Fine grained AZ31 produced by conventional thermo-mechanical processing

The magnesium alloy AZ31 was processed by severe hot rolling and annealing. This processing was optimised to produce recrystallised grain sizes as small as 2.2 μm. The texture of the processed plate was similar to that of the as-received material, with a strong basal alignment in the normal direction. Tensile testing of the fine grained material showed an increase in the strength and elongation compared to the as-received plate. It is  concluded that the improved mechanical properties of the fine grained material results from a refinement in the grain size and the elimination of shear bands that pre-exist in the as-received material.

Thermo-mechanical processing and the shape memory effect in an Fe–Mn–Si-based shape memory alloy

The effect of cold rolling and annealing on the shape memory effect (SME) in an Fe–Mn–Si-based alloy has been studied. It has been found
that the SME in these alloys can be significantly increased by the appropriate thermo-mechanical processing (TMP). The optimum conditions
were found to be 15% cold rolling followed by annealing at 800 ◦C for 15 min. This produced a total strain recovery of 4.5%. TEM showed that
this processing schedule produces a microstructure of evenly spaced, and well defined stacking faults throughout the parent phase. It is shown for
the first time that samples processed in this way produce a larger fraction of martensite compared to samples in the as-austenitized condition. It
is concluded that the stacking faults induced by TMP act as nucleation sites for martensite formation during deformation. The SME is improved
primarily as a result of the increased amount of martensite that is formed in this condition.

Effect of thermo-mechanical processing on precipitate formation in next generation HSLA steel

Development of advanced high strength steels (AHSS) using a conventional rolling setup is one of the biggest challenges to steel industry. It has been found that fine precipitation in a soft matrix, formed after hot rolling, can markedly improve the mechanical properties. In this work, three dimensional atom probe tomography (3D-APT) has been used to study the formation of precipitates in thermomechanically simulated steel. 3D-APT data reveals co-existence of numerous nano clusters with precipitates. Also, quantitative analysis of the nano clusters and precipitates shows clusters are as small as mm in size. Precipitates are found to be disc shaped with the composition of equilibrium precipitates (TiMo)C. Thus, 3D-APT is seen as an ideal technique to complement TEM to understand the nanoscale features in thermomechanically processed steel for further improvements in the mechanical properties of AHSS.

Thermo-mechanical processing of TRIP-aided steels

The effects of the partial replacement of Si with Al and the addition of P on the microstructure and mechanical properties of experimental TRIP-aided steels subjected to different thermo-mechanical cycles were studied. Based on the available literature and thermodynamics-based calculations, three steels with different compositions were designed to obtain optimum results from a relatively low number of experiments. Different combinations of microstructure were developed through three different kinds of thermo-mechanical-controlled processing (TMCP) routes, and the corresponding tensile properties were evaluated. The results indicated that partial replacement of Si with Al improved the strength-ductility balance along with providing an improved variation in the incremental change in the strain-hardening exponent. However, the impact of the P addition was found to depend more on the final microstructure obtained by the different TMCP cycles. It has also been shown that an increase in the volume fraction of the retained austenite ($$ V_{{\gamma_{\text{ret}} }} $$Vγret) or its carbon content ($$ C_{{\gamma_{\text{ret}} }} $$Cγret) resulted in an improved strength-ductility balance, which can be attributed to better exploitation of the TRIP effect.

Integrated model for thermo-mechanical controlled process in rod (or bar) rolling

This study presents an integrated model for computing the thermo-mechanical parameters (cross-sectional shape of workpiece, the pass-by-pass strain and strain rate and the temperature variation during rolling and cooling between inter-stands) and metallurgical parameters (recrystallisation behaviour and austenite grain size—AGS), to assess the potential for developing “Thermo-Mechanical Controlled Process” technology in rod (or bar) rolling, which has been a well-known technical terminology in strip (or plate) rolling since 1970s.

The advantage of this model is that metallurgical and mechanical parameters are obtained simultaneously in a short computation time compared with other models. The model has been applied to a rod mill to predict the exit cross-sectional shape, area and AGS per pass by incorporating the equations for AGS evolution being used in strip rolling. At the finishing train of rod mills, the strain rates reach as high as 1000–3000 s−1 and the inter-pass times are around 10–60 ms.

The results show that the proposed model is an efficient tool for evaluating the effects of process-related parameters on product quality and dimensional tolerance of the products in rod (or bar) rolling. The results of the simulation demonstrated that the equation for AGS evolution being used in strip rolling might have limitations when applied directly to rod rolling at a high strain rate.

Effect of composition and processing parameters on the formation of nano-bainite in advanced high strength steels

The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3M0-0.6Al-0.02Nb (wt%) steel after controlled thermo-mechanical processing. The microstructure for both steels consisted of bainite. The microstructural characteristics of bainite, such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers, as a function of steel composition and processing was studied using transmission electron microscopy (TEM). It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing. Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution. The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.

Effect of processing parameters on the magnetic properties and macrotexture of a Nd13.5Fe73.8Co6.7B5.6Ga0.4 alloy processed by equal channel angular pressing with back pressure

Equal channel angular pressing (ECAP) is a well-established thermo-mechanical processing technique, which could induce the c-axis texture of Nd2Fe14B in a melt-spun Nd13.5Fe73.8Co6.7B5.6Ga0.4 alloy. However, the effects of ECAP processing parameters, such as temperature, back pressure (BP), and multiple-pass ECAP routes, remain unknown for this alloy. In this paper, we have investigated the effects of these processing parameters on the c-axis texture formation. It is found by X-ray diffraction macrotexture analysis that the maximum intensity of (001) pole figures for the tetragonal-Nd2Fe14B phase (Imax) shows an increase from 2.7 to 4.1 m.r.d. (multiples of random distribution) by increasing the ECAP temperature from 723 to 823 K, while the difference in remanent magnetization between easy and hard directions (Δ Mr) rises from 24.0 to 41.5 Am2/kg. When the BP was increased from 0.25 to 0.5 GPa at 823 K, Imax showed an increase from 2.8 to 4.1 m.r.d. However, Imax saturated for BPs above 0.5 GPa, suggesting that BP has limited effect on the texture formation, although it is necessary for the compaction of the alloy powders. Two multiple-pass ECAP routes conventionally known as routes A and C were employed for two-pass ECAP at 823 K. It is found that route A processing is effective in enhancing the texture formation, while the texture is lost by a subsequent pressing when adopting route C. Therefore, the compaction of Nd13.5Fe73.8Co6.7B5.6Ga0.4 alloy powder using route A ECAP passes with 0.5 GPa BP at 823 K results in pronounced texture, which is beneficial for anisotropic hard magnetic properties.

In situ synchrotron X-ray diffraction studies of the effect of microstructure on tensile behavior and retained austenite stability of thermo-mechanically processed transformation induced plasticity steel

Transmission electron microscopy and in situ synchrotron high-energy X-ray diffraction were used to investigate the martensitic transformation and lattice strains under uniaxial tensile loading of Fe-Mn-Si-C-Nb-Mo-Al Transformation Induced Plasticity (TRIP) steel subjected to different thermo-mechanical processing schedules. In contrast with most of the diffraction analysis of TRIP steels reported previously, the diffraction peaks from the martensite phase were separated from the peaks of the ferrite-bainite α-matrix. The volume fraction of retained γ-austenite, as well as the lattice strain, were determined from the diffraction patterns recorded during tensile deformation. Although significant austenite to martensite transformation starts around the macroscopic yield stress, some austenite grains had already experienced martensitic transformation. Hooke's Law was used to calculate the phase stress of each phase from their lattice strain. The ferrite-bainite α-matrix was observed to yield earlier than austenite and martensite. The discrepancy between integrated phase stresses and experimental macroscopic stress is about 300 MPa. A small increase in carbon concentration in retained austenite at the early stage of deformation was detected, but with further straining a continuous slight decrease in carbon content occurred, indicating that mechanical stability factors, such as grain size, morphology and orientation of the retained austenite, played an important role during the retained austenite to martensite transformation.

Re-examination of the effect of NbC precipitation on shape memory in Fe-Mn-Si-based alloys

Current literature pertaining to the shape memory effect in the Fe–Mn–Si-based system is critically discussed. It is argued that the
enhanced shape memory previously attributed to NbC precipitation is mainly due to the associated thermo-mechanical treatments.
It is concluded that the thermo-mechanical processing of the alloy is the dominant factor that determines the shape memory effect in
this alloy system.

Study of the deformation behaviour of austenite via model alloys

This study compared two potential model alloys, 304 stainless steel and Ni-30wt.%Fe, to study the behaviour of austenite during the thermo-mechanical processing of steel. The deformation behaviour as well as the textural and microstructural evolution was characterised in detail over a wide range of deformation conditions.

Substructure development through dynamic recrystallisation in an austenitic Ni–30%Fe alloy

This data looks at the effect of grain boundary movement on the characteristics of substructure development within the DRX regime. Different thermo-mechanical processing routes were employed to produce a range of DRX grain sizes at a given deformation temperature. The development of dislocation substructure was investigated using electron back-scattered diffraction (EBSD) in conjunction with transmission electron microscopy (TEM).

Atom probe tomography as a tool to advance steels design and performance

Development of modern steels consisting of complex or nano-scale microstructures with advanced properties requires in-depth understanding of the mechanisms responsible for their microstructure/property relationships. The evolution of microstructure during processing is often associated with various changes taking place at atomic level. These include solute distribution between phases as a result of phase transformations, formation of atmospheres at dislocations, clustering and precipitation phenomena due to various thermo-mechanical processing schedules and/or heat treatments. Atom probe tomography (APT) is invaluable tool for gaining insight into events at atomic scale determining the steel properties. This technique also contributes to the fundamental understanding of phase transformations, which is essential for nano-scale engineering of modern steels and optimization of their performance. In this work application of APT to study solute segregation, clustering and precipitation in TRIP steels and nanostructured bainitic steels after isothermal heat-treatment and after thermomechanical processing will be discussed.