Microstructures and mechanical properties of NbCr2 and ZrCr2 Laves phase alloys prepared by powder metallurgy

Microstructures, mechanical properties and oxidation behavior were investigated on NbCr₂ and ZrCr₂ Laves phase alloys prepared by powder metallurgy (P/M), and also by arc-melting, i.e. ingot metallurgy (I/M). These properties were also evaluated, in terms of alloying, heat treatment and alloy stoichiometry. High-temperature yield strength and brittle ductile transition temperature (BDTT) were generally lower in alloys prepared by P/M process than in those prepared by I/M process while micro hardness and fracture toughness were higher in alloys prepared by P/M process than in those prepared by I/M process, irrespective of NbCr₂ or ZrCr₂ alloys. Also, high-temperature strength and micro hardness were higher in NbCr₂ alloys than in ZrCr₂ alloys while fracture toughness was lower in NbCr₂ alloys than in ZrCr₂ alloys, irrespective of P/M or I/M process. For oxidation behavior at 1223 K, NbCr₂ alloys showed linear increase with increasing time accompanied with irregular fluctuation, while ZrCr₂ alloys showed parabolic increase with increasing time. It was also found that alloy stoichiometry greatly affected micro hardness, fracture toughness and oxidation behavior in ZrCr₂ alloys.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃, is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.When microstructure and composition are optimized (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

Mechanical properties and oxidation resistance of C–B–Si coated silicon nitride fiber reinforced Si–N–C composites with cross-ply structure

To enhance the oxidation resistance of a ceramic matrix composite, a C–B–Si interface layer was applied between the fiber and the matrix. The layer was deposited on the fiber by chemical vapor deposition. Three types of coatings were prepared: A1, A2 (multilayers of graphite layer/B–C–Si crystalline layer/graphite layer) and B1 (monolayer of B and C containing graphite). The multilayer coated CMC retained 88–97% of the original strengths after oxidation at 1523 K for 36 ks. The monolayer coated CMC degraded to 55% of its original strength after oxidation, but had a high fracture toughness (28 MPa m^1/2) before oxidation. The differences of the oxidation resistance and fracture toughness were discussed in relation to the microstructure of the coatings.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Microstructures and fracture toughness of directionally solidified Mo–ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (Mo_SS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness K_Q evaluated by three point bending test using the single edge notched beam method is >13 MPa m^1/2 for AMC, 20 MPa m^1/2 for SMC and 9 MPa m^1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of Mo_SS phase in FZC leads to brittle {100} cleavage fracture associated with low fracture toughness.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Impact toughness and fractography of Al–Si–Cu–Mg base alloys

Critical automotive applications using heat-treatable alloys are designed for high impact toughness which can be improved using a specified heat treatment. The alloy toughness and fracture behavior are influenced by the alloy composition and the solidification conditions applied. The mechanical properties of alloys containing Cu and Mg can also be enhanced through heat treatment. The present study was undertaken to investigate the effects of Mg content, aging and cooling rate on the impact toughness and fractography of both non-modified and Sr-modified Al–Si–Cu–Mg base alloys. Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with dendrite arm spacing (DAS) values of 24 and 50 μm, respectively. Test bars were aged at 180 °C and 220 °C for 2–48 h. Charpy Impact test was used to provide the impact energy. It was observed that high cooling rates improve the impact toughness whereas the presence of Cu significantly lowers the impact properties which are determined mainly by the Al₂Cu phase and not by the eutectic Si particles. The addition of Mg and Sr were also seen to decrease the impact toughness. The crack initiation energy in these alloys is greater than the crack propagation energy, reflecting the high ductility of Al–Si–Cu–Mg base alloys.     

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 10⁷ cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120 MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses K_IC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3 MPa m^1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Enhanced sintering ability of biphasic calcium phosphate by polymers used for bone scaffold fabrication

Biphasic calcium phosphate (BCP), which is composed of hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] and β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂], is usually difficult to densify into a solid state with selective laser sintering (SLS) due to the short sintering time. In this study, the sintering ability of BCP ceramics was significantly improved by adding a small amount of polymers, by which a liquid phase was introduced during the sintering process. The effects of the polymer content, laser power and HAP/β-TCP ratios on the microstructure, chemical composition and mechanical properties of the BCP scaffolds were investigated. The results showed that the BCP scaffolds became increasingly more compact with the increase of the poly(l-lactic acid) (PLLA) content (0–1 wt.%) and laser power (6–10 W). The fracture toughness and micro-hardness of the sintered scaffolds were also improved. Moreover, PLLA could be gradually decomposed in the late sintering stages and eliminated from the final BCP scaffolds if the PLLA content was below a certain value (approximately 1 wt.% in this case). The added PLLA could not be completely eliminated when its content was further increased to 1.5 wt.% or higher because an unexpected carbon phase was detected in the sintered scaffolds. Furthermore, many pores were observed due to the removal of PLLA. Micro-cracks and micro-pores occurred when the laser power was too high (12 W). These defects resulted in a deterioration of the mechanical properties. The hardness and fracture toughness reached maximum values of 490.3 ± 10 HV and 1.72 ± 0.10 MPa m^1/2, respectively, with a PLLA content of approximately 1 wt.% and laser power of approximately 10 W. Poly(l-lactic-co-glycolic acid) (PLGA) showed similar effects on the sintering process of BCP ceramics. Rectangular, porous BCP scaffolds were fabricated based on the optimum values of the polymer content and laser power. This work may provide an experimental basis for improving the mechanical properties of BCP bone scaffolds fabricated with SLS.     

Formation mechanism of NbSi2–Al2O3 nanocomposite subject to mechanical alloying

Silicide compounds such as NbSi₂ have many desirable properties such as high melting point, high resistance to oxidation and suitable electrical conductivity. However, they have limited practical use because of low ductility. To overcome this limit, we produced NbSi₂ based nanocomposite containing Alumina second phase by an exothermic reaction between Al and Nb₂O₅ in mechanical alloying of Al–Nb₂O₅–Si system. Structural and phase evolution throughout milling were investigated by using X-ray diffraction and microscopy methods. It followed that after 10 h of MA, the reaction between Al and niobium oxide began in a gradual mode and after around 40 h of milling; the reaction was successfully completed. The final product consisted of NbSi₂ intermetallic compound and nanocrystalline Al₂O₃ with a grain size of 15 and 45 nm, respectively. Microhardness and fracture toughness of nanocomposite were also measured which are greater than NbSi₂ intermetallic. As the result of this research we showed that high strength together with increased ductility could be gained in nanocomposite compounds.     

Effects of Si addition on mechanical properties and superelasticity of Ti–7.5Nb–4Mo–2Sn shape memory alloy

Effects of 0–2.1 at.% Si additions on microstructure and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–7.5 at.%Nb–4 at.%Mo–2 at.% Sn (Ti–7.5Nb–4Mo–2Sn), were investigated. The alloys after annealing at 973 K mainly contain β and α″. As the concentration of Si is higher than 1 at.%, Ti₅Si₃ particles can be found in the alloys, and the number density of the particles increases with the increasing of silicon’s concentration. The addition of Si promotes the strength of the Ti–7.5Nb–4Mo–2Sn due to the Si solid solution strengthening effect and fine Ti₅Si₃ precipitates. However, as the Si concentration reaches 2.1%, the alloy exhibits a brittle fracture. The 0.5–1.6 at.% Si additions improve the superelasticity of the Ti–7.5Nb–4Mo–2Sn alloy by increasing the critical stress for inducing martensite (σ_SIM).     

Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels

A new alumina-forming austenitic stainless steel with greatly improved high-temperature oxidation resistance and strength was developed via alloying 3.0 wt.% Al in the Fe-25Ni-18Cr based alloy. Continuous, stable and exclusive alumina scale was formed in either dry air or air with 10% water vapor mixed environment at 800 °C. The long-term high-temperature oxidation performance is appreciably enhanced which is associated with the high density of the B2-NiAl precipitation phase maintaining the Al₂O₃ surface layer. Moreover, when tested at 750 °C in dry air environment, the new steel showed high yield and fracture tensile strength of 310–335 and 480–500 MPa, respectively.     

Mechanical behaviour of zirconia–mullite directionally solidified eutectics

Laser floating zone technique (LFZ) is used to grow directionally solidified eutectic (DSE) zirconia–mullite composite fibres (30:70 in wt.%). A notable increase in hardness is observed from 11.3 to 21.2 GPa as the pulling rate increases from 10 to 500 mm/h, due to the ultra-fine eutectics developed at very high growth rates. The indentation fracture toughness reaches a maximum value of 3.5 MPa m^1/2 for the fibre pulled at 100 mm/h, almost three times the value of 1.2 MPa m^1/2 determined for LFZ single-crystal mullite. The eutectic dendrites that develop along the growth direction are immersed in a glassy phase whose brittleness is counteracted by the beneficial ultra-fine morphology, giving a bending strength maximum of 534 MPa. Yet, the soft nature of the glassy matrix prevails at the high temperature testing (1400 °C), causing a decrease to about one-half of the RT value in the fibres with less glassy phase content.     

Effects of solids loading on microstructure and mechanical properties of HfB2–20 vol.% MoSi2 ultra high temperature ceramic composites through aqueous gelcasting route

HfB₂–20 vol.% MoSi₂ ultra high temperature ceramic composites were prepared through aqueous gelcasting route. The stability of HfB₂ and MoSi₂ suspensions were studied by zeta potential measurements, sedimentation tests and apparent viscosity measurements. The solids loading had significant effects on the green and sintered densities, microstructure and mechanical properties of HfB₂–MoSi₂ composites. The values of flexural strength of the green and sintered bodies ranged from 18.3 to 38.7, and 111.5 to 415.9 MPa, respectively, which were strongly dependent on the solids loading. The values of fracture toughness of the sintered bodies ranged from 2.18 to 4.24 MPa m^1/2. The highest relative density, mechanical properties and the most homogeneous microstructure was obtained when the solids loading was 45 vol.%. The highest green strength, flexural strength and fracture toughness were 38.7 ± 5.3 MPa, 415.9 ± 17.0 MPa and 4.24 ± 0.22 MPa m^1/2, respectively.     

Strength and toughness: The challenging case of TaC-based composites

This work aims at studying the relationships between strength and toughness of tantalum carbide (TaC) ceramics, a refractory ceramic used in aerospace and energy production sectors. The effect of different secondary phases was explored: (I) the addition of a transition metal silicide with suited thermo-elastic properties, TaSi₂, (II) the addition of SiC particles, platelets or fibers, and (III) chopped carbon fibers. Microstructural analyses, performed by scanning and transmission electron microscopy, were essential in revealing at nanoscale level the morphological changes occurred during sintering in the reinforcing phase and its interaction with matrix and sintering additive. Mechanisms of reinforcement evolution are suggested accordingly. Fracture toughness and flexural strength were measured and the values were compared to unreinforced materials and discussed in agreement to the microstructural features. Strength approaching 1 GPa was obtained upon addition of SiC particles, but residual thermal stresses prevented from notable increase of toughness, which fluctuated around 4 MPa √m. A good compromise between strength and toughness was found for addition of Hi-Nicalon SiC fiber, 550 MPa and 5.3 MPa √m, respectively. More refractory SiC fibers resulted not effective, owing to the rising of tensional state in the matrix. On the other hand, TaSi₂ led to a toughness of 4.7 MPa √m and strength around 680 MPa. Conversely, carbon fiber led to poor toughness due to unfavorable combination of coefficient of thermal expansion with the matrix.     

Toughening effect of short carbon fibers in the ZrB2–ZrSi2 ceramic composites

The toughening effect of the short carbon fibers in the ZrB₂–ZrSi₂ ceramic composites were investigated, where the ZrB₂–ZrSi₂ ceramics without carbon fibers were used as the reference. The mechanical properties were evaluated by means of flexural and SENB tests, respectively. The microstructure was characterized by SEM equipped with EDS. The results found that the short carbon fibers were uniformly incorporated in the ZrB₂–ZrSi₂ matrix and the relative density was about 97.92%. The flexural strength of short carbon fiber-reinforced ZrB₂–ZrSi₂ composites is 437 MPa; the fracture toughness and the work of fracture are 6.89 MPa m^1/2 and 259 J/m², respectively, which increased significantly in comparing with composites without fibers. The microstructure analysis revealed that the improved fracture toughness could be attributed to the fiber bridging, the fiber–matrix interface debonding and the fiber pullout, which consumed more fracture energy during the fracture process.     

Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys

Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the (CoCrCuFeNi)_100-xNb_x HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC (face-centered cubic) to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases. Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of (CoCrCuFeNi)_100-xNb_x HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% (no fracture) to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC (valence electron concentration) value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys.     

Improving the mechanical properties of Al2O3/Ni laminated composites by adding Ni particles in Al2O3 layers

The (Al₂O₃ + Ni) composite, (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials were prepared by aqueous tape casting and hot pressing. Results indicated that the (Al₂O₃ + Ni) composite had higher strength and fracture toughness than those of pure Al₂O₃. The fracture toughness of (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials was higher than not only those of pure Al₂O₃, but also those of Al₂O₃/Ni laminar with the same layer numbers and thickness ratio. It was found that the toughness of the Al₂O₃/(Al₂O₃ + Ni)/Ni laminated material with five layers and layer thickness ratio = 2 could reach 16.10 MPa m^1/2, which were about 4.6 times of pure Al₂O₃. The strength and toughness of the (Al₂O₃ + Ni)/Ni laminated material with three layers and layer thickness ratio = 2 could reach 417.41 MPa and 12.42 MPa m^1/2. It indicated the material had better mechanical property.