable reference for the 3D printing of high-performance thermoplastic composites.

Design of lightweight Ti3Zr1.5NbVx refractory high-entropy alloys with superior mechanical properties

Refractory high-entropy alloys (RHEAs) have attracted extensive attention due to their excellent mechanical properties. However, most RHEAs have high density and exhibit poor ductility at room temperature, which greatly limits their applications. In this work, a series of lightweight Ti3Zr1.5NbVx (x = 0, 1 and 2, respectively) RHEAs with high strength and good ductility were designed and prepared using vacuum arc melting. The effects of V content on the microstructure, mechanical properties and deformation mechanism of the as-cast Ti3Zr1.5NbVx RHEAs were investigated in details. The results showed that all the Ti3Zr1.5NbVx RHEAs exhibit a single body-centered cubic (BCC) phase and their densities are less than 6 g/cm3. With the increase of V content, the grain size of the Ti3Zr1.5NbVx RHEAs decreases from 436.1 to 81.2 μm, and the hardness increases from 199.7 to 297.5 HV. The Ti3Zr1.5NbV2 RHEA possesses optimal tensile mechanical properties with the yield strength of 974.0 MPa, fracture elongation of 6.3%, and a specific yield strength of 165.7 MPa cm3/g, better than most previously reported RHEAs. More importantly, the Ti3Zr1.5NbV2 RHEA also exhibits excellent elevated-temperature mechanical properties with yield strength of 770.9 MPa at 600 °C and 243.6 MPa at 800 °C. The deformation mechanism of the Ti3Zr1.5NbV2 RHEA is governed by dislocation slip, including planar slip bands, dislocation loops, and high density dislocation walls. The high strength of the Ti3Zr1.5NbV2 RHEA is mainly attributed to the solid solution strengthening effect, in which Zr and V elements play an important role.

The role of the Mg17Al12-phase in the high-pressure die-cast magnesium-aluminum alloy system

The Mg17Al12 β-phase is highly influential to the performance of magnesium-aluminum high-pressure die-casting alloys. Numerous studies have investigated the effect of this phase on the room temperature and elevated-temperature mechanical properties as well as the corrosion, flammability, thermal conductivity and castability performance. This work summarizes the results of these works, as well as the studies that describe control of the Mg17Al12-phase through alloying additions that modify the microstructure and resulting alloy performance. The results of this work can be used as a basis for the development of alloying additions that enable modifications to the performance of high-pressure die-cast magnesium-aluminum-based alloys.

Room- and elevated-temperature mechanical property of selective laser melting-fabricated Hastelloy X with different heat treatments

Superalloy alloy Hastelloy X was fabricated by selective laser melting (SLM), and heat treatments have been designed to optimize microstructures for better mechanical property at high temperature. Specimens of as-printed, aging treatment (AT), solution treatment (ST), and wrought conditions are of interests to this study, and their grains, dislocations, grain boundaries, precipitates, twins and defects have been characterized. Compared to the wrought specimens, the as-printed specimens showed sub-micron cellular microstructure, while M23C6-type carbides and Fe2Mo phases were found after AT condition with partial recrystallization. The ST treatment introduced a large number of high angle grain boundaries (HAGBs) and a high twin boundary ratio due to recrystallization. With this, their post-mortem microstructures at both room temperature (RT) and elevated temperature (ET) of 850 °C were easily correlated to the mechanical performance. AT yielded the highest ultimate tensile strength (UTS) of ∼1067 MPa, while ST gave the highest ductility of ∼51% at RT. At ET, the as-printed, AT, and ST SLM specimens showed improved UTS over the wrought counterparts, all with a reduced ductility. With these observations, a comprehensive heat treatment roadmap has been proposed to summarize their microstructure-property relationship, which rigorously included the traditional ST + AT heat treatment and proved SLM-fabricated Hastelloy X's incompatibility. This research not only deepens our understanding of its mechanical behavior after possible heat treatments, but also semi-quantitatively guides its feasible processing even for high-temperature applications.

Elevated-temperature mechanical property and constitutive model of Galfan-coated and full-locked steel cables

Steel cables are an essential stress-bearing member in pre-tensioned steel structures. Their exposure to fire may lead to prestress loss, which may cause structural failure or even overall collapse. This study used a non-contact strain measurement system to investigate the mechanical properties of 1670 MPa Galfan-coated steel cable and 1570 MPa full-locked steel cable at elevated temperatures. The test results showed that the mechanical properties of the two types of cable decayed significantly with the increase in temperature. At 700 °C, the ultimate strength of Galfan-coated steel cable was only 2.36% of that at room temperature, and that of full-locked steel cable was 3.56%. By analyzing the stress-strain relationships between the yielding and hardening stages at 9 test temperature levels, it was found that the high-temperature nominal yield strength could be taken as the stress at the strain level of 1.25% for both types of steel cables. Based on the full stress-strain curves at each temperature level, the calculation formulae for high-temperature reduction factors of elastic modulus, proportional limit, nominal yield strength, and ultimate strength of Galfan-coated and full-locked steel cables were proposed. Besides, the high-temperature constitutive models of the two cable types were also proposed based on the experimental data. The findings of this study can improve the fire safety of pre-tensioned steel structures.

Carbide-reinforced Re0.1Hf0.25NbTaW0.4 refractory high-entropy alloy with excellent room and elevated temperature mechanical properties

Refractory high-entropy alloy (RHEA) with high elevated-temperature strength is generally brittle at room-temperature, which greatly restricts the workability and application of the RHEA. Here, we report a novel Re0.1Hf0.25NbTaW0.4C0.25 RHEA with both excellent high-temperature strength and decent room-temperature ductility. Meanwhile, the microstructure and mechanical properties of Re0.1Hf0.25NbTaW0.4C0.25 RHEA were investigated. The microstructure of the alloy was composed of BCC matrix phase and eutectic phases (BCC+(MC + M2C)), and lots of secondary carbides were dispersedly precipitated in the BCC matrix. Due to the synergistic strengthening effect between the BCC matrix and the carbides. The alloy exhibited excellent compressive strength of 1026 MPa at 1450 °C and 523 MPa at 1700 °C, with a good compressive fracture strain of 10.2% at room-temperature. The Re0.1Hf0.25NbTaW0.4C0.25 RHEA has potential applications in the field of ultra-high-temperature structural materials.

Research on influence of grain boundaries on the mechanical properties of 460 MPa refractory steel used for high-strength building structures

In this paper, the microstructure of Mo-type seismic refractory steel for building g, as well as different boundary densities and boundary ratios, are combined with elevated-temperature mechanical properties analysis to explore the laws of boundary for high-temperature performance. The results show that salt water cooling (SWC) and water cooling (WC) can obtain lath bainite with a higher content, and oil cooling (OC) with a lower cooling rate can obtain the microstructure of multiphase bainite + bulk ferrite. The boundary characterization results show that when the sample contains more high angle grain boundaries (Block and High-Packet boundaries), and the dislocation density is high, it can make it have better mechanical properties at room temperature. When the content of low angle boundary and low interfacial energy twin boundary (Σ3 boundary, which is mainly composed of V1/V2 variant pair) is high, it will have better microstructure stability after high temperature tempering, and the boundary density and dislocation density will decrease slightly, ensuring that it has better refractory performance.

Enhanced elevated-temperature mechanical properties of hot-rolled Al–Cu alloys: effect of zirconium addition and homogenization

Enhanced elevated-temperature mechanical properties of hot-rolled Al–Cu alloys: effect of zirconium addition and homogenization

Refining mechanism and elevated-temperature mechanical properties of Al-Ce alloys solidified under super gravity field

We reported that the enhanced elevated-temperature mechanical property of Al-Ce alloy could be obtained by solidification under super-gravity field. The refinement of eutectic Al11Ce3 is detected, which can be ascribe to the enhancement of mass transfer under super-gravity field, affecting the nucleation rate and inhibiting the growth of eutectic phase for this irregular Al-Ce eutectic system. Interestingly, the refined eutectic Al11Ce3 is thermally stable even up to 400 °C thermal exposure without obvious coarsening, contributing to the enhancement of elevated-temperature mechanical properties (∼125.0 ± 9.1 MPa at 300 °C), which is ∼74.8% higher than that solidified under normal gravity field (∼71.5 ± 0.6 MPa). The refinement of eutectic Al11Ce3 with higher number density also contribute to the stress dispersion, the crack prefer to propagate through the eutectic colony, leading to the transformation of fracture mode from brittle to ductile.

A physically-based model for elevated temperature strength prediction of (TiBw + (Ti, Zr)5Si3)/Ti-6.5Al-2Zr-1Mo-1V composite

In this paper, effects of heat treatment temperature and soaking time on the microstructure and elevated temperature mechanical properties of the (TiBw+(Ti, Zr)5Si3)/Ti-6.5Al-2Zr-1Mo-1V composite were studied. A physically-based model capable of predicting microstructure and yield strength was established. Results show that acicular martensite forms in the transformed β microstructure, the size of (Ti, Zr)5Si3 increases obviously, while the morphology of TiBw remain unchanged. With the increase of heat treatment temperature, the fraction of primary α phase and (Ti, Zr)5Si3 decreases, while the fraction of βT increases. The yield strength and ultimate tensile strength of the composite are improved by 34% and 46% after heat treatment at 975 °C. The yield strength can be accurately predicted by the model. The mean relative error (MRE) of yield strength calculated by the model was 1.20% under the condition of heat treatment at 900 °C and 1050 °C. Using the model, martensitic strengthening accounts for the largest proportion in yield strength. The variation tendency of yield strength with temperature is predominantly decided by the variation tendency of martensitic strengthening. And the heat treatment temperature affects the yield strength through the factors including the fraction of βT and the lamellae thickness of martensite.

B2 to ordered omega transformation during isothermal annealing of refractory high entropy alloys: Implications for high temperature phase stability

Al containing refractory high entropy alloys or complex concentrated alloys (RHEAs or RCCAs) often exhibit a patterned BCC + B2 microstructure, resembling the FCC + L12 microstructure typically observed in many Ni/Co base superalloys. There is a rapidly growing interest in these BCC+B2 microstructures, especially due to their promising balance of elevated temperature mechanical properties. Unfortunately, the stability of such microstructures on long-term exposure to elevated temperatures is not well-understood and prompts the motivation of the present study. The current study traces the phase transformation pathway during the intermediate temperature annealing of such microstructures for extended time periods, and more specifically the formation of ordered omega type phases. In the case of a high strength RHEA, Al0.5Mo0.5NbTa0.5TiZr the relationships between the two structures strongly suggest a strong possibility that such ordered omega phases are essentially derived from a parent B2 phase. At the early stages of annealing, the solutionized condition of this alloy consists of a co-continuous mixture of BCC and B2 phases which transforms into a continuous B2 matrix with discrete BCC precipitates presumably via spinodal decomposition concomitant with ordering. With increasing annealing time at 800 °C, isolated islands of BCC phase embedded with discrete, fine-scale pockets of the B2 and hP18 (P63/mcm) phase develop, where the hP18 phase is an ordered derivative of the omega phase, commonly found in titanium and zirconium base alloys. The transformation of the parent B2 phase to the hP18 ordered omega type phase can be rationalized via a combined replacive and displacive transformation, including simultaneous rejection and enrichment of specific alloying elements to achieve the desired stoichuometry. First-principles based DFT calculations were also carried out to compare the stability of the B2 versus hP18 phases, as a function of composition. While long term annealing and CALPHAD based calculations both indicate towards an equilibrium phase field comprising of hP18, B2 and BCC phases, hP18 does not directly nucleate homogeneously within the BCC grains. Instead, it appears to form heterogeneously along grain boundaries and more importantly within the grains with B2 phase acting as an intermediate phase, both of which indicates a high nucleation barrier for its homogeneous nucleation. The universality of this phenomenon has been demonstrated using examples from other RHEAs, and the resultant impact on the high temperature stability of the BCC+B2 microstructure has been discussed.

Tailoring precipitates in Mg–12Gd-3Y alloys by Sn addition to obtain superior elevated temperature mechanical properties

The precipitation phase was tailored by adding different Sn contents to the Mg-12Gd-3Y alloy to achieve multiphase composite strengthening. Adding different Sn contents has different effects on the microstructure and mechanical properties. Adding 1.0 wt% Sn to the Mg-12Gd-3Y alloy produced Mg5RE, Sn3RE5, and β′ precipitates, which were uniformly diffusely distributed within the alloy. When Sn elements were added up to 1.5 wt%, Mg5RE and Sn3RE5 precipitates increased and aggregated into lumps. The prepared Mg-12Gd-3Y-1.0Sn alloy exhibited excellent room and elevated temperature (ET) mechanical properties. The ultimate tensile strengths (UTS) of Mg-12Gd-3Y-1.0Sn alloy after extrusion and aging (E-A) treatment were 476, 415, 383 MPa at RT, 200 °C, and 250 °C, respectively. The fine grain size inhibits dislocation sliding, and the uniformly diffuse multi-scale phase structure inhibits the expansion of grain boundary cracks, resulting in excellent strength of Mg-12Gd-3Y-1.0Sn at room temperature (RT). The excellent ET strength of the Mg-12Gd-3Y-1.0Sn alloy is derived from the homogeneous diffusion of multi-scale heat-resistant precipitates within the alloy.