Microstructure and improved mechanical properties of Mo-12Si-8.5B alloys with lanthanum oxide addition

Mo-12 Si-8.5 B alloys with different La_2 O_3 contents were fabricated by mechanical alloying and then hot pressing.The effects of La_2 O_3 on microstructure,room and elevated temperature mechanical properties of Mo-12 Si-8.5 B alloys were studied.The microstructure of Mo-12 Si-8.5 B alloy with La_2 O_3 additions exhibits a continuous α-Mo matrix,where the spherical Mo_3 Si and Mo_5 SiB_2 intermetallic phases are distributed inside the grains and along the grain boundaries.The detailed microstructure shows that some nanoscale La_2 O_3 particles are dispersed mainly in the a-Mo grains and partially in the intermetallics.These La_2 O_3 particles can refine the grain sizes of a-Mo matrix and intermetallic,but the refining effect is limited with the La_2 O_3 addition further increasing.The mechanical testing results show that the La_2 O_3 addition simultaneously improves the compression strength and fracture toughness of Mo-12 Si-8.5 B alloy,due to that theα-Mo matrix is strengthened and toughened at ambient temperature and intermetallics are strengthened at elevated temperatures.The enhancing effect is sensitive to the amount of La_2 O_3 additions,and the Mo-12 Si-8.5 B alloy can obtain a better combination of strength and toughness when the content of La_2 O_3 is 0.9 wt%.     

Oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy in temperature range of 650-850 ℃

The oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy (Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si) was studied in the temperature range of650-850℃. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) were used to identify the phase constituents and microstructure of the scales formed on the specimens after oxidation at different temperatures.Isothermal oxidation tests show that the oxidation rate of the alloyed Ti_2AlNb-based alloy is obviously reduced at all temperatures, and the mass gains are very low for this alloy in comparison with those of Ti-22Al-25Nb alloy. The alloying elements Mo, V, Zr and Si have an obvious affect on the oxidation products of the alloys. The improved oxidation resistance for the alloy is ascribed to the introduction of Mo, V, Zr and Si elements, which are beneficial to the selective oxidation of Al to form protective oxides while are disadvantage of the formation of poor oxidation resistance oxides such as AlNb0_4.     

INTERNAL OXIDATION BEHAVIOR OF Pd-40Ag-1M(M=Zr,Y)ALLOYS

The internal oxidation behavior of Pd-40Ag-1M(M=Zr,Y)alloy wires has been studied inair at 800—1200℃.The relationship between the internal oxidation depth ξ and the reactiontime t can be expressed as ξ= Kt~n,where n=0.5—0.75.The higher the temperature,thelarger the value of n is.The active elements Zr and Y show different internal oxidationcharacters.For the alloys eontaining Zr,the oxidation rate along the grain boundaries isabout twice as high as that in grains,and“lateral oxidation”exists along the grainboundaries.For the alloys containing Y,the oxidation rates in grains and along the grainboundaries are roughly the same,and there is no“lateral oxidation”along the grain bounda-ries.The activation energies of both alloys are in the range of 120—150kJ/mol.Some prop-erties for oxidized alloys were studied.The mechanisms of the internal oxidation were dis-cussed.     

Improvement of isothermal oxidation resistance of a γ'-strengthened Co-Al-W-Mo-Ta-B alloy at 800℃via doping Ce

Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at% and 0.20 at% Ce)exposed at 800℃ were compared.The 0.01 Ce and 0.05 Ce alloys were consisted of γ/γ' coherent micro structure,while the κ-Co_3 W compound precipitated at the grain boundary of the 0.1 OCe and 0.20 Ce alloys in addition to the γ/γ'microstructure.The oxidation kinetics curves of the Cedoped alloys exhibited a parabolic time dependence on the weight gain.With an increasing nominal Ce content,the weight gain of the Co-Al-W-Mo-Ta-B alloys monotonically decreased.An oxide scale composed of a dense and uniform outer Co_3 O_4+CoO layer,a middle CoAl_2 O_4 and CoWO_4 compound layer and an inner Al_2 O_3 layer.The excellent oxidation resistance of 0.2 Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al_2 O_3 layer and the thickest Al_2 O_3 layer during entire oxidation process.     

Corrosion behavior of NiCr alloys in HCl-containing oxidation atmosphere at 700-800℃

Corrosion behaviors of pure Ni and three NiCr alloys were investigated in an HCl-containing oxidizing at-mosphere at 700℃ and 800℃. All materials suffer from accelerated corrosion at both temperatures. NiCr alloys show an initial mass loss due to the formation of volatile CrCl3 and CrO2Cl2. Some chlorides are detected at the scale/substrate interface and many voids are also found there. NiCr alloys with higher chromium content have better corrosion resistance. However, Ni50Cr is inferior to Ni25Cr due to its two-phase structure, which makes it easy for chlorine to diffuse along grain boundary and to occur inner oxidation. The relevant corrosion mechanism was also discussed.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Influence of solution treatment on microstructure,corrosion resistance,and oxidation behavior of cast G-NiCr28W alloy

G-NiCr28W is a nickel-based cast alloy. Its microstructure consists of nickel-rich matrix phase and chromium-rich eutectic carbides. The solution treatment process can provide homogenous microstructure and desired mechanical/thermal properties for G-NiCr28W alloy. However, the solution treatment process affects the corrosion resistance of the alloy and it causes metal loss due to the occurrence of oxidation at atmospheric conditions. Therefore, determining the changes in the properties of the G-NiCr28W is important. For this purpose, G-NiCr28W specimens were solution treated at 1,040 ℃, 1,100 ℃ and 1,160 ℃ for 1 h and 8 h, respectively. The microstructures of the solution-treated samples were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis. Moreover, hardness, corrosion resistance and oxidation behaviors of the solution-treated samples were examined. The solution treatment process applied at 1,160 ℃ led to the formation of Fe_2W_2C blocky carbides, and hardness of the sample increased with the existence of blocky carbides, while corrosion resistance decreased. Furthermore, excessive metal loss occurred depending on oxidation due to the high process temperature at 1,160 ℃.     

Effect of 5%Cr on oxidation of Ni- 10Al in 0.1MPaO2 at 900 ℃ and 1 000 ℃

The effect of 5%Cr（mole fraction） on the scaling behavior of Ni-10Al was investigated at 900 ℃ and 1 000 ℃ with the aim to discuss the third element effect（TEE）. The oxidation rate of the ternary alloy containing Cr is much slower than the corresponding binary alloys at both temperatures. A dense external scale of NiO overlying a zone of internal oxide precipitates forms on Ni-10Al, while Ni-5Cr-10Al develops a protective layer. So the addition of Cr promotes the formation of alumina scale. The possible mechanism of Cr on the oxidation of Ni-Al alloy is discussed.     

Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni–Mo–Cr Superalloy

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents(0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys,only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.     

Age Hardening of Rapidly-Solidified Mo-Th-B Alloys

The splat foils with 30～35 μm thickness and the spun ribbon with 50～55 μm thickness of Mo-4Th-0.4B andMo-8Th-0.8B alloy were made by arc melt hammer-anvil and melt spinning techniques.Both alloys have highhardness and strong age hardening response,especially for Mo-8Th-0.8B alloy.The precipitates associated with agehardening were identified to be ThB_4(Tetragon) and Mo_2B(C16,BTC).The times of reaching age hardening peak forboth alloys are not the same,depended on the precipitation extent of Th and B solutes from metastable solid solution.Astrong grain refinement was observed.Present and previous work indicate that new high strength Mo alloys can be de-veloped by rapid solidification techniques.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Microstructure and dry sliding wear resistance of Moss-toughened Mo2Ni3Si metal silicide alloys

A wear resistant Mo₂Ni₃Si-based metal silicide alloy toughened by molybdenum-based solid solution (Mo_ss) was fabricated by the laser melting deposition (LMD) manufacturing process. Microstructure of the alloy is composed of Mo_ss primary dendrites and the matrix of the single phase Mo₂Ni₃Si. Wear resistance and friction coefficient of the alloys were evaluated under metallic dry sliding wear test conditions as a function of contact load. Results showed that the alloys have a low friction coefficient and outstanding wear resistance due to the high hardness of Mo₂Ni₃Si matrix and the high strength, ductility and toughness of Mo_ss dendrites. The wear rate and the friction coefficient of alloys are extremely insensitive to the contact load owing to the abnormal hardness–temperature relation of Mo₂Ni₃Si. The Mo_ss dendrite played the role of trapping micro-cracks and restraining brittle spalling of the Mo₂Ni₃Si matrix during wear process and improved the wear properties of Mo_ss-toughened Mo₂Ni₃Si alloy.     

The oxidation of micro-crystalline Co–Cr alloys under balanced oxygen pressure of Co2O3/Co powders

Surface micro-crystallizing was applied to Co–5Cr, Co–10Cr and Co–15Cr alloys by electro-spark deposition (ESD) and the oxidation behaviors of both cast and micro-crystalline alloys were investigated at 1000 °C under balanced oxygen pressure of Co₂O₃/Co-mixed powders. Surface and cross-section SEM morphologies, EDS and XRD analysis show that micro-crystallizing promotes both the external oxidation and internal oxidation of Co–5Cr alloy. Micro-crystallizing provides more grain boundaries so that the diffusion is enhanced. The inwards diffusion of O makes internal oxidation zone thicker while the outwards diffusion of Cr makes external oxidation more prominent for the micro-crystalline Co–5Cr alloy. For both the cast and micro-crystalline Co–10Cr and Co–15Cr alloys, 10 wt.% Cr is enough to form complex selective oxides (AO/AB₂O₄/BO) scales and 15 wt.% Cr is still not enough to form a single Cr₂O₃ oxide scale. Micro-crystallizing promotes the outwards diffusion of Cr and selective Cr₂O₃ scales are easier to form so that the external oxides scales of micro-crystalline Co–10Cr and Co–15Cr alloy are much thinner and their Cr/Co ratios are higher. The oxidation model of A–B alloy under oxygen pressure that both A and B can be oxidized is also proposed.     

TiB2/Al-Si-Mg-Er复合材料的组织与性能

采用重熔稀释法制备了Al-7Si-0.5Mg-0.1Er和0.5TiB₂/Al-7Si-0.5Mg-0.1Er合金,研究了TiB₂颗粒增强Al-Si-Mg-Er复合材料的组织性能。结果表明,复合材料铸态组织主要由α-Al基体、共晶Si相和TiB₂颗粒组成。TiB₂粒子的加入使Al-7Si-0.5Mg-0.1Er合金二次枝晶间距减小了7.1 μm。抗拉强度达到217.53 MPa,较Al-7Si-0.5Mg-0.1Er合金提升了12.1 %。TiB₂/Al-Si-Mg-Er复合材料的最优T6热处理工艺为530 ℃×12 h固溶+160 ℃×7 h时效,经该工艺处理后,TiB₂/Al-Si-Mg-Er复合材料抗拉强度达到319.49 MPa,相比热处理前提高了46.9%,相比Al-7Si-0.5Mg-0.1Er合金提高了5.9%;屈服强度达到266.75 MPa,相比热处理前提高了106.4%,相比Al-7Si-0.5Mg-0.1Er合金提高了14.9%。复合材料抗拉强度的提升主要源于TiB₂颗粒加入后产生的晶粒细化、变质和热处理强化。     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Oxidation behavior of Zr-Y alloyed Mo-Si-B based alloys

Three Mo-Si-B based alloys (Mo-12Si-10B, Mo-12Si-10B-1Zr and Mo-12Si-10B-1Zr-0.3Y (at.%)) were fabricated to study the effects of Zr and especially further addition of Y on the oxidation behaviors of alloys at 1250 °C. Mo-12Si-10B alloy shows good oxidation resistance due to the formation of a protective SiO₂ layer. However, as the result of the phase transformation of ZrO₂, triggering the breaking up of the protective SiO₂ layer, the addition of 1 at.% Zr (Mo-12Si-10B-1Zr alloy) leads to the catastrophic oxidation behavior. Further addition of 0.3 at.% Y (Mo-12Si-10B-1Zr-0.3Y alloy) eliminates this detrimental effect of Zr alloying by promoting the formation of monomorphic ZrSiO₄ instead of polymorphic ZrO₂ during oxidation. Besides, at the later oxidation stage (≥20 h), a continuous and compact Y₂Si₂O₇ layer forms on the scale surface, further improving the oxidation resistance.     

Hydrogen disproportionation by reactive milling and recombination of Nd2(Fe1−xCox)14B alloys

Stoichiometric Nd₂(Fe_1−xCo_x)₁₄B alloys (x=0, 0.25, 0.5, 0.75 and 1) have been disproportionated into NdH_2+δ and bcc–(Fe,Co) (0≤x≤0.75) or fcc–Co (x=1), respectively, by milling in hydrogen at enhanced temperatures. Reactive milling leads to the disproportionation of the thermodynamically very stable Nd₂Co₁₄B alloy. This reaction is not possible via the conventional hydrogenation disproportionation desorption and recombination (HDDR) process. Grain sizes of disproportionated and recombined Nd₂(Fe,Co)₁₄B materials were found to be <10 nm and 40–50 nm, respectively — approximately an order of magnitude smaller than those of conventional-HDDR processed alloys. The recombined Nd₂Co₁₄B alloy shows on average slightly smaller grain sizes than the Nd₂Fe₁₄B compound. A more effective exchange coupling leading to enhanced remanences, possibly due to the slightly smaller grain size, has been observed for Nd₂Co₁₄B powders recombined at 600–700°C.     

The molybdenum diphosphate Mo1.3O(P2O7), containing Mo2 and Mo3 clusters

A novel molybdenum diphosphate, Mo_1.3O(P₂O₇), was obtained by electrochemical lithium deintercalation. The diphosphate crystallises in space group I2/a with the lattice parameters a=22.88(1), b=22.94(2), c=4.832(1) Å, γ=90.36°, Z=8. Its original framework is built up from MoO₆ octahedra, P₂O₇ groups and also from MoO₄, Mo₂O₄ and Mo₃O₈ units containing Mo₂ and Mo₃ clusters. These polyhedra delimit large octagonal and z-shaped tunnels running along c, in which the inserted cations may be located.     

Synthese und kristallstruktur von KCr0,8Al0,2Mo2O8

Single crystals of KCr_0.8Al_0.2Mo₂O₈ were prepared and investigated by the X-ray diffractometer technique. It shows a structure type related to trigonal KAIMo₂O₈, monoclinic NaCrMo₂O₈ or orthorhombic KInMo₂O₈, space group C_2h⁶—C2/c; a=17.445 Å, b=5.649 Å, c=8.997 Å, β=119.37°; Z=4. KCr_0.8Al_0.2Mo₂O₈ is characterized by isolated MoO₄ tetrahedra, isolated (Cr/Al)O₆ octahedra and a distorted square antiprism around K⁺. The crystal structure is discussed with respect to those of related compounds.

Zusammenfassung

Einkristalle von KCr_0.8Al_0.2Mo₂O₈ wurden synthetisiert und mit Vierkreisdiffraktometertechnik röntgenographisch untersucht. Sie zeigen einen mit trigonal-KA1Mo₂O₈, monoklin-NaCrMo₂O₈ oder orthorhombisch-KlnMo₂O₈ verwandten Strukturtyp, Raumgruppe C_2h⁶—C2/c; a=17,445 Å, b=5,649 Å, c=8,997 Å, β=119,37°; Z=4. KCr_0.8Al_0.2Mo₂O₈ zeichnet sich durch isolierte MoO₄-Tetraeder, isolierte (Cr/Al)O₆-Oktaeder und ein verzerrtes quadratisches Antiprisma um K⁺ aus. Die Kristallstruktur wird mit solchen verwandter Verbindungen diskutiert.     

Preparation of W–Al–Mo ternary alloys by mechanical alloying

Single phase W_XAl₅₀Mo_50−X (X = 40, 30, 20 and 10) powders have been synthesized directly by mechanical alloying (MA). The structural evolutions during MA and subsequent as-milled powders by annealing at 1400 °C have been analyzed using X-ray diffraction (XRD). Different from the Mo₅₀Al₅₀ alloy, W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys were stable at 1400 °C under vacuum. The results of high-pressure sintering indicated that the microhardnesses of two compositions, namely W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys have higher values compared with W₅₀Al₅₀ alloy.