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It has been shown that it is possible to obtain significant room temperature tensile ductility in FeAl alloys using iron-rich deviations from stoichiometry. A comparison of the room temperature tensile and compressive behaviors of Fe-50 at. pct Al and Fe-40 at. pct Al shows that FeAl is brittle at higher Al contents because it fractures along grain boundaries before general yielding. Lower aluminum contents reduce the yield stress substantially and hence some ductility is observed before fracture. Addition of boron results in measurable improvements in ductility of Fe-40 at. pct Al and is accompanied by an increase in transgranular tearing on the fracture surface, suggesting a grain boundary strengthening mechanism. Increasing the cooling rate following annealing at 1273 K results in a large increase in the yield strength and a corresponding decrease in ductility.
Fatigue process analysis through crack initiation and propagation and final fracture, considering ductility, tensile strength and fracture toughness roles
A room-temperature tensile elongation of about 2.5 percent, where the only slip vector observed was the 100, has been determined for a double-extruded B2 Ni-30Al-20Fe alloy consisting of recrystallized equiaxed grains about 25 microns in diameter; these results suggest that 100-slip does not preclude limited ductility in polycrystalline B2 alloys in tension at low temperatures. A suppression of ordering through resort to rapid solidification, in other words, is not necessary for ductility, since the cast and as-extruded alloy presently examined is ordered.
This study explores the use of machine learning (ML) as a computational tool to accelerate the design of multi-principal element alloys (MPEAs) with improved tensile elongation. An ML model was trained using available experimental data from the literature along with theoretically derived features to predict yield strength (YS) and ductility. A subset of ML-predicted compositions—CoNiVFe, CoNiVTi, CoNiVTiFe, and CoCrNiVTi—was synthesized and evaluated through tensile testing. The ML model underpredicted YS by approximately 20–30 % and overpredicted ductility by 60–70 % for Ti-containing alloys. Microstructural analysis revealed that Ti segregation at interdendritic regions contributed to early fracture, leading to discrepancies in ductility predictions. Ti segregation at these regions likely drives the increased YS due to segregation strengthening. In contrast, the CoNiVFe alloy showed good agreement with both experimental YS and elongation, with prediction errors of ∼10.2 % and ∼20.7 %, respectively. Microstructural characterization revealed minimal segregation in this alloy, suggesting that the ML model can reliably predict the properties of alloys with little to no segregation. These findings highlight the capability of ML in predicting YS with good accuracy but underscore its limitations in capturing defect-driven failure mechanisms such as segregation-induced embrittlement.
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Sodium exposure tests were conducted on the first commercial heat, H58776 of A709 steel. Six different processing and heat treatment conditions were investigated, including three processing conditions, namely, Argon-Oxygen-Decarburization (AOD), Electro-slag Remelting (ESR), and Electro-slag Remelting with subsequent homogenization (HOMO), and two heat treatment conditions, i.e. solution annealing at 1100°C (SA1100°C) or 1150°C (SA1150°C). Specimens of AOD-SA1100°C, AOD-SA1150°C, ESR-SA1100°C, ESR-SA1150°C, HOMO-SA1100°C, and HOMO-SA1150°C were tested in sodium at 650°C and for times up to 10,152 h. Specimens in the AOD-SA1100°C and AOD-SA1150°C conditions were also tested in sodium at 550 and 600°C. Parallel thermal aging experiments were carried out for comparison with sodium exposure tests to separate the thermal and sodium effects. It was found that A709 H58776 generally showed a weight loss after sodium exposures at 650°C except a few data points of the ESR-SA1100°C specimens. The corrosion rate showed large variations in the initial stage and decreased with increasing exposure time. Long-term exposure tests are needed to determine its steady-state corrosion rate. The AOD-SA1100°C and HOMO-SA1100°C specimens showed similar tensile responses after thermal or sodium exposures at 650°C. The yield stress increased, the ultimate tensile strength decreased, and the uniform and total elongations were reduced after thermal or sodium exposures. A longer exposure time had a minimal effect on the tensile strength but further reduced the tensile ductility. The sodium-exposed specimens had higher tensile strength than the thermally-aged specimens. In contrast, the ESR-SA1100°C specimens showed an increase in the yield stress and the ultimate tensile strength after thermal or sodium exposures at 650°C and ~10,000 h. The AOD-SA1150°C and ESR-SA1150°C specimens showed similar tensile behavior after thermal or sodium exposures at 650°C.Thermal or sodium exposures at 650°C increased the yield stress, decreased the ultimate tensile strength, and reduced uniform and total elongations. There were minimal differences in tensile properties between the sodium-exposed and thermally-aged specimens after an ~3000 h exposure. After an exposure for ~10,000 h the sodium-exposed specimens showed higher yield stress and ultimate tensile strength and lower uniform and total elongations than the thermally-aged specimens. For the HOMO-SA1150°C specimens, thermal or sodium exposures at 650°C significantly increased the yield stress but had a minimal effect on the ultimate tensile strength. Both uniform and total elongations were significantly reduced at an exposure time of ~3,000 h, and they were further reduced after an exposure of ~10,000 h. The sodium-exposed specimens had lower tensile ductility than the thermally-aged specimens. Before exposure, the stress-strain curves of the specimens in all six processing-heat treatment conditions showed serrations indicating the dynamic aging effect at 650°C. Thermal aging or sodium exposures at 650°C after ~10,000 h completely removed the serrations of the flow curves in A709 H58776, indicating strong precipitation during thermal or sodium exposures. Longer-term sodium exposure tests are needed to determine the steady-state corrosion rates in sodium for A709 H58776 and fully understand the influence of sodium exposures on its mechanical performance.
While the ultrahigh tensile ductility and superior durability of Engineered Cementitious Composites (ECC) have been demonstrated, the relatively high energy and carbon intensity, as well as high material cost present potential impediments to broader ECC applications. The objective of this research is to develop a more sustainable and cost-effective ECC. The ordinary Portland cement (OPC) and the commonly used PVA fiber in conventional ECC were replaced by limestone calcined clay cement (LC3) and polypropylene (PP) fiber, respectively. The ECC compressive strength, tensile stress-strain relationship, and microcrack self-healing behavior were studied at three water to binder ratios (0.3, 0.2, 0.16). The novel LC3-PP-ECC showed a tensile strain capacity of greater than 6% and an intrinsically tight crack width below 82 μm when loaded to 1% tensile strain. Further, the LC3-PP-ECC demonstrated efficient recovery of the composite tensile ductility and ultimate tensile strength through self-healing. Compared to typical ECC made with OPC and PVA fiber, the material cost, embodied energy and carbon footprint of LC3-PP-ECC are reduced by 61%, 45%, and 48%, respectively. Here, the superior mechanical properties and durability combined with the low environmental impact and cost for material production promote LC3-PP-ECC as a sustainable material for structural and non-structural applications.
Austenitic stainless steels are used extensively in harsh environments, including for high-pressure gaseous hydrogen service. However, the tensile ductility of this class of materials is very sensitive to materials and environmental variables. While tensile ductility is generally insufficient to qualify a material for hydrogen service, ductility is an effective tool to explore microstructural and environmental variables and their effects on hydrogen susceptibility, to inform understanding of the mechanisms of hydrogen effects in metals, and to provide insight to microstructural variables that may improve relative performance. In this study, hydrogen precharging was used to simulate high-pressure hydrogen environments to evaluate hydrogen effects on tensile properties. Several austenitic stainless steels were considered, including both metastable and stable alloys. Room temperature and subambient temperature tensile properties were evaluated with three different internal hydrogen contents for type 304L and 316L austenitic stainless steels and one hydrogen content for XM-11. Significant ductility loss was observed for both metastable and stable alloys, suggesting the stability of the austenitic phase is not sufficient to characterize the effects of hydrogen. Internal hydrogen does influence the character of deformation, which drives local damage accumulation and ultimately fracture for both metastable and stable alloys. While a quantitative description of hydrogen-assisted fracture in austenitic stainless steels remains elusive, these observations underscore the importance of the hydrogen-defect interactions and the accumulation of damage at deformation length scales.
Traditional PVA fiber-reinforced Engineered Cementitious Composites (ECC) show high tensile ductility and superior durability with tight crack width, but the high cost and embodied carbon can hinder its wider application in infrastructures. The objective of this study is to develop a better understanding of the fresh and hardened properties of an ECC that employs a lower embodied-carbon binder, Limestone Calcined Clay Cement (LC3), and lower-cost PP fiber that is widely available. Specifically, the interrelations between material processing, microstructure, and composite properties were studied experimentally. The results showed that ECC with high tensile ductility up to 9% tensile strain and tight crack width with 50 μm at 2% tensile strain can be achieved. It was found that a matrix paste with higher viscosity generally enhanced fiber dispersion uniformity and robustness in tensile strain-hardening. The paste viscosity is increased when OPC is replaced by LC3 and can be tuned with superplasticizer content. Larger maximum flaw size leads to lower first crack strength, beneficial for microcrack initiation and multiple cracking. This study generates fundamental knowledge linking processing-microstructure-performance of PP-LC3-ECC. This class of low embodied carbon ECC with tight crack width is expected to contribute to reducing the carbon footprint of the built environment.
The deformation properties of an extruded Ni-30Al-20Fe-0.05Zr (at. pct) alloy in the temperature range 300-1300 K were investigated under initial tensile strain rates that varied between 10 exp -6 and 10 exp -3/sec and in constant load compression creep between 1073 and 1300 K. Three deformation regimes were observed: region I, occurring between 400 and 673 K, which consisted of an athermal regime of less than 0.3 percent tensile ductility; region II, between 673 and 1073, where exponential creep was dominant; and region III, between 1073 and 1300 K, where a significant improvement in tensile ductility was observed.
High-performance refractory alloys with ultrahigh strength and ductility are in demand for a wide range of critical applications, such as plasma-facing components. However, it remains challenging to increase the strength of these alloys without seriously compromising their tensile ductility. Here, we put forward a strategy to “defeat” this trade-off in tungsten refractory high-entropy alloys by stepwise controllable coherent nanoprecipitations (SCCPs). The coherent interfaces of SCCPs facilitate the dislocation transmission and relieve the stress concentrations that can lead to premature crack initiation. As a consequence, our alloy displays an ultrahigh strength of 2.15 GPa with a tensile ductility of 15% at ambient temperature, with a high yield strength of 1.05 GPa at 800 °C. The SCCPs design concept may afford a means to develop a wide range of ultrahigh-strength metallic materials by providing a pathway for alloy design.
Carbonated MgO-based Engineered Cementitious Composite (ECC) has ultrahigh tensile ductility and tight crack width control behavior. However, the expectation of improved fire-resistant has not been confirmed. This study explored the alterations to mechanical and microstructure characteristics of this material after exposure to temperatures up to 500 °C. Material mass loss, compressive strength, tensile strength, strain capacity, and matrix fracture toughness were measured. Scanning electron microscopy, mercury intrusion porosimetry, thermogravimetric analysis, and X-ray diffraction were used to probe the degradation of the cement matrix and fibers. The effect of elevated temperature on carbonated MgO-based ECC was further assessed via examination of the micromechanical behavior of the fiber-matrix. The objective of this research was to assess the performance of carbonated MgO-based ECC exposed to fire hazards. The tensile ductility of carbonated MgO-based ECC was found to be enhanced when exposed to ~100 °C as compared with those at room temperature ~20 °C. Further increase in exposure temperature, however, posed a negative impact on the composite compressive strength, ultimate tensile strength, and ability to control crack width. Here the results provide a useful database for further investigations into carbonated MgO-based ECC for fire safety enhancement.
Eighteen plasma sprayed coating systems, nine based on the NiCoCrAly chemistry and nine based on the CoCrAly composition, were evaluated to identify coating systems which provide equivalent or superior life to that shown by the electron beam physical vapor deposited NiCoCrAly and CoCrAly coatings respectively. NiCoCrAly type coatings were examined on a single crystal alloy and the CoCrAly based coatings were optimized on the B1900+ Hf alloy. Cyclic burner rig oxidation and hot corrosion and tensile ductility tests used to evaluate the various coating candidates. For the single crystal alloy, a low pressure chamber plasma sprayed NiCoCrAly + Si coating exhibited a 2x oxidation life improvement at 1394 K (2050 F) over the vapor deposited NiCoCrAly material while showing equivalent tensile ductility. A silicon modified low pressure chamber plasma sprayed CoCrAly coating was found to be more durable than the baseline vapor deposited CoCrAly coating on the B1900+ Hf alloy.
The effect of substituting the Mg metal in Mg-B composites by a Mg-14 wt pct Li solid solution on the ductility of the resulting composite was investigated using elastic modulus measurements on the P/M composite material prepared with a dispersion of B particles (in a vol pct range of 0-30) in a matrix of Mg-14 wt pct Li-1.5 wt pct Al. It was found that the elastic modulus of the composites increased rapidly with increasing boron, with specific stiffness values reaching about two times that of most structural materials. The values of the compression and tensile strengths increased significantly with boron additions. Good tensile ductility was achieved at the level of 10 vol pct B. However, at 20 vol pct B, the Mg-Li composite exhibited only limited tensile ductility (about 2 percent total elongation).
Eighteen plasma sprayed coating systems, nine based on the NiCoCrAlY chemistry and nine based on the CoCrAlY composition, were evaluated to identify coating systems which will provide equivalent or superior life to that shown by the electron beam physical vapor deposited NiCoCrAlY and CoCrAlY coatings respectively. NiCoCrAlY-type coatings were examined on a single crystal alloy and the CoCrAlY based coatings were optimized on the B1900 + Hf alloy. Cyclic burner rig oxidation and hot corrosion and tensile ductility tests were used to evaluate the various coating candidates. For the single crystal alloy, a low pressure chamber plasma sprayed NiCoCrAlY + Si coating exhibited a 2X oxidation life improvement at 1121 C (2050 F) over the vapor deposited NiCoCrAlY material while showing equivalent tensile ductility. A silicon modified low pressure chamber plasma sprayed CoCrAlY coating was found to be more durable than the baseline vapor deposited CoCrAlY coating on the B1900 + Hf alloy.