Ambient to high-temperature fracture toughness and cyclic fatigue behavior in Al-containing silicon carbide ceramics

A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt.%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth resistance at both ambient (25 °C) and elevated (1300 °C) temperatures. Indeed, no evidence of creep damage, in the form of grain-boundary cavitation, was seen at temperatures at 1300 °C or below. The enhanced toughness of the higher Al-containing materials was associated with extensive crack bridging from both interlocking grains (in 3ABC) and uncracked ligaments (in 5ABC); in contrast, the 7ABC SiC showed no such bridging, concomitant with a marked reduction in the volume fraction of elongated grains. Mechanistically, cyclic fatigue-crack growth in 3ABC and 5ABC SiC involved the progressive degradation of such bridging ligaments in the crack wake, with the difference in the degree of elastic vs. frictional bridging affecting the slope, i.e. Paris law exponent, of the crack-growth curve.     

High-temperature cyclic fatigue-crack growth behavior in an in situ toughened silicon carbide

The growth of fatigue cracks at elevated temperatures (25–1300°C) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with Al–B–C additions (termed ABC–SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements from an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equivalent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1200°C; behavior at 1300°C, however, was similar to that at 1200°C. Moreover, no effect of frequency was detected (between 3 and 1000 Hz), nor evidence of creep cavitation or crack bridging by viscous ligaments or grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1200–1300°C that was essentially identical to that at room temperature, i.e. predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attributed to a process of grain-boundary microstructural evolution at elevated temperatures, specifically involving crystallization of the amorphous grain-boundary films/phases.     

Deformation behavior of a two-phase Mo–Si–B alloy

Mo–Si–B alloys are being considered as possible candidates for high-temperature applications beyond the capabilities of Ni-based superalloys. In this paper, the high-temperature (1000–1400 °C) compression response over a range of quasi-static strain rates, as well as the monotonic and cyclic crack growth behaviors (as a function of temperature from 20 °C to 1400 °C) of a two-phase Mo–Si–B alloy containing a Mo solid solution matrix (Mo(Si,B)) with ∼38 vol% of the T2 phase (Mo₅SiB₂) is discussed. Analysis of the compression results confirmed that deformation in the temperature–strain-rate space evaluated is matrix-dominated, yielding an activation energy of ∼415–445 kJ/mol. Fracture toughness of the Mo–Si–B alloy varies from ∼8 MPa√m at room temperature to ∼25 MPa√m at 1400 °C, the increase in toughness with temperature being steepest between 1200 °C and 1400 °C. S–N response at room temperature is shallow whereas at 1200 °C, a definitive fatigue response is observed. Fatigue crack growth studies using R = 0.1 confirm the Paris slope for the two alloys to be high at room temperature (∼20–30) but decreases with increasing temperature to ∼3 at 1400 °C. The crack growth rate (da/dN) for a fixed value of ΔK in the Paris regime in the 900–1400 °C range, increases with increasing temperature.     

Structure and fracture mechanism of a two-phase chromium–nickel alloy during high-temperature deformation

The structure and mechanical properties of a two-phase Kh65N33V2FT alloy has been studied after tests at room and high temperatures. The morphology of the main phases, namely, solid solutions of nickel in chromium (α) and chromium in nickel (γ), is changed depending on temperature. The lattice parameters of the main phases have been determined. The main mechanism of deformation for this alloy is shown to be grain-boundary sliding. Bulk and grain-boundary diffusion creep and self-regulating diffusion-viscous flow is possible in the γ phase during high-temperature deformation. The heat resistance of this alloy is restricted to 1000°C because of the formation of a γ-phase percolation cluster.     

Effect of Tempering Temperature on the Microstructure and Properties of Fe-2Cr-Mo-0.12C Pressure Vessel Steel

To obtain the high-temperature strength and toughness of the medium–high-temperature–pressure steel, the microstructure evolution and mechanical properties of Fe-2Cr-Mo-0.12C steel subjected to three different tempering temperatures after being normalized were investigated. The results show that the microstructure of the sample, tempered in the range 675-725 °C for 50 min, did not change dramatically, yet the martensite/austenite constituents decomposed, and the bainite lath merged together and transformed into polygonal ferrite. At the same time, the precipitate size increased with an increase in tempering temperature. With the increase in the tempering temperature from 675 to 725 °C, the impact absorbed energy of the Fe-2Cr-Mo-0.12C steel at ?40 °C increased from 257 to 325 J, and the high-temperature yield strength decreased; however, the high-temperature ultimate tensile strength tempered at 700 °C was outstanding (422-571 MPa) at different tested temperatures. The variations of the properties were attributed to the decomposition of M/A constituents and the coarsening of the precipitates. Fe-2Cr-Mo-0.12C steel normalized at 930 °C and tempered at 700 °C was found to have the best combination of ductility and strength.     

Microstructural evolution and mechanical properties of as-cast and directionally-solidified Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys at room and high temperatures

Arc-melting (AC) and directional solidification (DS) techniques were used to prepare Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys (hereafter referred as to 2Cr and 14Cr alloys, respectively), and the microstructural evolution and mechanical properties, including Vickers hardness, room temperature fracture toughness and high temperature strength, of the two AC and DS alloys were compared. The results showed that with heat-treatment at 1350 °C for 50 h, the AC-2Cr alloy composed of Nb solid solution (Nb_SS) and α-Nb₅Si₃ silicide, while Laves C15-Cr₂Nb phase arose in the 14Cr alloy. With two-phase Nb_SS/α-Nb₅Si₃ microstructure, the AC-2Cr alloy showed excellent room-temperature fracture toughness (K_Q: 14.2 MPa m^1/2) and 0.2% yield strength at 1250 °C (σ_0.2: 315 MPa) and 1350 °C (σ_0.2: 294 MPa), better than the AC-14Cr alloy with tri-phase Nb_SS/α-Nb₅Si₃/C15-Cr₂Nb microstructure (K_Q: 9.4 MPa m^1/2, σ_0.2: 189 MPa at 1250 °C and 87 MPa at 1350 °C). The DS technique was found not to change the phase constituent of each alloy, but it made the microstructure slightly orient to the growth direction, resulting in a significant improvement in room-temperature fracture toughness (by ∼43%) and high-temperature yield strength σ_0.2 (by ∼55%), as compared with the AC samples.     

Damage tolerance of wrought alloy 718 Ni- Fe-base superalloy

The influence of a modified heat treatment (MHT) and the standard heat treatment (SHT) on the damage tolerance of alloy 718 turbine disk material has been studied over a range of temperatures— from room temperature to 650 °. The influence of these heat treatments on creep, low-cycle fatigue (LCF), notch sensitivity, cyclic stability, and fatigue crack growth rate (FCGR) properties has been studied. The microstructure developed through the MHT sequence is shown to be damage tolerant over the temperature range studied. Shot peening leads to a marked improvement in the LCF crack initiation life of the MHT material relative to the SHT material at 650 °. Serrated grain boundaries formed through controlled precipitation of grain-boundary 5 phase are beneficial to elevated- temperature FCGRs. The S-phase precipitates formed at an angle to the grain boundaries do not make the material notch sensitive.     

Improvement of mechanical properties of the Ti–45Al–5Nb–1Mo–0.2B (аt %) intermetallic alloy by means of microstructure controlling

The effect of heat and thermomechanical treatments conditions on the microstructure and main mechanical characteristics (obtained by tensile, high-temperature long-term strength, fracture toughness, and high-cycle fatigue tests) of the Ti–45Al–5Nb–1Mo–0.2B (аt %) alloy was studied. Before the treatments, the sequence of phase transformations in the alloy after its solidification was determined by testquenching method. The obtained data were used to develop conditions for the heat and thermomechanical treatments. It was found that a small but stable increase in the plasticity and strength of the cast alloy is observed after three-stage annealing at temperatures that correspond to the (α + γ)- and (α₂ + β(В₂) + γ)-phase region. The thermomechanical treatment at temperatures corresponding to the (α(α₂) + β(В₂) + γ)-phase region and subsequent two-stage annealing at temperatures that correspond to the (α + β(В₂) + γ)- and (α₂ + β(В₂) + γ)-phase region lead to the formation of fine-grained duplex structure. This determined the substantial improvement of the low-temperature plasticity and strength (δ = 3.1% and σ_u = 860 MPa at 20°C, respectively) and retained high creep resistance to 700°C.     

Study of characteristics and properties of spark plasma sintered WC with the use of alternative Fe-Ni-Nb binder as Co replacement

Most of all WC-based cemented carbides use cobalt as binder due to the excellent strength and ductility that this combination provides. Motivators to find alternative binders have been related to factors such as the shortage and price oscillations of the cobalt and toxicity of the WC-Co system. In this work, Fe-Ni-Nb was used as alternative binder for WC sintered via spark plasma sintering (SPS) technique. The composites were sintered at different sintering temperatures (1100 °C, 1200 °C and 1300 °C). In addition, WC-Co was sintered at 1200 °C via SPS for comparison purposes. X-ray diffraction and Scanning electron microscopy (SEM) were employed as characterization methods to investigate the crystalline phase's formation, sintering effectiveness, porosity and phase distribution. Mechanical properties such as Vickers hardness, fracture toughness, nanohardness, elastic modulus and thermal properties (thermal expansion coefficient) were evaluated. The results demonstrate Fe-Ni-Nb as a viable alternative binder to cobalt in hardmetal applications.     

The superior thermal stability and tensile properties of hot rolled W-HfC alloys

Herein, a W-0.5wt%HfC (WHC05) alloy is fabricated by conventional sintering and multi-step hot-rolling. The high-temperature stability and tensile properties at different temperatures, ranging from room temperature to 600 °C, are studied to demonstrate the influence of HfC addition. The results reveal that the WHC05 alloy has a higher recrystallization temperature (1400 °C–1500 °C) than the previously reported as-rolled pure W (1200 °C) and as-rolled W-0.5wt%ZrC (WZC05 ~ 1300 °C) alloy. Moreover, after recovery and recrystallization (annealing at 1600 °C), the WHC05 alloy maintained a high ultimate tensile strength of ~300 MPa and exhibited a desirable increase in total elongation (>35%), which is ~1.6 times higher than the recrystallized WZC05 at 500 °C. The superior thermal stability and excellent high-temperature mechanical properties can be ascribed to the unique microstructure and uniform dispersion of nano-sized HfC particles in W matrix. The influence of annealing temperature on grain structure, grain orientation and distribution of nano-sized HfC particles has been studied to unveil the possible mechanism of enhanced thermal stability and superior mechanical properties.     

Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.     

Tensile,Creep, and Fatigue Behaviors of 3D-Printed Acrylonitrile Butadiene Styrene

Acrylonitrile butadiene styrene (ABS) is a widely used thermoplastics in 3D printing. However, there is a lack of thorough investigation of the mechanical properties of 3D-printed ABS components, including orientation-dependent tensile strength and creep fatigue properties. In this work, a systematic characterization is conducted on the mechanical properties of 3D-printed ABS components. Specifically, the effect of printing orientation on the tensile and creep properties is investigated. The results show that, in tensile tests, the 0° printing orientation has the highest Young’s modulus of 1.81 GPa, and ultimate strength of 224 MPa. In the creep test, the 90° printing orientation has the lowest k value of 0.2 in the plastics creep model, suggesting 90° is the most creep resistant direction. In the fatigue test, the average cycle number under load of 30 N is 3796 cycles. The average cycle number decreases to 128 cycles when the load is 60 N. Using the Paris law, with an estimated crack size of 0.75 mm, and stress intensity factor is varied from 352 to 700 \(N\sqrt m\), the derived fatigue crack growth rate is 0.0341 mm/cycle. This study provides important mechanical property data that is useful for applying 3D-printed ABS in engineering applications.     

The effect of W and N addition on the mechanical properties of 10Cr steels

The effect of W and N on the creep properties and microstructural degradation in 10Cr steels was studied. Creep testing was performed to determine the creep rupture strength and minimum creep rate. Transmission electron microscopy was used to observe the microstructural degradation during the creep deformation. W and N which were added to the 10Cr steel increased the creep rupture strength and decreased the minimum creep rate. As W and N were added, the thermal stability of the subgrain and carbide was improved, thus the growth of the subgrain and carbide during creep deformation was restricted. In W added steel, the Laves phase played an important role in increasing creep rupture strength. But the impact toughness was rapidly degraded by the addition of W after aging at 600°C for 5000 hours. So one must evaluate more accurately the effect of the Laves phase on long term creep and impact properties. In N added steel, V(C, N) was precipitated in the lath boundary and within the lath. The size of the precipitates was 20–50 nm. The increase of creep rupture strength in N added steel may be due to the precipitate of the V(C, N). Future tests are required to clarify the effect of N on creep and impact properties.     

Stability of composite materials NiAl-refractory metal with cellular structure

The structure and mechanical properties of NiAl-Wand NiAl-W-Mo composite materials (CM) obtained by sintering from powders are studied. Comparative analysis of the effect of hot compressive deformation of a compact material at 1000–1300°C on the integrity of the microspecimens and of the tungsten shells on NiAl granules in CM with cellular structure is performed. The thermokinetic stability of the grain structure of unalloyed nickel aluminide NiAl and of a NiAl-W composite material with cellular structure is investigated. The temperature of the beginning of recrystallization of the NiAl intermetallic is determined. A map of structural states is plotted in the “temperature-operating time” coordinates for CM with cellular structure. The local chemical composition of the “NiAl-refractory metal” phase boundary is studied in CM with cellular structure and without it. The effect of the structural state of CM on the yield strength in compressive tests at 1000°C is determined. The oxidation resistance at 1000–1300°C is studied and a treatment approaching the oxidation resistance of CM with cellular structure at 1000–1300°C to the level of high-temperature strength of unalloyed NiAl and of its alloy with 4 wt.% Hf is suggested. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 25–30, June, 2006.     

Microstructural evolution and mechanical properties of forged β-solidified γ-TiAl alloy by different heat treatments

The microstructural evolution and tensile properties of a forged Ti?42Al?5Mn alloy subjected to different heat treatments were studied. The results showed that, when the forged alloy was aged at 800 °C for 24 h, the interlamellar spacing (λ) and γ grain size at colony boundaries are generally coarsened. Whereas, when the alloy was first annealed at 1300 °C and then aged at 800 °C for 24 h, this coarsening of related microstructures appears less pronounced. The suggested annealing temperatures for the forged Ti?42Al?5Mn alloy are in the range of 1250?1300 °C. It was found that, on the condition of the same annealing system, both the strength and ductility were improved as the aging temperature changed from 1000 to 800 °C. The secondary precipitated β_o (β_o,sec) at colony boundaries could be responsible for improving the strength, and the γ phase at colony boundaries with the grain size about 6 μm might be one of the main reasons for the better ductility.     

Phase reactions in the Nb–N system below 1400°C

In continuation of a recent study on high-temperature nitridation of niobium the phase equilibria of the Nb–N system were investigated for T≤1400°C by means of diffusion couples, electron probe microanalysis (EPMA) and differential scanning calorimetry (DSC). The γ-Nb₄N_3±x→δ-NbN_1−x phase transition was investigated as a function of composition and occurs at temperatures between 1070°C (45.2 at.% N) and 1225°C (38.9 at.% N). The equilibrium composition of γ-Nb₄N_3±x in this temperature interval is—depending on the temperature—only 42–44.0 at.% N, hence the transition occurs also in samples with a non-equilibrium composition. It can only be investigated by using in situ methods such as DSC and high-temperature X-ray diffraction because of the fast transition rate which does not permit quenching and which is responsible for the contradictory results in the literature. The congruent transformation η-NbN→δ-NbN_1−x was observed between 1300 and 1320°C. Homogeneity ranges were measured from the phase bands of the diffusion couples by means of EPMA and the results are presented in the form of a phase diagram.     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Influence of heat treatment on mechanical properties of 51CrV4 high strength spring steel

Abstract

The durability of the springs is limited by plastic deformation, fatigue and fracturing. From this point of view, the use of spring steel with following properties is recommended: high ductility and toughness at operation temperatures from ?40°C to +50°C, good hardenability that provides required mechanical properties even at maximum dimensions. For the manufacturers of springs, the information relating to the heat treatment of specific spring steel is important. This paper describes the influence of heat treatment parameters on tensile strength R_m, yield strength R_p0·2, fracture toughness K_Ic, impact toughness, Charpy-V as a function of tempering temperature in the range from 350 to 700°C for a specific austenitising temperature. Also the difference between the properties given by the mathematical modelling of heat treatment using the computer software Hardenability and the properties obtained by testing the heat treated samples are presented.     

Effects of deviations from stoichiometry on the deformation behaviour of FeAl at intermediate temperatures

The stress–strain behaviour and creep of polycrystalline and monocrystalline FeAl phases with various deviations from stoichiometry was studied at temperatures up to 1000°C with emphasis on intermediate temperatures between 600°C and 1000°C. The results revealed strong effects of impurity precipitates besides the strong effects of the point defects. The transition of FeAl from low-temperature behaviour to high-temperature behaviour occurs at about 1000°C for 10⁻³ s⁻¹ compression rate and at lower temperatures for lower rates. Creep was observed with subgrain formation and stress exponents primarily in the range 4–5. The apparent activation energy for creep is of the order of the activation energy for bulk diffusion only above 950°C, whereas much higher activation energies were observed for creep at lower temperatures. Stress–strain behaviour with yield points was observed in the intermediate temperature range 600–1000°C.     

Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect

Due to its high strength and low temperature ductility, tungsten fibers (W_f) have been widely used as reinforcement elements in metallic, ceramic and glass matrix composites to improve the strength, toughness and creep resistance. Materials designed for future fusion reactors also utilize the option of W_f reinforcement, i.a. with a copper (W_f/Cu) or tungsten (W_f/W) matrix. W_f/W composites are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to embrittlement induced by neutrons resulting from fusion reaction. Operation of W_f/W in high temperatures (up to 1300 °C and even higher) fusion environment implies a risk of recrystallization and grain growth, which dimishes the attractive properties of tungsten fibers. In this work, we assess this modification of micro-mechanical and microstructural properties of tungsten fibers by means of nanoindentation, scanning electron microscopy, electron back-scattering diffraction analysis and corelate it with the ultimate tensile strength and fracture modes observed in the tensile tests. Both pure W and pottasium doped wires in the as-fabricated and annealed states are investigated and the results are compared with bulk tungsten, also exposed to several annealing temperatures. The results highlight the postive impact of potassium doping which shifts the threshold temperature for the grain growth by about 600 °C compared to pure tungsten wire. The results of the nanoindentation revealed systematic linear correlation with the ultimate tensile strength, which therefore offers a complimenatary way of micro-mechanical testing linking it with macro-scale properties of the wires.