Materials Science and Technology

The mechanical properties of aluminium alloys with non-equilibrium microstructures necessary for heat treatment simulation are not available. Therefore, compression tests of undercooled aluminium alloys such as AlSi1MgMn and AlZn4·5Mg1 have been performed in a quenching and deformation dilatometer with varying quenching rates and quenching finish temperatures. The compressive load on quenching finish temperature was applied immediately after quenching. The mechanical properties such as yield strength and strain hardening are strongly dependent on quenching rates and finish temperatures of quenching. Flow curves depending on quenching rates and quenching finish temperatures have been implemented in a quenching simulation by the finite element method. For the cooling process of extrusion profiles in water and in a gas nozzle field, the simulation results are presented.

The present study focuses on the grain refining of NiCrMoV steel. All the samples were austenitised at 1230°C for 10 h, followed by holding at 350°C for 20 h and then experienced different heat treatment processes to refine the grain. It is proved that the isothermal treatment newly proposed in the present paper is much more effective than the conventional normalising process due to the formation of pearlite, which can cut off the structural heredity between parent austenite and martensitic or ferritic product in NiCrMoV steel.

Microstructural evolution in thermomechanical processing is of great importance to obtain good final properties for high strength low alloy steels. This work presents a microstructure based physical model to describe the microstructural evolution of Nb microalloyed steels under controlled rolling. Several factors of dislocation density, recovery, recrystallisation and precipitation have been coupled in the model. The dynamic recrystallisation (DRX) behaviour of Nb microalloyed steels has been investigated to derive the model. The recrystallisation is described as a nucleation grain growth process, and critical radius of subgrain has been used to estimate the occurrence of dynamic recrystallisation. Predictions of microstructural evolution under various hot rolling conditions using this model agree well with reported experimental results. The correlation among evaluation time, DRX rate and recrystallisation grain size can be effectively predicted by this model under different deformation conditions of hot rolling.

Multilayered metal composites consisting of alternating metals or alloys have been well developed to obtain the superior mechanical properties different from those in any of the constituent materials. The layer integrated steel and metals research project sponsored by Japan government started from 2006 and aimed to fabricate steel sheets with high strength (>1200 MPa) and superior elongation (>20), through the composite design of high strength steels and ductile steel layers. In the present work, two kinds of hot rolling bonded multilayered composites consisting of austenitic stainless steel/bainite (martensite) steel are focused. The effects of hot rolled microstructure and the annealed microstructure on the tensile behaviour of a 25-layer laminated composite steel are discussed. On the other hand, the tensile deformation microstructure of the hot rolling bonded and annealed 15-layer composite steel is systematically investigated in order to find the reason for the enhanced plasticity of the brittle layer in the composite.

The crystal plasticity finite element method has been adopted to study the plastic deformation behaviour, such as the slip system activation, the number and magnitude of the actived slip system, the rotation character and the macromechanics response of face centred cubic (fcc) single crystal metal during uniaxial tension. Five typical crystalline orientations, including four types of typical fcc texture orientation: Goss orientation, copper orientation, S orientation and brass texture, have been considered, and its effects to plastic deformation have been studied and compared. The results show that the orientation between crystalline and loading direction will influence the slipping character and the macromechanical response: the S crystalline orientation (60, 32 and 65°) has the least number of active slip system (just only 3); the brass texture (35, 45 and 0°) has the smallest magnitude of crystalline lattice rotation; and the brass texture orientation (35, 45 and 0°) has the highest yield stress value.

First principles thermodynamic models based on the cluster expansion formalism, lattice dynamic calculations and quantum mechanical total energy calculations are employed to compute the thermal stability of metastable hardening precipitations in hcp structure α-Mg–Gd binary alloys. It shows that vibrational entropy reverses the energetic preference and plays a critical role in hardening precipitation at different aging temperatures in Mg–Gd binary alloys. In addition to energetics, the analysis of bonding charge density reveals that the metastable β′ phase is responsible for the high strength during subsequent isothermal treatment. Our results are found to be in good agreement with experimental measurements and helpful in clarifying the metastable precipitation sequence in Mg–Gd binary alloys.

The Mg–Li alloys with Li content of 5–11 mass- will exhibit a dual phase structure of α (hcp) and β (bcc) phases. These dual phase Mg–Li alloys have excellent formability, as well as extra low density. However, the Mg–Li alloys still have disadvantages of poor corrosion and wear resistance. In the present study, the AlN films deposited on Mg–Li alloy by radio frequency magnetron sputtering were investigated. The microstructure, surface morphology and corrosion resistance of the AlN films were examined by means of X-ray diffraction, SEM, TEM, and potentiodynamic polarisation. Experimental results show that deposited AlN films exhibit an amorphous structure, which can effectively improve the corrosion resistance of Mg–Li alloy. Meanwhile, the sputtering parameters, such as the radiofrequency power and working pressure, are found to have significant effects on the corrosion behaviour of deposited AlN films on Mg–Li alloy.

The corrosion process of Al–5Zn–0·02In–1Mg–0·05Ti (wt-) alloy was investigated by electrochemical noise. The corroded surfaces of the alloy were observed by scanning electron microscopy. The electrochemical noise data obtained from the corrosion process were analysed using shot noise theory. The pitting and uniform corrosions were confirmed by cumulative probability F [F = (fn)] versus the frequency of events fn. The Weibull probability plots show the three corrosion stages for the alloy: pitting, pitting and dissolution–precipitation and uniform corrosion. The physical model for the corrosion process has been developed.

Owing to their large and curved shape, blade castings, a key component for heavy hydro turbines, are susceptible to deformation during casting and heat treatment. In the present paper, the stress analysis of a blade casting during both casting and heat treatment is performed. The coupled thermo-stress and thermo-phase transformation stress models are used for casting and heat treatment respectively. Machining allowance distribution is used as the deformation criterion and an algorithm of inverse deformation determination is presented. The mechanical properties of the martensitic stainless steel ZG₀Cr₁₃Ni₄Mo (13Cr–5Ni–1Mo) at different temperatures are measured under as cast and heat treated status. Finally, the inverse deformation of the blade during both casting and heat treatment processes is obtained, and a series of sections of the blade casting with inverse deformation design are given for pattern making. The calculated deformation results are compared to the measured one, and they are basically in agreement.

A coupled thermomechanical three-dimensional finite element model was developed for friction stir welding in the ABAQUS environment using Johnson–Cook material law and Johnson–Cook failure model. The temperature evolution during the plunge, dwell and moving stages of a friction stir welded 7050 aluminium alloy and the effect of heat conduction by the back plate were investigated. Results show that the temperature almost symmetrically distributes across the plate cross-section, and the temperature contour in the weld nugget zone presents a V type shape after the plunge stage. In the dwell stage, the frictional heat conducts around to preheat the plate. While in the moving stage, the heat gradually accumulates until a quasi-stable temperature field is formed. Moreover, it is shown that the heat conduction through the back plate has a significant effect on the temperature field. With the increasing heat convective coefficient of the back plate, the temperature field remarkably shrinks.

To develop new steels that exceed the ballistic resistance, strength and toughness of current naval ship steel plates, we designed optimally quenching–lamellarising–tempering treated low carbon 10Ni steel plates. These plates displayed exceptional properties in all aspects; when compared to the widely used HSLA-100 steel plates, they showed superior toughness and an improvement of over 15 in strength and 20 mm fragment simulation projectile ballistic limit V₅₀. A preliminary vibrating sample magnetometer study on the effect of quenching–lamellarising–tempering process and dynamic deformation on the microstructure evolution of the 10Ni steel during ballistic impact was valuable to our understanding of the steel. The data led us to conclude that dynamic deformation during ballistic perforation induced an austenite to martensite phase transformation, which improves the global dynamic plasticity and ballistic resistance of the steel target. This mechanism, similar to the transformation induced plasticity effect, can be appropriately named ballistic induced plasticity. Characteristics of the microhardness maps of the sectioned craters created by ballistics are consistent with ballistic induced plasticity.

The effects of ferrite–pearlite and ferrite–martensite starting microstructures on ferrite grain sizes and carbide particle sizes after cold rolling and prolonged annealing treatment have been investigated. Ferrite–martensite starting microstructures showed finer grain and particle sizes and improved tensile properties after cold rolling annealing cycle compared to ferrite–pearlite starting microstructures. A ‘fibrous’ martensite morphology developed by intermediate quenching treatment is more beneficial in that respect compared to the ‘blocky’ martensite morphology obtained from the step quenching treatment.

In order to reduce the cost of alloying elements, Mn series ultralow carbon bainitic (ULCB) steels have been developed. The different cooling conditions have been used to improve the mechanical properties of Mn series ULCB steels in the present study. The hot rolled ULCB steel plates with 26 mm thickness were water quenched to different finishing quenching temperatures (550, 450 and 350°C) followed by air cooling. Results show that the as rolled ULCB steel plates exhibit high yield strength of 775 MPa and low energy transition temperature of −55°C when the finishing quenching temperature is chosen at 450°C. Low temperature impact toughness was improved through control of the finishing quenching temperature. Microstructure observations show that a fine granular bainite structure is obtained, and the granular bainite structure exhibited shorter crack propagation path than both granular ferrite and lath martensite structures, which contributed to the improved combination of strength and toughness. Furthermore, the formation process of fine granular bainite structure was investigated.

Au−30 at-Sn eutectic alloy was fabricated by sequentially pulse electroplating Au and Sn films on Si chips. Three kinds of Au/Sn/Au triple layer films were prepared in the present work: Au/Sn/Au (6/6/1 μm) films, Au/Sn/Au (6/6/6 μm) films and Au/Sn/Au (8/6/1 μm) films. The microstructure and phase transformation in Au/Sn/Au films during aging and reflow soldering were investigated. For Au/Sn/Au (6/6/1 μm) films during aging at 100 and 150°C, the layered AuSn/AuSn₂/AuSn₄ structure formed in the reaction region. Furthermore, the Sn film was completely consumed, and AuSn₄ finally transformed into AuSn and AuSn₂ after aging at 150°C for 15 h. For Au/Sn/Au (6/6/6 μm) films during aging at 150°C, the electroplating sequence had an important effect on the formation of Au−Sn phases. An Au₅Sn layer was present at the Au II/Sn interface but not at the Au I/Sn interface. For Au/Sn/Au (8/6/1 μm) films, the micropores that formed preferentially along the Au₅Sn/AuSn interface remarkably decreased with increasing reflow temperature from 280 to 310°C. After reflowing for 10 s, the microstructure was not an Au−Sn eutectic; however, after reflowing for 60 s, coarsened primary Au₅Sn phase and typical Au−30 at-Sn eutectic microstructure of fine eutectic phases (AuSn+Au₅Sn) formed.

Microstructure refinement of ternary eutectic Fe₄₀Ni₄₀B₂₀ alloy in undercooling and rapid cooling solidification has been studied. In natural cooling conditions, two kinds of microstructure refinement take place with increasing undercooling. For melts with the same undercooling, the as solidified microstructure refines first and then coarsens with increasing cooling rate. On this basis, repeatable experiments show that the combination of melt undercooling at ∼50 K with Ga–In alloy bath rapid cooling produces very fine eutectic cells (∼5 μm in diameter with the lamellae spacing <100 nm). This indicates that an appropriate initial melt undercooling combined with a proper cooling rate is a potential way to produce a bulk nanostructure in ternary Fe₄₀Ni₄₀B₂₀ alloy.

Fatigue related failures of small bore piping caused by vibration of socket welds often occur at nuclear power plants. The purpose of the present study is to evaluate the integrity of the socket weld in nuclear piping under the condition of vibration testing. The specimens were bolted to the shaker table and shaken simultaneously near their resonant frequencies to produce the desired stress. The test results show that the failures at higher stress tend to originate at the toe, while that for the case of lower stress failures tends to occur at the root. The effects of penetration depth and radial gap were also investigated by finite element method. Results show that the higher penetration depth and radial gap can decline the stress distribution in the weld root, which is beneficial to fatigue life. Further, the desired radial gaps can significantly reduce the von Mises stresses, which decrease from 150·4 to 82·7 MPa with the width of radial gap from 0·1 to 0·4 mm.

Isothermal compression tests are carried out on 20CrMnTiH steel using a Gleeble-1500 thermal simulator in the temperature range of 1223–1423 K and in the strain rate range of 0·01–10 s⁻¹. The flow behaviours of 20CrMnTiH steel are described, which are based on the analysis of true stress–true strain curves. The effects of hot deformation on the microstructure are investigated. The results show that there is a typical dynamic recrystallisation behaviour during deformation. A constitutive model coupling flow stress with strain, strain rate and temperature to 20CrMnTiH steel is proposed using a hyperbolic sinusoidal type equation. In the constitutive equation, the material constants α, n, ln A and Q are found to be functions of strain. The validity of the constitutive equation is assessed by the correlation coefficient (r), the average absolute relative error and the mean squared error.

This paper presents a new micromechanical model for the prediction of the tensile modulus and strength of natural fibre reinforced polymer matrix composites. The model addresses issues linked to the statistical variation inherent in fibre reinforcements extracted from plants. The new model introduces a fibre area correction factor (FACF). Modulus and strength are estimated and compared to experimental data for a jute–epoxy composite. The predictions of tensile modulus and strength using the FACF show improvements over those from other micromechanical models presented in the literature.

The present paper is the first out of two papers in which thermal criteria for modelling and optimising, i.e. minimising, the formation of A segregates are investigated. One specific thermal criterion has been incorporated into a transient three-dimensional thermal fluid model inside a commercial simulation software package. It is then used for predicting A segregates inside a large steel casting, i.e. a forging ram. In part I, experimental data obtained from a foundry serve to validate the given criterion and to evaluate the critical value for A segregate initiation for one alloy composition on the forging ram. In part II, the criterion forms the basis of shape optimisation of the original casting layout for the ram. More specifically, unknown optimal shapes and sizes of the top riser and chills are sought by means of autonomous optimisation to establish a better solidification pattern, which would eliminate the likelihood of centreline porosity and A segregates.

The effects of thermal aging and step cooling embrittlement on the impact toughness of a reactor pressure vessel steel SA533B quenched and tempered (QT) with and without post-weld heat treatment (PWHT) have been studied. Charpy impact testings were conducted on the aged plates at 350°C for 5000 h to evaluate whether the embrittlement was induced by step cooling heat treatment. The results show that thermal aging increases the ductile–brittle transition temperature in both QT and PWHT states but dramatically decreases the upper shelf energy in QT state and has less effects on the PWHT state. By comparing the correlation between thermal aging embrittlement and step cooling embrittlement for both QT and PWHT states in steel, it is found that the step cooling heat treatment can obviously promote further embrittlement of the base metal in QT state but has little influence on the impact toughness in PWHT and thermal aged state. Further analysis indicates that the step cooling heat treatment cannot promote steel embrittlement at some heat treatment states. Finally, a new method is proposed to evaluate the degree of step cooling embrittlement of the pressure vessel steel.

Applying the fluxing method, hypercooling was achieved in Co₈₀Pd₂₀ alloys. After recalescence, the rapidly solidified microstructures of hypercooled Co₈₀Pd₂₀ alloys were subjected to rapid quenching. Applying transmission electron microscopy, high densities of substructures, e.g. dislocations and subgrains, and migration of high angle grain boundaries suggest the occurrence of partial recrystallisation in the hypercooled Co₈₀Pd₂₀ alloys.

Further comments regarding the papers ‘On preventing HAZ cracking in laser welded DS Rene 80 superalloy’ by Osoba et al. and ‘The hot ductility of Nb/V containing high Al, TWIP steels’ by Kang et al.