Evaluation and review of simultaneous transformation model in high strength low alloy steels

Abstract

This work briefly describes and evaluates one of the most complete transformation models, which deals with the non-isothermal decomposition of austenite. The model, that does not consider the effect of precipitation on phase transformations, has been experimentally validated in high strength low alloy steels in order to evaluate how it works for microalloyed steels, where precipitation may play an important role. It has been found that the simultaneous transformation model is able to predict with an excellent agreement in microalloyed steels the formation of microstructures consisting of ferrite plus pearlite. However, the bainite formation is not successfully described by the model. The calculations incorrectly predict the formation of martensite instead of bainite in many situations.     

Structure evolution in annealed and hot deformed Al–V2 O5 composite

Abstract

Structural observations and hot deformation tests were carried out on mechanically alloyed Al-10 wt-%V₂ O₅ composite. Initial annealing experiments revealed a hardening of the material during the first stage of annealing. The material hardness increased from 114 HB for as extruded material to 167 HB after annealing at 873 K for 6 h. Differential scanning calorimetry tests conducted on as extruded material confirmed the development of an exothermic reaction during heating of the material within the temperature range 650–870 K. The amount of heat released was reduced with increasing annealing time at 873 K. Transmission electron microscopy (TEM) and X-ray analysis of annealed material revealed new intermetallic grains and very fine aluminium oxide particles, which resulted from the chemical reaction between the aluminium matrix and vanadium oxides. The development of voids in long aged specimens was found to be an undesirable effect of local specific volume reduction during the course of the chemical reaction that was not fully compensated by the local volume increase due to the growth of intermetallic particles. As a result, the material hardness was reduced in long time annealed specimens. The mechanical properties of as extruded and annealed specimens were investigated by means of hot compression testing within the temperature range 623–903 K. These tests revealed that the flow stress of as extruded material was reduced from 180 to 22 MPa when tested at 623 and 903 K, respectively. Annealed specimens exhibited higher flow stresses of 195 and 32 MPa at the same temperatures. The results indicate that the strength of the material can be effectively increased owing to a change of material structure as a result of the chemical reaction taking place during high temperature annealing.     

Reactive sintering of boron-doped Ni76Al24 intermetallic

A preliminary investigation into the formation of boron-doped nickel-rich Ni₃Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni₃Al) from elemental powders by reaction synthesis was carried out. The application of reaction synthesis was seen as a low-energy alternative to the production of Ni₃Al/boride composite suitable for wear applications. X-ray diffraction, Neutron diffraction, SEM/EDS,WDS, Image analysis, Archimedes principle and Rockwell hardness measurements; were used to study the effect of boron addition on the final microstructure, average grain size, bulk density and hardness of as-prepared Ni₇₆Al₂₄. Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni₃Al, however, at a boron content of 0.5 wt% an apparent transition from a single phase microstructure to a two-phase intermetallic/boride composite microstructure was observed, which dominated when the boron content increased, up to 2 wt%. The two-phase microstructure was identified as Ni₃Al (particles) within an Ni₄₁Al₅B₁₂ boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni₃Al via two mechanisms. Below the solubility limit, the increase in hardness was due to solution hardening. Above 0.5 wt%B, solution hardening in addition to the formation of the harder boride phase, were found to amount to up to 50% increase in the hardness compared with boron free Ni₃Al. The extrusion of semi-molten beads at the surface of the compact at high B-content may be a limiting factor, in the formation of Ni₃Al/boride composites via this route.     

Effect of superimposed oscillations on creep behaviour of Al – 2 4·.5Cu and Al – 2 4·5Cu – 2 0·1In (wt-%) alloys containing θ′ precipitates

Abstract

The behaviour of both transient and steady state creep of Al – 4·5Cu and Al – 4·5Cu – 0·1In (wt-%) alloys was investigated using a constant stress where torsional oscillations of different frequencies and shear strain amplitudes were operated at various working temperatures. An augment in both transient and steady state creep rates was observed by increasing both frequency and shear strain amplitude of the applied oscillations. The mean values of the activation energy of both alloys for both creep stages were found to be equal to that quoted for dislocation intersection mechanism.     

Interfacial microstructure and mechanical behaviour in laser clad TiCp/Ni alloy coatings

Abstract

Coatings of TiC_p reinforced composite have been produced by laser cladding. Two kinds of coating with different TiC_p origins were investigated, i.e. undissolved TiC_p and in situ TiC_p. For undissolved TiC_p, epitaxial growth of TiC, precipitation of CrB, and a chemical reaction occur at phase interfaces, and nanoindentation loading curves show pop in marks caused by the plastic deformation associated with crack formation or debonding of TiC_p from the matrix. As for in situ TiC_p, no pop in mark appears. Meanwhile, in situ TiC_p produces hardness and elastic modulus values that are higher than those produced by the coating that contains undissolved TiC_p.     

Interfacial reaction and its effect on strength of Al18 B4O33 /Al composites

Abstract

Aluminium alloy 6061, AC8A, Al–1Mg, Al–9Cu and pure aluminium composites reinforced with aluminium borate whiskers were fabricated by a squeeze casting process. The interfacial reaction in the composites and its effect on the bending strength are discussed, together with the results from SEM, TEM, and X-ray diffraction. A slight interfacial reaction is favourable for composite strength as it has the effect of anchoring the whiskers. A T6 treatment can enhance the strength of an Al–9Cu matrix composite, but is not efficient for magnesium containing 6061 and AC8A matrix composites. Furthermore, if heated at temperatures higher than 793 K for a long time, the composite strength drops rapidly owing to whisker damage and shortening during the interfacial reaction. It is suggested that the interface in an Al₁₈ B₄O₃₃ /Al alloy composite is stable below 623 K which is the temperature requirement for automobile engine components.     

Influence of boron on the microstructure and mechanical properties of Al-11.6Si-0.4Mg casting alloy modified with strontium

Abstract

The influence of the addition of Al-1B master alloy on the microstructure and mechanical properties of Al-11.6%Si-0.4%Mg alloy modified with 0.030%Sr has been investigated. The mechanical properties and fracture behaviour of the alloys as cast and after T6 heat treatment with three different melt treatments (no treatment; 0.030%Sr modifying treatment; and 0.030%Sr + 0.028%B combined melt treatment) were also compared. Al-1B master alloy has a strong action in refining the dendritic structure in near eutectic Al-Si casting alloys modified with Sr. The Sr+B combined melt treatment can improve considerably the mechanical properties of the alloys, both as cast and after T6 heat treatment. Fracture modes of the alloys with the Sr modifying treatment and the Sr+B combined melt treatment are typically ductile. However, fractographs indicate that the alloy with combined melt treatment suffered greater ductile deformation before fracture. The Sr+B combined melt treatment significantly improves the mechanical properties of near eutectic Al-Si casting alloys.     

Mechanical behaviour of Zr doped Ni3Al alloy: effects of heat treatment,environment, and temperature

Abstract

The mechanical behaviour of polycrystalline Ni₇₆Al₂₃Zr alloy was studied as a function of heat treatment, environment, and temperature. It was found that the tensile ductility was very sensitive to temperature, the alloy showing low ductility at temperatures from 700 to 1000°C both in air and vacuum. Environmental embrittlement could be alleviated for those specimens with elongated grains. The ductile transgranular fracture was explained by stress concentration at the intersection of slip bands and grain boundaries. It was also found that an oxide layer, formed during tensile testing at elevated temperature, affected the environmental embrittlement of Ni₃Al(Zr) alloy. An adherent Al rich oxide film was effective in protecting the underlying alloy from oxygen penetration.     

Creep and stress rupture behaviour of Be/Al composite material Albemet 162

Abstract

Creep and stress rupture tests were conducted on Albemet 162, a Be particulate reinforced Al matrix composite with a 70% volume fraction of reinforcement. A power law relationship was observed between steady state creep rate, stress rupture life, and applied stress. Considerable differences were seen between transverse and longitudinal specimens in terms of creep and rupture behaviour. Fractography of the specimens revealed that particle agglomeration of the Be phase is highly susceptible to creep damage and the primary cause of creep failure, which was observed in specimens of both orientations.     

Effect of composition on kinetics of athermal martensite formation in plain carbon steels

Abstract

The kinetics of martensite formation in three Fe–C–Mn alloys has been determined using dilatometry, and is compared with the data for other steels reported in literature. Each curve can be well described by the Koistinen–Marburger equation using a composition dependent start temperature and rate parameter α _m. The empirical relationship derived for α _m as a function of the chemical composition can improve predictions with the Koistinen–Marburger model of the volume fraction martensite at a certain temperature.     

Morphology and constitution of the phases in as-welded microstructure of austempered ductile iron

Abstract

It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.     

Effects of temperature and grain size on deformation and fracture in recrystallized Ni3Al doped with boron

The effects of temperature and grain size on the deformation and fracture behaviour of recrystallized Ni₃Al doped with boron were investigated by tensile tests at temperatures up to 973 K as a function of grain sizes from 1.6 to 105m. The yield stress showed a positive temperature dependence to a peak temperature in somewhat different manners depending on the grain size. For coarse-grained specimens, a rapid drop in elongation was observed with increasing temperature. The predominant fracture mode changed with temperature from the transgranular fracture of {1 1 1} cracking to brittle intergranular fracture. This embrittlement at elevated temperatures was considered to occur by a high stress concentration at grain boundaries arising from increased flow stress level and the occurrence of grain boundary sliding (GBS). In contrast, the elongation was not so markedly decreased with temperature for intermediate- and fine-grained specimens which exhibited ductile intergranular fracture and cavitation fracture, respectively, at elevated temperatures, and a slant-type fracture and cup-cone fracture, respectively, at low temperatures. The suppression of serious high-temperature embrittlement for intermediate-grained specimens was explained in terms of the slow propagation of a crack formed by GBS, owing to stress relaxation by dynamic recrystallization (DR) and plastic deformation. In the case of ultra-fine-grained specimens a large elongation was developed at elevated temperatures, which was interpreted as that the further occurrence of DR with increasing volume fraction of grain boundaries reduces the cavitation promoted by GBS, and that the limited sliding length due to extremely small grain diameter raises the stress for cavity formation.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Effect of structural relaxation on critical fieldsH c andH c2 and on resistivity in sputtered amorphous alloys Zr76Cu24 and Zr76Ni24

We report critical field results on sputtered amorphous Zr₇₆Cu₂₄ and Zr₇₆Ni₂₄ alloys. TheH _c (T) obtained from calorimetric measurements is well described by assuming that both systems are weak coupling superconductors. A similar good agreement is not found forH _c2 (T) obtained by a resistive method; tentative explanations of this discrepancy are suggested. We also report resistivity measurements in the normal state on Zr₇₆Ni₂₄ alloys. Results are well interpreted in the framework of the Faber-Ziman diffraction model. An alternative explanation, the Kondo-like model for two-level systems, is also discussed.Also at the Service National des Champs Intenses.     

Ni3Al合金热/机械疲劳裂纹扩展行为研究

对ＩＣ１０定向凝固高温合金进行了４５０￣９９０℃热／机械疲劳裂纹扩展行为的试验研究。分别研究了相位角、保持时间、温度、频率对ＩＣ１０合金裂纹扩展行为的影响,并建立了热／机械疲劳裂纹扩展速率预测模型。研究发现：温度的升高和频率的降低均会加速裂纹扩展;同相位热／机械疲劳裂纹扩展速率大于反相位热／机械疲劳裂纹扩展速率,而且它们两者介于最大温度和最小温度的等温度裂纹扩展速率之间;载荷保持加速裂纹扩展。     

Characterisation and formation of boundary in laser cladding AZ91D with Al + Al2O3

Abstract

The laser surface cladding of AZ91D magnesium alloy with Al + Al₂O₃ powders was investigated. The growth morphology of the boundary zone between the laser surface cladding layer and AZ91D substrate, and the distribution of Al₂O₃ particles in the laser surface cladding layer, were observed by optical microscope and scanning electron microscope. Furthermore, the element line and mapping scans analyses on the boundary zone were carried out with energy dispersive spectrum. The phases in the laser cladding layer and the substrate of AZ91D magnesium alloy were identified by X-ray diffraction. The results showed that the distribution of the Al₂O₃ particles was homogeneous in the laser surface cladding layer and the growth morphology of the boundary zone was in a unique parallel branching feature. The formation of the parallel branching dendrite is mainly related to the temperature gradient, the ultimate rate of dendrite growth and the rate of pool solidification on the growing fronts.