Microstructural and mechanical characterisation of ODS ferritic alloys produced by mechanical alloying and spark plasma sintering

Abstract

Powders with nominal composition Fe–14Cr–2W–0·4Ti were mechanically alloyed (MA) with Y₂O₃ in a planetary ball mill at two different rotational speeds. Consolidation of the as milled powders was performed by spark plasma sintering (SPS). As milled powders showed a highly deformed microstructure with elongated nanometre grains and, depending upon the rotational speed, different stages of the nanocluster evolution were observed to be produced. In the case of consolidated materials, grain growth occurred during the SPS process and it was possible to observe the influence of the MA parameters, with larger and more homogeneously distributed grains at the higher rotational speed. Additionally, Ti was observed to be incorporated to the nanoclusters after SPS, indicating a further step in their evolution during consolidation. The mechanical behaviour of the SPS compacts was evaluated by tensile and small punch testing also showing the influence of the MA parameters in the material behaviour.     

Ultra-high temperature Mo-Si-B alloys — Synthesis, microstructural and mechanical characterization

Molybdenum boron silicides containing between 18 and 31 vol.% α-Mo are prepared by reactive hot-pressing from Mo-Si-B powder mixtures. Their microstructures and mechanical properties are investigated. The multiphase alloys consist of Mo₃Si, Mo₅SiB₂ and discontinuous α-Mo. The results demonstrate that the volume fraction of α-Mo and B content exerts a significant effect on the flexural strength and fracture toughness of the multiphase alloys.     

Crystallization kinetics of mechanically alloyed amorphous Fe-Ti alloys during annealing

The crystallization behaviors of mechanically alloyed amorphous Fe-Ti alloys were studied. The α-Fe phase formed during annealing as a result of the devitrification of the amorphous phase. According to the Kissinger plot, the crystallization activation energy (E) was obtained as 171 kJ/mol, which is close to the activation energy for the diffusion of Fe in α-Ti. According to the non-isothermal Johnson–Mehl–Avrami (JMA) analysis and obtaining the volume fraction of the crystalline phase from the differential scanning calorimetry (DSC) plots, the average Avrami exponent (n) was determined as 1.61 and 5.67 for low and high heating rates, respectively. Moreover, a method for obtaining the value of E based on the non-isothermal JMA analysis was proposed. The value of E was determined as ~185 and 191 kJ/mol respectively for low and high heating rates, which are consistent with the value determined from the Kissinger plot for all heating rates.     

Structural,mechanical and tribological characterization of Zn25Al alloys with Si and Sr addition

The ZA-27 alloy is a zinc–aluminium casting alloy that has been frequently used as the material for sleeves of plain bearings. It has good physical, mechanical and tribological properties. However, one of the major disadvantages is its dimensional instability over a period of time (ageing). To overcome this, copper in the alloy may be replaced with silicon. Coarsening of silicon particles can be controlled by a suitable addition of strontium. In this paper, the commercial ZA-27 alloy and six different Zn25Al alloys (with 1 and 3 wt.% silicon; and with 0, 0.03 and 0.05 wt.% strontium) were obtained by casting in the preheated steel mould. Casting of the alloys was carried out at a laboratory level. In the alloys containing silicon, a finer dendritic structure was noticed compared to the structure of the commercial ZA-27 alloy. The addition of strontium influenced the size and distribution of primary silicon particles. Needle-like particles of eutectic silicon were changed into the fibrous ones. The presence of silicon and strontium did not significantly affect mechanical properties of the obtained Zn25Al alloys compared to mechanical properties of the commercial ZA-27 alloy. Wear rate of the alloys containing silicon was lower than that of the ZA-27 alloy. The addition of strontium further lowers the wear rate and slightly increases the coefficient of friction.     

Machine learning of phases and mechanical properties in complex concentrated alloys

The mechanical properties of complex concentrated alloys (CCAs) depend on their formed phases and corresponding microstructures.The data-driven prediction of the phase formation and associated mechanical properties is essential to discovering novel CCAs.The present work collects 557 samples of various chemical compositions,comprising 61 amorphous,167 single-phase crystalline,and 329 multi-phases crystalline CCAs.Three classification models are developed with high accuracies to category and understand the formed phases of CCAs.Also,two regression models are constructed to predict the hard-ness and ultimate tensile strength of CCAs,and the correlation coefficient of the random forest regression model is greater than 0.9 for both of two targeted properties.Furthermore,the Shapley additive expla-nation (SHAP) values are calculated,and accordingly four most important features are identified.A significant finding in the SHAP values is that there exists a critical value in each of the top four fea-tures,which provides an easy and fast assessment in the design of improved mechanical properties of CCAs.The present work demonstrates the great potential of machine learning in the design of advanced CCAs.     

Influence of matrix structure on the fatigue properties of an alloyed ductile iron

Rotary bending fatigue tests were conducted on ductile iron containing 1.25 wt% nickel, 1.03 wt% copper and 0.18 wt% molybdenum with various matrix structures. Several heat treatments were applied to obtain ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic structures in the matrix of a pearlitic as-cast alloyed ductile iron. The tensile properties (ultimate tensile strength, 0.2% yield strength and percent elongation), the hardness and the microstructures of the matrixes were also investigated in addition to fatigue properties. Fractured surfaces of the fatigue specimens were examined by the scanning electron microscope. The results showed that the lowest hardness, tensile and fatigue properties were obtained for the ferritic structure and the values of these properties seemed to increase with rising pearlite content in the matrix. While the lower ausferritic structure had the highest fatigue strength, the upper ausferritic one showed low fatigue and tensile properties due to the formation of the second reaction during the austempering process.     

Microstructure and mechanical properties of hot rolled TiNbSn alloys

Titanium alloys with lower elastic modulus and free from toxic elements such as Al and V have been studied for biomedical matters. Ti–Nb–Sn alloys showed up as presenting great potential for the aforementioned purpose. The current study got Ti–35Nb-XSn alloys (x = 2.5; 5.0; 7.5) by applying the following techniques: arc melting, homogenizing and cooling in furnace, homogenizing and water quenched, hot rolling and water quenched. According to each step of the study, the microstructures were featured by means of optical microscopy, by applying a scanning electron microscopy (SEM) analysis as well as X-ray diffraction. The mechanical properties were gotten by means of: Vickers microhardness, tensile and ultrasonic tests. Their ratio between tensile strength and elastic modulus as well as the ductility were compared to other biomedical alloys already available in the literature. The mechanical behavior of the Ti–Nb alloys directly depends on the Sn rates that constitutes the phases as well as on the thermomechanical background to which the alloy was submitted to. The hot rolled Ti–35Nb–2.5Sn alloy showed high ratio between strength and elastic modulus as well as high ductility, just as high as those of some cold rolled Ti alloys.     

Microstructure and mechanical properties of FexCoCrNiMn high-entropy alloys

The microstructure and tensile properties of Fe_xCoCrNiMn high-entropy alloys (HEAs) were investigated. It was found that the Fe_xCoCrNiMn HEA has a single face-centered cubic (fcc) structure in a wide range of Fe content. Further increasing the Fe content endowed the Fe_xCoCrNiMn alloys with an fcc/body-centered cubic (bcc) dual-phase structure. The yield strength of the Fe_xCoCrNiMn HEAs slightly decreased with the increase of Fe content. An excellent combination of strength and ductility was achieved in the Fe_xCoCrNiMn HEA with higher Fe content, which can be attributed to the outstanding deformation coordination capability of the fcc/bcc dual phase structure.     

Thermal expansion of cementite and thermoelastic stresses in white cast iron

With X-ray diffraction the three microscopic differential thermal expansion coefficients of pure cementite were determined with high precision in the temperature range 30–320 °C. These data were used to calculate the average three-axial thermoelastic stresses in white cast iron with differently shaped and oriented cementite inclusions. Between 30 and 300 °C the orientation distribution of the (211) interplanar spacing of the ferrite phase in white cast iron was determined. An evaluation of these data according to the formalism of conventional X-ray stress analysis, however, yields completely wrong results because this formalism is based on the assumption that only biaxial average stresses parallel to the surface exist in the thin surface layer which is penetrated by the X-rays. It is demonstrated that what were observed in reality were the three-axial average phase-specific stresses from the interior of the specimen, which are complicated because of the preferred orientation of the cementite plates. The average phase-specific stress component σ₃₃ perpendicular to the irradiated surface drops to zero only in a sub-micrometre range below the surface. Because of the equilibrium conditions it is exactly zero at the external surface.     

Thermal conductivity and mechanical properties of Sm-containing Mg-Zn-Zr alloys

This study investigated the effect of Sm additions on the microstructure, thermal conductivity, and mechanical properties of Mg-Zn-Zr alloy. The results indicate that the addition of Sm led to the formation of a rare-earth phase at the grain boundaries, and the grain size was significantly refined in the extruded state. The thermal conductivity of Mg alloy increased with the increase in Sm content because of the formation of a rare-earth phase that helps to dissolve the Zn atoms in the α-Mg matrix. Moreover, the as-extruded Mg alloy exhibited a higher thermal conductivity (up to124?W?(m?K)^?1) than its as-cast counterparts. The Sm-containing as-extruded Mg alloy showed excellent yield strength of up to 254?MPa, and also a good plastic deformation ability.     

Relationship between structure and mechanical properties in high manganese alloyed ductile iron

Abstract

Measurements of ultimate tensile strength, 0·2% proof stress, elongation, and impact energy are reported for an alloyed ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg,for a range of austempering temperatures and times after austenitising at 920°C for 120 min. It is shown that the mechanical properties satisfy the high strength grades of the standard AST MA897 M:1990, but fail to satisfy the higher ductility grades because of poor ductility. This is attributed to overlapping of the stage I and II reactions and the occurrence of the transformation induced plasticity mechanism during deformation, particularly in irons austempered at higher temperatures.

MST/3054     

Structure and mechanical properties of rapidly solidified magnesium based Mg-Al-Zn-RE alloys consolidated by extrusion

Magnesium based Mg-9Al-lZn-5RE (RE = La or Nd) alloys were rapidly solidified by chill block melt spinning. The resulting ribbons were cold packed into an aluminium alloy can and extruded at temperatures of 230 and 340°C and ratios of 20:1 or 25:1. Tensile and hardness tests of the extruded and heat treated materials were carried out. The tensile strength and elongation to fracture of the as extruded material were 478 MN m^?2 and 6·5% respectively and those of the material heat treated for 2 h at 350° C were 420 MN m^?2 and 20% respectively. The microstructure of these specimens was studied by X-ray diffraction and transmission electron microscopy. Intermetallics of Al₁₁ La₃ or Al₂Nd were found at grain boundaries and in the matrix which had a grain size of between 0–26 and 0–8 μm, while Mg₁₇Al₁₂ precipitates were present in the specimens extruded at a lower temperature (230° C). Yield strengths were consistent with the Hall-Petch relationship with grain size established earlier for this class of material.

MST/3495     

Nitrogen addition to iron powder by mechanical alloying

Nitrogen was alloyed into iron (a) by mechanical processing in a nitrogen gas environment, and (b) by mechanically alloying with iron-nitride powders to characterize resulting nano-structure and nitrogen distribution. Although the infused nitrogen concentration was significantly greater than the thermodynamic equilibrium solubility of iron, no nitrides formed, even for nitrogen concentrations as high as 4.1 wt.% However, a bctFe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bctFe nitrogen concentrations was significantly less than the total measured nitrogen concentration. A considerable portion of the mechanically infused nitrogen was determined to be associated with nanograin boundaries.     

Fabrication and characterization of novel biodegradable Zn-Mn-Cu alloys

Zn-Mn-Cu alloys with micro-alloying of Mn and Cu in Zn are developed as potential biodegradable metals. Although the as-cast alloys are very brittle, their ductilities are significantly improved through hot rolling. Among the as-cast and the as-hot-rolled alloys, as-hot-rolled Zn-0.35 Mn-0.41 Cu alloy has the best comprehensive property. It has yield strength of 198.4 ± 6.7 MPa, tensile strength of 292.4 ± 3.4 MPa,elongation of 29.6 ± 3.8% and corrosion rate of 0.050-0.062 mm a~(-1). A new ternary phase is characterized and determined to be MnCuZn18, which is embedded in MnZn13, resulting in a coarse cellular/dendritic MnZn13-MnCuZn18 compound structure in Zn-0.75 Mn-0.40 Cu alloy. Such a coarse compound structure is detrimental for wrought alloy properties, which guides future design of Zn-Mn-Cu based alloys.The preliminary research indicates that Zn-Mn-Cu alloy system is a promising candidate for potential cardiovascular stent applications.     

Structure and mechanical properties of age-hardened directionally solidified AlSiCu alloys

The structure of directionally solidified Al-Si hypoeutectic alloys are generally composed of Al-matrix and Si-reinforcing phase. The growth direction of the both phases was near 200. The strength properties of an Al-Si alloy with additions of 2 wt.% and 4 wt.% copper have been investigated. These alloys were solution treated, quenched in water and aged at 200°C. Large Al₂Cu precipitates present in D.S. samples dissolved partly, and after ageing, they precipitated as the Θ′ platelets, significantly increasing the mechanical properties of the alloys. Hardness, strength and elongation were measured in the course of ageing. The structure was investigated by means of: XSAS, X-ray phase analysis, lattice parameter measurements and scanning microscopy.     

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.     

Corrosion and mechanical properties of titanium alloyed with aluminum, iron, and molybdenum

We study the influence of Fe, Al, and Mo on the corrosion and mechanical properties of titanium. It is shown that the corrosion resistance of titanium becomes 3.5 times lower as the concentration of iron increases from 0.1 to 5%. The microalloying of Ti-Al-Fe-Mo alloys with molybdenum inhibits corrosion. We develop new low-alloy structural and corrosion-resistant titanium alloys of the Ti-Al-Fe-Mo system. Their innovative character is confirmed by patents of the Ukraine. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 41–44, September–October, 2006.     

Relationship between mechanical properties and structure in austempered alloyed compacted graphite cast iron

Abstract

Austempering kinetic measurements and mechanical property measurements are presented for a compacted graphite iron of composition Fe-3.4C-3.5Si-0.25Mn-0.50Mo-0.50Ni (wt-%) austempered at 375°C after austenitising at 890 and 940°C. Analysis of the austempering kinetics shows that alloying elements have a similar effect on the processing window as in ductile irons. The mechanical properties show optimum values at austempering times within the processing window. However, the graphite morphology limits the mechanical property enhancement achieved by austempering. Nevertheless, it is possible to double the strength of the as cast compacted graphite iron without loss in ductility.     

Enhanced viscoelastic properties of nickel–aluminum–bronze alloyed with indium or its alloys

Engineering structures may be sensitive to vibrations such that their normal functioning is impaired due to the presence of vibrations. A material capable of dissipating energy of vibration by means of heat tends to have better mechanical damping properties. However, due to the demands on structural stability, the material needs to have sufficient stiffness to be useful structurally. Thus, the material must have both high stiffness and high mechanical damping. This paper discusses a variety of materials that were uniquely developed to achieve the aforementioned objectives. The new nickel–aluminum–bronze (UNS C95800)-based samples were classified according to alloying composition by weight percentage with different indium alloys. A dynamic mechanical analyzer was used to find the complex modulus (|E*|) and loss tangent (tan δ) values of the samples. These values were used to evaluate and compare the figure-of-merit (FOM) across different frequencies at a particular temperature and strain. The samples which have FOM equal to or greater than 0.6 GPA include the ones alloyed with pure indium (5 wt%), eutectic solder (5 wt%), indium–bismuth eutectic (10 wt%), bismuth–tin eutectic (5 and 10 wt%), and indium–tin eutectic (5 wt%). The newly alloyed high-damping metals can be used to reduce vibration while maintaining their stiffness.