Room Temperature Yield Research Articles

Development of Fire Resistance Steel for Structural Applications

Three grades of steel were chosen to evaluate for their fire resistance performance. The steel # 1 (0.18%C-1.37%Mn-0.35%Si-0.12%Cr-0.113%V) is a vanadium micro alloyed steel, while Steel #2 (0.24%C-0.81%Mn-0.19%Si-0.97%Cr-0.2%Mo-0.01%Ti-0.055%Nb) is a titanium and niobium micro-alloyed steel, whereas steel #3 (0.19%C-0.50%Mn-0.19%Si-1.23%Cr-0.40%Mo-0.018%Nb) is a Nb micro alloyed steel. The room temperature and high temperature (@600oC) were evaluated for the fitness for fire resistance steel. It was found that the steel #1 is having the yield ratio (ratio of yield strength at 600oC to room temperature yield strength) of 0.74 while the steel #2 and #3 are having yield ratio of 0.96 and 0.97 respectively. The higher yield ratio of the steel #2 and #3 are due to the presence of Cr, Mo as the solid solution strengthening in addition to the Nb or Nb & Ti micro-alloying. All the three steels with ferrite pearlite microstructure were qualified for the fire resistance norms (2/3=0.67, of the yield strength at 600oC) and however considering the higher carbon equivalent of the steel#2 and #3 only steel#1 can be used for fire resistance structural applications for the replacement of E350 grade.

Microstructure and mechanical properties of WE43 magnesium alloy fabricated by wire-arc additive manufacturing

Wire-arc additive manufacturing (WAAM) is emerging as a revolutionary method for fabricating heat-resistant WE43 magnesium alloy (WE43-Mg) components and multiple other Mg alloys. In this study, we successfully utilized cold metal transfer WAAM technology to fabricate high quality single-pass thin-walled WE43-Mg structures. The results show that the microstructure is mainly composed of equiaxed grains, and Mg14Nd2(Y,Gd) is the main second phase distributed along the grain boundaries. The relative density of the sample reaches 99.96 %, and the main defects are oxide inclusions. The room temperature yield strength, ultimate tensile strength and elongation are 147.0 MPa, 221.9 MPa and 7.2 %, respectively, with anisotropy rates of 1.9 %, 2.2 % and 3.7 %. High-temperature tensile tests conducted at 250 °C showed a slight increase in ultimate tensile strength and a significant increase in elongation from 7.2 % to 16.8 %, representing a 133 % increase. Under these conditions, the proportion of substructure increased significantly, and the proportion of low-angle grain boundaries rose from 3.2 % to 69 %. The stress-strain curves exhibited pronounced serrated flow behavior, which is attributed to the interaction between solute atoms and dislocations.

Microstructure and mechanical properties of an elevated temperature L12-type Ni-Co-Cr-Al-Ti-V-B-based high-entropy intermetallic

In this study, a novel L12-type Ni46.6Co23.3Cr10Al9Ti6V5B0.1 high-entropy intermetallic (HEI) with the excellent strength-ductility combination at 25 ℃ and 800 ℃ was prepared by vacuum arc melting. An ultrahigh volume fraction of cubical L12-type nanoparticles (∼81 %) was formed uniformly inside the grains after annealing. The intragranular L12 phase, FCC nanolayers, and intergranular blocky FCC precipitations constituted the heterogeneous structure. The annealed HEI exhibits high room-temperature yield strength (∼711 MPa) and ultimate tensile strength (∼937 MPa) with large tensile elongation (∼17 %). The room-temperature plasticity is superior to that of many other simple ordered intermetallics. The yield strength can still be maintained at ∼740 MPa with a total strain of about 6.7 % when tested at 800°C, and its strength is higher than many L12 nanoparticle-strengthened high-entropy alloys. High strength is primarily attributed to the elevated volume fraction of L12 nanoparticles and the significant increase in the antiphase boundary energy caused by the addition of Ti and V elements. The stacking-fault (SF) networks, L-C locks, and K-W locks provide sustained work-hardening capacity, resulting in excellent yield strengths at 25 ℃ and 800 ℃. In addition, the favorable ductility is due to the formation of intragranular disordered FCC nanolayers and grain boundary FCC precipitations, which can avoid premature intergranular fracture.

Influence of Si on the Elevated-Temperature Mechanical and Creep Properties of Al–Cu 224 Cast Alloys during Thermal Exposure

The influence of Si content (0.1–0.8 wt.%) on the development of precipitation microstructures and the resultant mechanical and creep properties during thermal exposure, up to 1000 h at 300 °C, in Al–Cu 224 cast alloys, was systematically investigated. The room and elevated temperature yield strength (YS) increased with increasing Si content under the T7 condition, which was attributed to the fact that the Si promoted the precipitation of fine θ′. However, Si increased the coarsening of θ′ during thermal exposure at 300 °C, and the alloys with low Si exhibited a higher YS and creep resistance at elevated temperatures than high Si alloys. The mechanical strength and creep resistance were mainly controlled by the precipitation strengthening of the predominant θ′ phase. Because of the high mechanical strength and creep resistance of the 0.1Si alloy during long-term thermal exposure, the Si level in Al–Cu alloys should be maintained at a low level of 0.1 wt.% for high-temperature applications. The strengthening mechanisms were quantitatively analyzed, based on the characteristics of the precipitate. The predicted YS values under different exposure conditions agreed well with the experimentally measured values.

Microstructural refinement of an Al-Ce-Mg alloy via Shear Assisted Processing and Extrusion

Al-Ce alloys have attracted recent interest because of their high thermal stability due to the low solubility of Ce in the Al matrix. The Al11Ce3 eutectic phase gives excellent strain hardening behavior and moderate high-temperature strength in the as-cast state. However, its strengthening effect is limited by its coarse as-cast structure. Therefore, alternative manufacturing methods such as additive manufacturing or equal channel angular pressing have been applied to refine the Al11Ce3 phase to good effect. However, these techniques are both expensive and time-consuming. Therefore, this study aims to use Shear Assisted Processing and Extrusion (ShAPE), an emerging solid phase processing technique that is more easily scalable than the previously mentioned methods. ShAPE can produce useful cross-sections of an Al-8Ce-4Mg alloy while refining the Al11Ce3 phase to produce a higher strength material. It was found that a low temperature ShAPE process can improve the room temperature yield strength by ∼60 % compared to a binary Al-4Mg alloy. Additionally, the high-temperature yield strength of the Al-Ce alloys increased by 20 %, with a simultaneous 15 % improvement in ductility compared to the binary Al-Mg alloy. These results highlight the potential for ShAPE as a processing technique for Al-Ce alloys.

Enhanced an eutectic carbide reinforced niobium alloy by optimizing Mo and W alloying elements

Niobium alloys play an indispensable role in aerospace technology. However, traditional niobium alloys has unsatisfactory or high-temperature strength or limited room-temperature. This work introduces Nb2MoxWyC0.25 alloys with strength-plasticity balance by optimizing refractory alloy elements and eutectic carbides drawing on the design concepts of eutectic high-entropy alloys. The influence of Mo and W contents on the microstructure and mechanical properties of the alloys was studied. The Nb2MoxWyC0.25 alloy contain body-centered cubic (BCC) primary phase and eutectic structures composed of BCC and carbide phases with semi-coherent interfaces. Appropriate additions of Mo and W refine the grain size of the primary BCC phase and cause the carbide phase to evolve from Nb2C to NbC. The Nb2Mo0.5W0.5C0.25 hypoeutectic niobium alloy composed of BCC and Nb2C phases with a relatively small lattice mismatch has a room-temperature yield strength of 1.27 ± 0.04 GPa, compressive strength of 2.03 ± 0.06 GPa, and a fracture strain of 17.8 ± 2.2 %. Solid solution strengthening in the BCC phase and second-phase strengthening of the carbide phase simultaneously enhance the alloy. The fine grain strengthening, reduced crack origination at low mismatch interface, and the crack tip by soft BCC at the phase interface improve the plasticity simultaneously. This paper provides a method to improve the room-temperature plasticity and strength of refractory niobium alloys, laying the foundation for the industrial application of refractory niobium alloys.

Regulating the mechanical properties of (μm B4C+nm SiC)/7075Al composites via nano-SiC content

In this study, micron B4Cp (m-B4Cp) and nanometer SiCp (n-SiCp) reinforced 7075Al composites were prepared by high-energy ball milling, hot press sintering, and hot extrusion. The microstructures and mechanical properties of composites were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile testing, and synchrotron radiation computed microtomography (SR-μCT). The results showed that n-SiCp and m-B4Cp were uniformly distributed in the aluminum matrix without interfacial reaction. The optimal grain refinement was achieved by adding 1.5 wt% n-SiCp, improving the strength and ductility of composites. The room temperature yield strength (YS) and ultimate tensile strength (UTS) of M5N1.5 composites were increased from 350 MPa and 401 MPa of 7075Al to 460 MPa and 502 MPa, respectively. The results revealed that the YS and UTS of composites were decreased, but elongation was increased with increasing temperature. When the temperature increased to 200 °C, the elongation of 7075Al and M5N1.5 was increased from 5.9 % and 4.7 % to 8.7 % and 10.9 %, respectively. Quantitative analysis revealed that dislocation strengthening and grain refinement strengthening caused by the addition of n-SiCp are the dominating strengthening mechanisms at both room and high temperature, while the contribution of m-B4Cp to the strengthening of the composites is very small.

Solution temperature influence on CRSS of the secondary γ′ phase in nickel-based superalloy FGH4096

Nickel-based superalloy components are capable of being manufactured by direct hot isostatic pressing (HIP) process with near-net shape and nearly 100 % raw materials utilization. However, they cannot be applied directly, due to the poor mechanical properties of HIPed components. Fortunately, heat treatment can improve the properties via regulating the microstructure. Therefore, the effects of varied solution temperatures on the tensile properties of FGH4096 nickel-based superalloy disk at room and high temperatures were studied. The results revealed that the disk after sub-solution treatment (1130 °C) has higher tensile properties (room temperature yield strength: 1194 MPa, elongation: 15.5 %; high temperature yield strength: 1075 MPa, elongation: 10.5 %). The antiphase boundary (APB) shear stress, Orowan bypass stress, stacking faults (SF) shear stress, and micro-twins (MT) shear stress affected by the size and distribution of the secondary γ′ phase were calculated. The results show that the main strengthening mechanism at room temperature is APB shear, and the APB shear stress of solution treatment at 1190 °C is the highest. The high temperature strengthening mechanism is due to SF shear and MT strengthening effects. The difference in SF shear stress between various solution temperatures is minimal. 1190 °C has a higher MT strengthening effect than 1090 °C. This is consistent with the actual result that the tensile properties at 1190 °C are superior to those at 1090 °C. The conclusion is that the combination of high strength reflected by several critical resolved shear stresses and considerable plasticity reflected by the MT strengthening effect can lead to a comprehensive improvement in alloy tensile properties.

Strengthening a non-equiatomic quaternary high-entropy alloy via concurrent gradient structures of nanotwin and dislocation cell

One main drawback of FCC high-entropy alloys (HEAs) is their insufficient room temperature yield strength. In this work, concurrent gradient structures of nanotwins and dislocation cells have been introduced into a non-equiatomic quaternary Fe40Ni20Co20Cr20 (at. %) high-entropy alloy by ultrasonic shot peening to meet this challenge. The gradient alloy exhibits a significantly enhanced yield strength, i.e., from the original 204 MPa to 559 MPa, combining with a well-retained ductility of 29 % and a good work hardening ability. Strengthening mechanisms at different depths (5, 30, 200, 400 μm) are also carefully discussed. The current work presents a strategy for hardening FCC HEAs significantly while maintaining the single solid solution microstructure.

Optimization of the grain boundary network and tensile properties of powder metallurgy-hot isostatic pressed Ni-based alloy Inconel 625

Powder metallurgy-hot isostatic pressing (PM-HIPing) is an advanced manufacturing technology for large-sized and complex-shaped components for nuclear applications. In the present study, a novel PM-HIPing strategy was proposed to optimize the grain boundary network and tensile properties of Ni-based alloy Inconel 625, and it was verified by a series of experiments. Samples with 69 % and 74 % of coincidence site lattice (CSL) grain boundaries were obtained when the narrow-size ranged powder was consolidated at 1140 °C and 1180 °C, respectively. The fraction of CSL grain boundaries was further increased to ∼78.5 % after a high temperature solution treatment. Room temperature yield strength and ultimate tensile strength of PM-HIPed alloy 625 were comparable to that of the wrought specimens, elongation of the specimens was ∼56–60 %, the samples all featured with dimple ductile dominant fracture mode.

Improving fatigue resistance of ultrafine bainitic steel by exploiting segregation-induced bands

An ultrafine bainitic steel was designed with segregation-induced martensitic/austenitic (MA) bands. The steel comprises almost 21 % retained austenite (RA); the rest is mainly bainitic ferrite and a small fraction of martensite. The steel exhibits room temperature (RT) yield strength of ∼914 MPa, ultimate tensile strength of ∼1969 MPa, and total elongation to failure of ∼11.5 %. Low cycle fatigue (LCF) behavior was investigated under a fully reversed strain-controlled waveform at RT, with a loading-axis parallel to the MA bands. LCF stress response reveals initial cyclic hardening followed by softening or saturation, depending on the applied strain amplitude. This is attributed to the interplay of strengthening mechanisms, which include RA transforming to martensite and dislocation multiplication/interactions and the softening effects of dislocation annihilation and crack formation/propagation. Damage studies reveal a mixed fracture mode, displaying brittle striations, secondary cracks, quasi-cleavage facets, and dimples. The secondary cracks mainly appear to originate from the main crack and propagate predominantly along the matrix-bands interphase regions or within MA bands. Additionally, surface-initiated cracks along their propagation path were deflected and branched around the interphase regions. This tortuosity reduces the crack propagation rate. Overall, the significant work hardening capability of the steel, along with its banded microstructure, contributes to its superior fatigue resistance compared to non-banded bainitic steels reported in the literature.

Absorption and fluorescence spectroscopy of cold proflavine ions isolated in the gas phase.

Proflavine, a fluorescent cationic dye with strong absorption in the visible, has been proposed as a potential contributor to diffuse interstellar bands (DIBs). To investigate this hypothesis, it is essential to examine the spectra of cold and isolated ions for comparison. Here, we report absorption spectra of proflavine ions, trapped in a liquid-nitrogen-cooled ion trap filled with helium-buffer gas, as well as fluorescence spectra to provide further information on the intrinsic photophysics. We find absorption- and fluorescence-band maxima at 434.2 ± 0.1 and 434.7 ± 0.3nm, corresponding to a Stokes shift of maximum 48cm-1, which indicates minor differences between ground-state and excited-state geometries. Based on time-dependent density functional theory, we assign the emitting state to S2 as its geometry closely resembles that of S0, whereas the S1 geometry differs from that of S0. As a result, simulated spectra involving S1 exhibit long Franck-Condon progressions, absent in the experimental spectra. The latter displays well-resolved vibrational features, assigned to transitions involving in-plane vibrational modes where the vibrational quantum number changes by one. Dominant transitions are associated with vibrations localized on the NH2 moieties. Experiments repeated at room temperature yield broader spectra with maxima at 435.5 ± 1nm (absorption) and 438.0 ± 1nm (fluorescence). We again conclude that prevalent fluorescence arises from S2, i.e., anti-Kasha behavior, in agreement with previous work. Excited-state lifetimes are 5 ± 1ns, independent of temperature. Importantly, we exclude the possibility that a narrow DIB at 436.4nm originates from cold proflavine cations as the band is redshifted compared to our absorption spectra.

Anisotropic Tensile Properties of a 14YWT Nanostructured Ferritic Alloy: On the Role of Cleavage Fracture

Two plates of nanostructured ferritic alloy NFA-1 were processed by ball milling atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt.%) with FeO powders, canning, and hot-extrusion at 850 °C, followed by annealing and multipass cross-rolling at 1000 °C. This produces a severe (001) brittle cleavage texture on planes running parallel to the plate faces. In the first plate (P1), pre-existing microcracks (MCs) formed on the cleavage planes during cross-rolling. The second plate (P2) contained far fewer, if any, MCs. Here, we compare the tensile data for out-of-plane (S) and in-plane (L) tensile axis orientations, at temperatures from −196 °C to 800 °C. We also assess the tensile property differences between P1 and P2, and the effect of specimen size. The L-orientation strength and ductility were excellent; for example, the room temperature (RT) yield stress, σy ≈ 1042 ± 102 MPa, and the total elongation, εt ≈ 12.9 ± 1.5%. In contrast, the S-orientation RT σy ≈ 708 ± 57 MPa, and εt ≤ 0.2%. These differences were due to cleavage on the brittle (001) planes. Cleavage leads to beneficial delamination toughening, but is deleterious to deformation processing and through-wall heat transfer. Therefore, it is important to quantitatively characterize the pronounced NFA-1 strength anisotropy due to severe crystallographic texturing and cleavage fracture.

Microstructural evolution and mechanical behavior of novel TiZrTaxNbMo refractory high-entropy alloys

The refractory high-entropy alloy (RHEA) with a body-centered-cubic (BCC) solid-solution structure has excellent high-temperature softening resistance, which has attracted wide interest in the field of high-temperature alloys. However, its limited room-temperature plasticity has greatly hindered its engineering application. To obtain an excellent strength-plasticity matching relationship, in this reported study, the Ta content in the high-entropy alloy (HEA) was adjusted to rectify this shortcoming. A series of TiZrTaxNbMo (x = 1.0, 0.9, 0.8, 0.7, and 0.6 at. percent, at%) RHEAs were prepared using the vacuum arc-melting technique, and the microstructure and mechanical properties of these RHEA alloys were systematically investigated. The experimental results show that the TiZrTaxNbMo RHEAs are composed of main BCC1 and minor BCC2 phases, which exhibit a dendritic structure. By reducing the Ta content, the elemental segregation caused by the non-equilibrium solidification is reduced. In terms of mechanical properties, with the decrease of Ta content, the hardness and room-temperature yield strength of the alloy decreases slightly, but the room-temperature plasticity increases significantly. The Ta0.7 alloy has the highest plasticity (34.8 %), which is about twice that of the equimolar Ta 1.0 alloy, while the yield strength remained at 1297 MPa. The excellent mechanical properties of the alloys can be attributed to solid-solution strengthening and the formation of moderate amounts of interdendritic regions. The interaction between slip bands and dislocations formed during compression of the Ta0.7 alloy decreased its work hardening. Moreover, the theoretical model of solid-solution strengthening elucidates that the calculated values of the alloy’s yield strength are consistent with that obtained experimentally.

Refined microstructure to improve the mechanical properties of the as-cast NiCoCr multi principal element alloy via mechanical vibration

The NiCoCr multi principal element alloy (MPEA) show excellent ductility at cryogenic temperatures and corrosion-resistance properties under the condition that it prepared by thermal–mechanical processing. However, the coarse as-cast grains negatively impact mechanical properties significantly and grain refinement is necessary for forming complex castings. Most grain refining methods need extra alloying elements, which may result in loss of ductility by brittle second phases. In this work, a significant microstructure refinement of the as-cast NiCoCr MPEA was achieved by pollution-free vacuum mechanical vibration, with the average grain size refined from the initial value of 620 μm to 140 μm. As a result, the room-temperature yield strength was increased by 54 % (from 255 MPa to 392 MPa) without loss of elongation compared to the as-cast condition without vibration.

Cryo-pre-straining contributes to achieving high yield strength in high-entropy alloys

In this study, we propose an effective and feasible strategy to significantly improve the room-temperature yield strength of the Co35Cr20Fe15Mn15Ni15 HEA by 338%, from 281 MPa (as-recrystallized, before CR) to 1231 MPa (30% CR) through cryo-pre-strain-inducing defects such as dislocations, stacking-faults (SFs), nano-twins, Lomer-Cottrell (L-C) locks, and HCP martensite, while maintaining good ductility (16.4% elongation in 30% CR). Additionally, the contribution of these defects to the yield strength enhancement was quantified. This study offers a new perspective for the preparation of advanced FCC high-entropy alloys.

Study of microstructure evolution and strengthening mechanisms in Mox(Nb3TaTi3Zr)100−x high-entropy alloys

A new series of high-entropy alloys denoted as Mox(Nb3TaTi3Zr)100−x, has been designed for aero-engine applications. The study focused on investigating the microstructural evolution and strengthening mechanisms of the alloys in response to varying Mo content. Results indicate that the alloys primarily display a BCC crystal structure. Notably, an FCC phase precipitates in the alloy when the Mo content reaches 30%. Furthermore, as the Mo content increases, the strength of the alloys significantly increases while their plasticity diminishes. The room-temperature yield strength of the alloys shows a substantial increase from 729 MPa (x = 0) to 1202 MPa (x = 25), while the yield strength at 1200 °C rises from 62 MPa (x = 0) to 498 MPa (x = 30). Concurrently, the microhardness of the alloys also increases from 247 HV (x = 0) to 471 HV (x = 30). The observed strengthening mechanisms include solid solution strengthening, fine grain strengthening, and precipitated phase strengthening.

Effect of Al content on microstructure and mechanical properties of Alx(Nb3TaTi3Zr)100-x refractory high-entropy alloys

In this study, seven refractory high-entropy alloys of Al x (Nb3TaTi3Zr)100-x (x = 0, 5, 10, 15, 20, 25, 30) were synthesized by vacuum arc melting and characterized by various techniques, including XRD, SEM, EDS, compression tests at both 1200 °C and room temperature, as well as hardness tests. The analysis revealed that the alloys exhibit a single BCC structure when the Al content is between 0 and 15%, while the B2 phase appears at 20% and a mixed structure of BCC + B2 + σ phase at 30%. As the Al content increases, the hardness, stiffness, and room temperature yield strength of the alloy increases, while the plasticity decreases. Notably, the alloy with 25% Al content displayed the highest yield strength of 1400 MPa and Young’s modulus of 75.94 GPa at room temperature. Moreover, the hardness of the seven alloys increased from 247 HV to 490 HV with Al content from 0 to 30%. Furthermore, the alloy containing 30% Al exhibited a notable elevated-temperature yield strength of 143 MPa at 1200 °C.

Unique Yttria Nanoparticle Strengthening in an Inconel 718 Superalloy Fabricated by Additive Manufacturing

AbstractOxide dispersion strengthened (ODS) nickel (Ni)‐based superalloys are advanced materials known for their outstanding tensile and creep performance at temperatures exceeding 1000 °C. Nevertheless, their conventional synthesis presents a longstanding challenge in cost‐effectively producing intricate components for critical applications. In this work, electrostatic self‐assembly (ESA) of powders with the laser powder bed fusion (LPBF) process are successfully combined to produce yttria ODS Inconel 718 (IN718) alloy for the first time. This approach demonstrates a significant contribution of yttria to the strength of IN718 after the solid solution heat treatment, as evidenced by ≈50% improvement in room temperature yield strength with a 0.5 wt.% yttria addition. The addition of yttria by this process leads to a heterogeneous microstructure. This heterogeneous microstructure comprises two distinct grain areas with varying amounts of yttria nanoparticles and dislocation storage. The results show that the yield strength increase can be predicted by the combination of both Y2O3 dispersion strengthening and dislocation strengthening mechanisms. These findings offer an effective approach to tailor heterogeneous microstructures, unlocking new opportunities for cost‐effectively producing high‐performance ODS Ni‐based superalloy products with excellent mechanical properties.

Unusually high room and elevated-temperature tensile properties observed in direct aged wire-arc directed energy deposited Inconel 718

Wire-arc directed energy deposition (DED) processed Inconel (IN) 718 is known to have coarse columnar grains, strong texture, and significant chemical and microstructural inhomogeneity in the as-fabricated condition. Homogenization treatment is commonly used prior to aging to eliminate the inhomogeneity and detrimental precipitation for better mechanical properties. In this study, however, direct aging (DA) at 700 °C without homogenization has resulted in room-temperature yield strength, ultimate tensile strength (UTS), and elongation that are comparable to wrought condition and among the highest reported properties for wire-arc DED IN718. The DA samples at between 650 and 750 °C aging also demonstrates remarkable ductility when deformed at elevated temperatures. In addition, when aged below 750 °C the DA IN718 possesses significantly higher UTS compared to those with homogenization treatment. These superior mechanical properties are highly likely due to the non-uniform and hierarchical precipitation consisting of disk-shaped γ″ in diameter from a few to tens of nm in the dendritic core area and micron-sized Laves phase and carbides in the inter-dendritic region.