Development of high strength in Ni–42Cr–7·7Al (at.-%) alloy

Abstract

An investigation has been made of the aging of supersaturated nickel based solid solution in a Ni–42Cr–7·7 Al (at.-%) alloy. Solution treatment of the alloy in the range 1150–1300°Cfollowed by quenching and aging at ~550–750°C produces high values of hardness and strength, e.g. 835 HV on aging at 600°C. This hardening results from a discontinuous precipitation reaction producing a fine scale lamellar structure consisting of nickel based and chromium based solid solutions; interlamellar spacings of ~60 nm were obtained. The rate of precipitation is substantially greater than that in a binary Ni–42 at.-%Cr alloy.

MST/736     

Liquid-phase bonding of silicon nitride ceramics using Y2O3–Al2O3–SiO2–TiO2 mixtures

Hot-pressure sintered β-Si₃N₄ ceramic was bonded to itself using Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures. Reactive behavior at interface between Si₃N₄ and Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures during silicon nitride ceramic joining was studied by means of scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray diffraction (XRD) and auger electron spectroscopy (AES). The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA, AES and XRD analyses show that the liquid glass solder reacts with silicon nitride at interface, forming the Si₃N₄/Y–Si–Al–Ti–O–N glass/TiN/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increase of bonding temperature and holding time, the joint strength increased reaching a peak, and then decreased. The maximum joint strength of 200 MPa measured by the four-point bending tests is obtained for silicon nitride bonded at 1823 K for 30 min.     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO₂ laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α + β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482 °C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α + β microstructures in the basket-weave form.     

Prediction of the mechanical properties of forged Ti–10V–2Fe–3Al titanium alloy using FNN

In this paper, a fuzzy neural network (FNN) prediction model has been employed to establish the relationship between processing parameters and mechanical properties of Ti–10V–2Fe–3Al titanium alloy. In establishing these relationships, deformation temperature, degree of deformation, solution temperature and aging temperature are entered as input variables while the ultimate tensile strength, yield strength, elongation and area reduction are used as outputs, respectively. After the training process of the network, the accuracy of fuzzy model was tested by the test samples and compared with regression method. The obtained results with fuzzy neural network show that the predicted results are much better agreement with the experimental results than regression method and the maximum relative error is less than 7%. And the optimum matching processing parameters can be quickly selected to achieve the desired mechanical property based on the fuzzy model. It proved that the model has a good precision and excellent ability of predicting.     

Synthesis of (Fe,Cr)3Al–Al2O3 nanocomposite through mechanochemical combustion reaction induced by ball milling of Cr,Al and Fe2O3 powders

In this work, (Fe,Cr)₃Al matrix nanocomposite reinforced by 47 vol.% Al₂O₃ was synthesized by mechanochemical reaction of Cr, 3Al and Fe₂O₃ powders mixture. The structural evaluation of powder particles during milling was done by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential thermal analysis (DTA). The results showed that at the early stage of milling, the thermite reaction between Fe₂O₃ and Al occurred and Fe and Al₂O₃ phases were formed. Then, the remaining Al and Cr were alloyed with Fe, leading to (Fe,Cr)₃Al–Al₂O₃ nanocomposite structure. Further investigations indicated that the presence of diluents (excess Al and Cr) did not change the modality of thermite reaction and the formation of (Fe,Cr)₃Al–Al₂O₃ nanocomposite proceeded with combustion process. The (Fe,Cr)₃Al–Al₂O₃ nanocomposite powder exhibited the hardness value of 1140 Hv which is significantly higher than 935 Hv obtained for (Fe,Cr)₃Al.     

Brazing continuous carbon fiber reinforced Li2O–Al2O3–SiO2 ceramic matrix composites to Ti–6Al–4V alloy using Ag–Cu–Ti active filler metal

C_f/LAS composites and TC4 alloy were brazed successfully by vacuum brazing using Ag–Cu–Ti active filler metal. The interfacial microstructure was characterized by a scanning electron microscope, energy dispersive spectrometer and X-ray diffraction. The effects of brazing temperature on the interfacial microstructure and joint properties were investigated in details. Various phases including TiC, TiSi₂, Ti₃Cu₄, Cu (s,s), Ag (s,s), TiCu and Ti₂Cu were formed in the brazed joints. Interfacial microstructure varies greatly with the increase of brazing temperature, while the amount of Ti₂Cu reduced, but no new phase is generated. The optimal shear strength of the joint is 26.4 MPa when brazed at 890 °C for 10 min. Shear test indicated that the fracture of the brazed joints went through the TiSi₂ + TiC layer close to the C_f/LAS composites interface.     

The mechanical alloying mechanism of various Fe2O3–Al–Fe systems

The mixed powders of in situ Al₂O₃ particles and Fe (Al) solid solution were prepared via self-propagating combustion reaction initiated by mechanical alloying (MA), and the MA mechanism of several Fe₂O₃–Al–Fe systems with different Al₂O₃ mass fractions were studied. The adiabatic temperature (T_ad) of each system was calculated to estimate whether the self-propagating combustion reaction could be initiated in theory. The microstructure of the mixed powders was investigated by SEM, EDS and TEM. The phase analysis was evaluated by XRD, and the Fe lattice parameter was calculated from the XRD patterns. The results showed that with the addition of Fe during the MA process, the activation period was prolonged and the sharp increase of temperature occurred, and when the Al₂O₃ mass fraction was decreased to 10.94%, the self-propagating combustion reaction could not occur in theory and practice. When there was no added Fe, the final product was homogeneous Fe (Al) solid solution.     

High strength Al–Al2O3p composites: Optimization of extrusion parameters

Composite aluminium alloys reinforced with Al₂O_3p particles have been produced by squeeze casting followed by hot extrusion and a precipitation hardening treatment. Good mechanical properties can be achieved, and in this paper we describe an optimization of the key processing parameters. The parameters investigated are the extrusion temperature, the extrusion rate and the extrusion ratio. The materials chosen are AA 2024 and AA 6061, each reinforced with 30 vol.% Al₂O₃ particles of diameter typically in the range from 0.15 to 0.3 μm. The extruded composites have been evaluated based on an investigation of their mechanical properties and microstructure, as well as on the surface quality of the extruded samples. The evaluation shows that material with good strength, though with limited ductility, can be reliably obtained using a production route of squeeze casting, followed by hot extrusion and a precipitation hardening treatment. For the extrusion step optimized processing parameters have been determined as: (i) extrusion temperature = 500 °C–560 °C; (ii) extrusion rate = 5 mm/s; (iii) extrusion ratio = 10:1.     

Three-dimensional shape of the Y2BaCuO5 pattern in melt-textured Y–Ba–Cu–O oxide

Crystallographic orientations of the Y211 tracks that were trapped within the melt-textured Y123 domains were determined from the orientation relationship among the Y211 pattern, twin traces and a microcrack. Analysing the orientation of the two-dimensional Y211 patterns that were observed on various crystallographic planes of the Y123, the three-dimensional shape of the Y211 pattern was inferred as follows: only one x-type Y211 pattern was included within one Y123 domain and three sets of diagonal Y211 tracks met with the corners of the Y123 domain. The diagonal Y211 tracks lie on the (1 1 0), ( 1 0), (0 1 1), (0 1), (1 0 1) and (1 0 ) planes of the Y123 domain. The planes where the Y211 tracks are located are considered to be the boundary planes where local growing parts of the Y123 domain impinge upon each other. The formation mechanism of the Y211 pattern is discussed on the basis of anisotropic growth behaviour and the shape of the Y123 domain. This revised version was published online in November 2006 with corrections to the Cover Date.     

Short communication Low temperature forgeability of Ti–48Al–2Cr–2Nb alloy

Abstract

A two phase (γ + α) titanium aluminide alloy Ti–48Al–2Cr–2Nb, at.-% was isothermally forged along all the three axes in succession at an initial forging temperature of 1273 K, followed by another two sets of forgings at lower temperatures of 1173 and 1123 K. The combined effects of heavy multiaxial strains (? = 2·07, per set of forgings), progressively lower temperatures, and a moderate strain rate (10^-3 s^-1 ) transformed the microstructure to a fine and equiaxed shape. The microstructure developed is suitable for working at a low temperature.     

Development of high performance Mg–Al2O3 composites containing Al2O3 in submicron length scale using microwave assisted rapid sintering

Abstract

In the present study, magnesium composites reinforced with different volume fraction of submicron size Al₂O₃ particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al₂O₃ particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al₂O₃ composites was found to decrease with increasing amount of submicron Al₂O₃ particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the effect of the presence of submicron alumina and its increasing amount with the microstructural, physical and mechanical properties of magnesium.     

Selection of eutectic systems in Al2O3–Y2O3 ceramics

There are two eutectic reactions in the Al₂O₃-rich portion of the Al₂O₃–Y₂O₃ pseudo-binary system; one is the equilibrium Al₂O₃–YAG eutectic reaction at 1826°C, and the other is the metastable Al₂O₃–YAP eutectic reaction at 1702°C. Selection of the Al₂O₃–YAG and the Al₂O₃–YAP eutectics was examined in terms of cooling rate, nucleation temperature and maximum melt temperature. When the melt was cooled from 2100°C at any cooling rate, it always nucleated below the Al₂O₃–YAP eutectic temperature, therefore the Al₂O₃–YAP eutectic was selected. The Al₂O₃–YAG eutectic was selected when the melt was cooled from 1900°C at a cooling rate of less than 1 K s⁻¹. The selection of the two eutectic systems was determined by the nucleation temperature, although the maximum holding temperature of the melt and the cooling rate significantly affected the nucleation temperature. The structure of the melt, such as coordination of oxygen and chemical order when being heated to 2100°C may affect the nucleation behavior.     

Bonding of Al to Al2O3 via Al–Cu eutectic method

The Al oxidation layer in the manufactures of direct aluminum bonded Al₂O₃ substrates (DAB) has been a long-term trouble for industries. In this work we propose a new method for fabricating the DAB substrates with no requirement of high vacuum or active O₂-getters. The new method comprises two stages: (i) Cu-film is bonded onto Al₂O₃ ceramic surface via DBC method; (ii) Al foil is joined to the DBC substrate by Al–Cu eutectic method at 600 °C in pure N₂ atmosphere. KF–AlF₃ flux was used to disrupt the Al–oxide layer on the surface of Al foil. The wetting ability was significantly enhanced due to the diffusion of Cu into Al and the dissolving of Al. The final contact angle is achieved of 22.10°. Microstructure and composition of the interface between Al and Al₂O₃ substrate were analyzed. The XRD, SEM and EDS results show that two new phases Al₂Cu and CuAlO₂ were formed, leading to a strong bonding along the interface. The thermal cycling reliability and adhesion strength of DAB substrates were also evaluated. The results show that the DAB substrates can satisfy application requirements completely.     

Sensitivity of novel 460 MPa proof strength titanium–vanadium microalloyed high strength low alloy steels to annealing linespeed and soak temperature

High strength low alloy steels are characterised by predominantly ferritic microstructures, strengthened by grain boundary and precipitation strengthening. Both of these strengthening mechanisms traditionally arise from the niobium addition. Increasing the niobium addition would theoretically increase strength. However, increasing niobium content above ~ 0.04 wt.% is not recommended in industrial practice due to narrowing of the annealing process window. Two novel grades exhibiting different additions of titanium and vanadium in place of the traditional niobium addition were investigated. Sensitivity to annealing linespeed and soak temperature was investigated to conclude whether a practically achievable process window exists and moreover, to conclude whether proof strength in excess of 420 MPa could be achieved while satisfying the maximum ultimate tensile strength and minimum total elongation specifications of CEN Grade HC420LA under European Standard EN 10268:2006. One of the two novel grades, exhibiting higher manganese and vanadium contents, met the minimum proof strength target, while almost satisfying the maximum ultimate tensile strength and minimum total elongation specifications. However, the annealed microstructure was found to be partially recrystallised, which is not recommended in industrial practice. Moreover, sensitivity to annealing linespeed and soak temperature was considered too great to obtain a practically achievable process window.     

Consolidation of Al2O3–GdAlO3 eutectic powder prepared from induction-melted solid and strength at high temperatures

A eutectic powder of Al₂O₃–GdAlO₃ was melted using a Mo crucible by induction heating. The melt was slowly solidified, resulting in a eutectic solid with coarse Al₂O₃ and GdAlO₃ phases. The eutectic solid was ground and sieved into 3–44 μm and 64–124 μm particles. The powders were consolidated to produce a eutectic composite by spark plasma sintering. Mechanical properties of the consolidated eutectic composite were measured at room temperature. High temperature strength was obtained at temperatures up to 1673 K. Superplastic deformation of the eutectic composite was not observed on stress–strain curves at 1673 K, but did occur in the case of a conventional composite at 1573 K.     

Dynamic recrystallization behavior of Ti–5Al–5Mo–5V–1Cr–1Fe alloy

Dynamic recrystallization process was considered as an important method to fabricate titanium workpieces with desired properties. The objective of this study was to investigate dynamic recrystallization behavior of Ti–5Al–5Mo–5V–1Cr–1Fe alloy through isothermal compression experiments. The volume fraction of dynamic recrystallization was quantified with the net softening effect by dynamic recrystallization (DRX). The saturated stress during hot deformation process was analyzed based on dislocation evolution influenced by working-hardening and dynamic recovery. The linear relationship between the saturated stress and peak stress has been obtained. The dependence of DRX process on deformation parameters has been discussed in detail and a model based on Avrami kinetics has been proposed to track DRX process with strain. A constitutive model incorporating DRX process has been proposed to describe the flow curves at large strains.