Mechanical anisotropy and stress–strain rate characteristics in superplastic deformation of titanium alloys

Abstract

Uniaxial tension and compression tests have been carried out on two titanium based alloys, Ti–6Al–4V in the form of extruded tubes and forged plates and Ti–3Al–10V–2Fe sheets, to study anisotropic behaviour during superplastic deformation. The following were observed: (i) originally round cross-section became elliptical after deformation; (ii) the flow stresses and strain rates were dependent on the orientation of the specimens; and (iii) the strain anisotropy became less severe as the strain rate increased. These characteristics of anisotropy were related to the original microstructure (e.g. the mechanical fibring of the α grains) and the microstructural evolution during superplastic deformation. New constitutive equations for describing anisotropic superplastic deformation have been proposed to explain the effect of strain rate or stress on anisotropy.     

Evaluation of hot strip mill backup roll cladding

Abstract

A new cladding material based on the Fe–Cr–Mo–V–C alloy system, suitable for submerged arc welding, has been designed for the refurbishment of forged and cast backup rolls used in the finishing stands of hot strip rolling mills. The work undertaken includes mechanical analysis, mechanical testing, and microstructural characterisation. The mechanical analysis indicated the nature and level of stresses operating near the surface of rolls; mechanical testing allowed material performance to be anticipated. An optimal post-weld heat treatment procedure, which maximises strength while minimising material strain hardening, was subsequently chosen. The microstructure of the candidate cladding material is a mixture of lower bainite and martensite, containing a very fine distribution of molybdenum carbides. In situresults have shown that welded rolls outperform traditional rolls, as the amount of steel rolled per millimetre of cladding material is 40% higher than with forged rolls and double that obtained with cast rolls.     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Development of high performance powder metallurgy steels by high-energy milling

High-energy milling is considered to be one of the most efficient techniques for producing materials that have a well-controlled chemical composition and microstructural features that are difficult to obtain using other synthesis routes. In this study, the mechanical alloying technique (MA) was used to develop special powder metallurgy (PM) steels with two different types of properties. The use of this technique is essential for obtaining the target microstructure, which ensures the desired performance of the resulting material. Oxide dispersion strengthened (ODS) ferritic steels were produced using MA based on the prealloyed grade Fe–20Cr–5Al and Fe–14Cr–5Al–3 W steels with the addition of Ti and Y₂O₃ as reinforcements. The incorporation of Y₂O₃ enables a homogeneous dispersion of nano-oxides and nano-clusters in a submicron-grained structure that should enhance the mechanical properties up to 600 °C. In addition, the base alloying system, Fe–Cr–Al (Ti), should enable the development of protective oxide layers through high-temperature treatments, which improves the compatibility with the environment, avoids liquid–metal embrittlement and contributes to the increase of the mechanical response up to 600 °C. Furthermore, microalloyed powders can be obtained using MA and consolidated with a pressure-assisted sintering process in an attempt to control the final grain size, to achieve microalloyed steels, and to achieve an extraordinary balance of properties.     

Laser surface remelting of Cu–Cr–Fe contact material

Abstract

Laser surface remelting/resolidifying treatment on a powder metallurgically manufactured Cu–Cr–Fe contact material was studied. Test results showed that a compact remelting/resolidifying layer was obtained with appropriate laser treatment conditions and a suitable surface absorption coating. After such treatment, the Cu–Cr–Fe microstructure was greatly refined and the Cr phase was uniformly dispersed in the Cu rich matrix with fine spherical or near spherical form. Improved compactness and microstructure of the laser remelted Cu–Cr–Fe material yielded increased hardness (by ~80%), wear resistance, and a reduced friction coefficient compared with the base material. The mechanism of laser strengthening is discussed in relation to the microstructural features of the Cu–Cr–Fe material.     

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

Abstract

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.     

Factors affecting dynamic recrystallization of metals and alloys

Abstract

The high-temperature mechanical behaviour of copper, Cu–Al alloys, and nickel has been examined using torsional testing with hollow testpieces in conjunction with microstructural observations on deformed and quenched specimens using both optical and electron microscopy. Dynamic recrystallization occurred in these materials as the restoration process during high-temperature deformation. The factors influencing dynamic recrystallization have been considered, including materials of high stacking fault energy. It was found that the regime of dynamic recrystallization and the transition in flow stress behaviour could be reasonably represented in terms of the Zener–Hollomon parameter. In Cu–Al solid solution alloys, although the addition of the solute aluminium into copper lowered the stacking fault energy, dynamic recrystallization was retarded to higher strains due to the reduced mobility of the grain boundary. By mechanical and microstructural analysis of the behaviour of various single phase metals and alloys during dynamic recrystallization, the factors influencing the behaviour (i.e. stacking fault energy (solute elements), Zener–Hollomon parameter (deformation condition), and strain) can be summarized on a three dimensional schematic.

MST/587     

Influence of prior straining on superplastic behaviour of an Fe–Cr–Ni alloy

Abstract

The superplastic behaviour of a microduplex Fe–Cr–Ni (25·7Cr–6·6Ni) alloy was investigated in the as-worked, annealed, and prestrained conditions. In the early stages of deformation, flow stress depends significantly on strain, and also on the instantaneous microstructural state in the case of as-worked and annealed specimens. Under these conditions, the empirical parameters of the constitutive equation for superplastic deformation were found to depend systematically on strain. At 1000°C, strain hardening predominates, and this could be accounted for by grain growth and by the hardening produced by the noticeable dislocation activity. After suitable prestraining, steady-state deformation conditions may be attained; this may facilitate the collection of σ–ε data, which could then be used to assess the relative importance of the appropriate deformation mechanisms.

MST/125     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

Microstructure characterisation and tensile properties of 18Cr-4.5Al-0.3Zr- oxide dispersion strengthened steel

The 18Cr–4.5Al–0.3Zr–oxide dispersion strengthened (ODS) steel was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) technique. A microstructural characterisation was performed on an 18Cr–4.5Al–0.3Zr–ODS steel using high angle annual dark field (HAADF) and synchrotron small angle X-ray scattering (SAXS). HAADF and SAXS results showed that high-density nanoscale oxides are formed in 18Cr–4.5Al–0.3Zr–ODS steel. The oxides in the specimen can be roughly divided into two categories according to their compositions: (1) core/shell structure oxides with Al–O oxide cores and Y shells; (2) nm-scale trigonal-phase Y₄Zr₃O₁₂ oxides. In addition, tensile testing results revealed that the specimen exhibited better tensile strength and ductility as compared with another commercial ODS steels with similar composition.     

Influence of powder metallurgical processing on production and properties of rapidly solidified Al–5Cr–2Zr,Al–6·43Cr–1·67Zr,and Al–4Cr–1Fe extrudates

Abstract

Rapidly solidified Al–5Cr–2Zr, Al–6·43Cr–1·67Zr, and Al–4Cr–1Fe alloy powders were processed using cold compaction and hot extrusion. It was found that the iron containing alloy was more easily extruded than the zirconium containing alloys, and this was attributed to phase transformations occurring during deformation of the latter. By obtaining extrudates in the form of rectangular bars, mechanical properties could be studied for both the transverse and the longitudinal directions. Compared with the longitudinal direction, no significant decrease of strength was detected in the transverse direction for any of the alloys, whereas significant decreases of ductility were recorded, especially for Al–6·43Cr–1·67Zr alloy. Fracture was observed to occur along primary powder particle boundaries. The relationship between microstructure and mechanical properties was also investigated. Compared with the addition of iron, it was found that additions of zirconium are more beneficial in that they promote formation of fine intermetallic phases. In addition, between the Al–Cr–Zr alloys, a reduction of chromium content yields a more homogenous and fine microstructure, which combined with the beneficial effects of increased additions of zirconium results in superior properties.

MST/1118     

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

Abstract

Transient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are generally larger than those produced when the alloy is bonded in the recrystallised condition. The differences in microstructural developments across the joint are discussed in terms of stored strain energy of the parent metal grains.     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Thermodynamics of duplex stainless steels

Abstract

The thermodynamic predictions of microstructures in the multicomponent alloys used for duplex stainless steels are discussed in detail in this paper. The production of the duplex ferrite–austenite microstructure by high temperature (1323–1423 K) annealing is considered. Phase diagrams are calculated for Fe–Cr–Ni ternary alloys and the more complex multicomponent systems. It is shown that there is good agreement between the predictions of the thermodynamic models and the measurements carried out on heat treated alloys. The low temperature aging behaviour of Fe–Cr–Ni ternary alloys is discussed and the limitations of activation energy approaches to the modelling of the aging process in these alloys are shown.

MST/1201     

Tailoring the secondary phases and mechanical properties of ODS steel by heat treatment

The oxide dispersion strengthened (ODS) steel with the nominal composition of Fe–14Cr–2W–0.3Ti–0.2V–0.07Ta–0.3Y₂O₃ (wt%) was fabricated by mechanical alloying and hot isostatic pressing (HIP). In order to optimize the relative volume fraction of secondary phases, the as-HIPed ODS steel was annealed at 800 °C, 1000 °C, 1200 °C for 5 h, respectively. The microstructures and different secondary phases of the as-HIPed and annealed ODS samples were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The tensile properties of all the ODS steels at room temperature were also investigated. The results indicate that annealing is an effective way to control the microstructure and the integral secondary phases. The annealing process promotes the dissolution of M₂₃C₆ particles, thus promoting the precipitation of TiC. No obvious coarsening of Y₂Ti₂O₇ nanoparticles can be observed during annealing. The tensile results indicate that the annealed ODS sample with the optimized secondary phases and high density possesses the best mechanical properties.     

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.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

High temperature deformation behaviour of Al-Fe-V-Si alloys

Abstract

The high temperature deformation behaviour of three rapidly solidified dispersion strengthened Al-Fe-V-Si materials with three different compositions has been investigated. The three materials contain high volume fractions, ranging from 16 to 36%, of small round dispersoids. The stress exponent and the activation energy for deformation are anomalously high and are temperature dependent. The material with the highest volume fraction of dispersoids showed the highest stress exponent dependence on temperature. At the same time, microstructural characterisation revealed that this material becomes heterogeneous and loses creep resistance as a result of microstructural changes. A correlation is found between the stress exponent and the microstructural changes. A ‘skeleton line’is defined for creep behaviour analysis which allows comparison of the investigated materials and contributes to their characterisation.     

等离子熔覆铁基涂层的组织及冲蚀磨损研究

采用等离子熔覆法制备了铁基涂层.研究了涂层的组织结构,测试了涂层的显微硬度及耐冲蚀磨损性能,并利用扫描电镜对涂层显微组织、冲蚀表面形貌进行了分析.结果表明:涂层显微硬度是基体材料不锈钢1Cr18Ni9Ti的2倍,最高达到550,涂层冲蚀后质量损失是不锈钢对比试样1Cr18Ni9Ti和0Cr13Ni5Mo的1/2左右.     

Preparation and structural investigation of nanostructured oxide dispersed strengthened steels

Although capability of steels has been improved in the past by thermomechanical treatment, utilization of powder metallurgy provides more controlled microstructure, a homogeneous dispersion of nanosized oxide particles in the metal matrix and tailored properties in terms of strength and radiation resistance. This article is summarizing recent results on preparation, structural, and mechanical investigation of oxide dispersed strengthened steel (ODS). Two commercial steel powders, austenitic 17Cr12Ni2.5Mo2.3Si0.1C and martensitic Fe16Cr2Ni0.2C powders have been used as starting materials. Nanosized yttria dispersed martensitic and austenitic sintered steel samples have been realized by powder metallurgical methods. An efficient dispersion of nano-oxides in ODS steels was achieved by employing high efficient attrition milling. A combined wet and dry milling process of fine ceramic and steel particles is proposed. Spark Plasma Sintering (SPS) was applied to realize nanostructured steel compacts. Grains with 100 nm mean size have been observed by SEM in sintered austenitic ODS. In comparison, the sintered martensitic dry milled and martensitic dry and combined milled ODS microstructure consisted of grain size with 100–300 nm in each case. A brittle behavior is shown in all of the cases. The martensitic ODS is two times harder than the austenitic ODS. The bending strength high as 1806.7 MPa was found for the martensitic ODS, whereas 1210.8 MPa was determined for the austenitic ODS. The combined milling assured higher strength and hardness compared to dry milling.