Strain-induced Precipitation Research Articles

The Interdependence of the Degree of Precipitation and Dislocation Density during the Thermomechanical Treatment of Microalloyed Niobium Steel

In this paper, thermomechanical processing of niobium microalloyed steel was performed with the purpose of determining the interaction between niobium precipitates and dislocations, as well as determining the influence of the temperature of final deformation on the degree of precipitation and dislocation density. Two variants of thermomechanical processing with different final rolling temperatures were carried out. Samples were studied using electrochemical isolation with an atomic absorption spectrometer, transmission electron microscopy, X-ray diffraction analysis, and universal tensile testing with a thermographic camera. The results show that the increase in the density of dislocations before the onset of intense precipitation is insignificant because the recrystallization process takes place simultaneously. It increases with the onset of strain-induced precipitation. In this paper, it is shown that niobium precipitates determine the density of dislocations. The appearance of Lüders bands was noticed as a consequence of the interaction between niobium precipitates and dislocations during the subsequent cold deformation. In both variants of the industrial process performed on the cold deformed strip, Lüders bands appeared.

Effect of warm rolling process on microstructures and tensile properties of 10 Mn steel

The influence of warm rolling processes on the microstructures and tensile properties of 10 Mn steel was studied. Strength appeared to increase with the rolling temperature but strengthening mechanisms varied. The increase of warm rolling temperature from 250 °C to 600 °C leads to enhanced recrystallization in martensite during the intercritical annealing (IA) at 620 °C for 5 h. As a result, both ultimate tensile strength (UTS) and total elongation (TE) increase. However, the size of relatively coarse recrystallized austenite grains and the resultant yield strength (YS) remain almost constant in this temperature range. The further increase of rolling temperature to 700–800 °C causes a considerable amount of pearlite to be formed during the IA, and then martensite is formed after the IA, resulting in dramatical increases in both YS and UTS but at the great loss of ductility. The warm rolling at 600 °C with 63% thickness reduction can produce the steel with the best mechanical combination of 1.2 GPa UTS and 35% TE, due to the formation of many ultrafine austenite grains and strain-induced cementite precipitates. This demonstrates that the mechanical combination of non-V-alloyed medium Mn steel can be improved to an equivalent level of 0.7% V alloyed 10 Mn steel just via the economic strain-induced cementite precipitation.

Tracing Microalloy Precipitation in Nb-Ti HSLA Steel during Austenite Conditioning

The microalloying with niobium (Nb) and titanium (Ti) is standardly applied in low carbon steel high-strength low-alloy (HSLA) steels and enables austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. In that respect, it is important to better understand the precipitation kinetics as well as the precipitation sequence in a typical Nb-Ti-microalloyed steel. Various characterization methods were utilized in this study for tracing microalloy precipitation after simulating different austenite TMCP conditions in a Gleeble thermo-mechanical simulator. Atom probe tomography (APT), scanning transmission electron microscopy in a focused ion beam equipped scanning electron microscope (STEM-on-FIB), and electrical resistivity measurements provided complementary information on the precipitation status and were correlated with each other. It was demonstrated that accurate electrical resistivity measurements of the bulk steel could monitor the general consumption of solute microalloys (Nb) during hot working and were further complemented by APT measurements of the steel matrix. Precipitates that had formed during cooling or isothermal holding could be distinguished from strain-induced precipitates by corroborating STEM measurements with APT results, because APT specifically allowed obtaining detailed information about the chemical composition of precipitates as well as the elemental distribution. The current paper highlights the complementarity of these methods and shows first results within the framework of a larger study on strain-induced precipitation.

Effect of Deformation on Precipitation Behaviors of Carbide in Ti-Nb-Mo Steel

The precipitation behaviors of (Ti, Nb, Mo)C in specimens of Ti-Nb-Mo steel hold at 700°C under no deformation condition and deformation condition with engineering strain 0.4 imposed in austenite region have been studied. The results indicate: under no deformation condition, the rows of interphase precipitation show straight and/or curve morphology, and the precipitation behaviors include interphase precipitation, random precipitation and fibrous precipitation; under deformation condition, the rows of interphase precipitation are straight, and the precipitation behaviors include interphase precipitation and strain-induced precipitation. Furthermore, the dispersion of precipitate particles formed under deformation condition is more uniform than those formed under no deformation condition.

Dynamic recrystallization behavior and interfacial bonding mechanism of 14Cr ferrite steel during hot deformation bonding

In this study, hot compression bonding was first applied to join 14Cr ferrite steel at temperatures of 950–1200 °C and strains of 0.11–0.51 under strain rates of 0.01–30 s−1. Subsequently, tensile tests were performed on the joints to evaluate the reliability of the joints formed. Detailed microstructural analyses suggest that two different competing dynamic recrystallization (DRX) mechanisms occur during the bonding process depending on the strain rate, and the joints obtained at different strain rate exhibits distinct healing effect. At a low strain rate (0.01 s−1), continuous DRX occurs, as expected in high-stacking-fault-energy materials, and is characterized by the progressive conversion of the sub-boundaries into larger-angle boundaries, which involves very limited grain boundaries migration. In addition, strain-induced precipitation (SIP) is sufficient under this condition, further impeding the healing of bonding interface. Hence, the joints obtained at low strain rate fractured at the bonding interface easily. Whereas discontinuous DRX is activated at high strain rates (10 and 30 s−1). Under this condition, the formation of sub-boundaries is severely suppressed, resulting in the piling-up of dislocations and hence the storage of a greater amount of stored energy for nucleation and subsequent nuclei growth via the long-distance grain boundaries migration. Meanwhile, the SIP process is sluggish, making the conditions much more favorable for grain boundaries migration which plays a key role in the healing of the original bonding interface. Thus, the joints can be successfully bonded when a high strain rate is applied, with the joints exhibiting tensile properties similar to that of the base material.

Microstructure Evolution and Recrystallization of D36 Steel during Ultrasonic Impact Assisted with Electropulsing and Heat

Ultrasonic impact treatment (UIT) combined with high-energy electropulsing (EP) was applied to D36 low-carbon steel with three different electrical regimes. Submicron crystalline was obtained on the superficial region after the treatment due to continuous dynamic recrystallization. The cementite experienced strain-induced decomposition and precipitation. The microstructure is significantly determined by the current density and temperature. A strengthened layer with a maximum hardness of 285 HV was obtained in EP-UIT, in comparison with the hardness of 227 HV resulted from UIT solely. Alongside with high hardness, the strengthened layer extended to a remarkable depth of nearly 2 mm due to acoustic softening, electroplasticity and thermal softening engaged simultaneously. A 3-μm oxide layer in average consisting of magnetite and hematite formed on the treated surface. Joule heat and athermal effect of EP are the factors inducing these phenomena.

Study on Deformation-Induced Pearlite Transformation in Vanadium Microalloyed Eutectoid Steel

In this paper, the hot compression tests were performed to study on deformation-induced pearlite transformation in vanadium microalloyed eutectoid steel. The results showed that volume fraction of deformation -induced pearlite were higher and the pearlite were spheroidized better under lower strain rate and higher strain in vanadium microalloyed steel. Ferrite grains and granular cementites were further refined through vanadium microalloying combined with deformation-induced pearlite transformation .Vanadium dissolved in γmatrix could retard deformation-induced pearlite transformation under low strain, vanadium carbides precipitated due to strain-induced precipitation eliminate the retardation when the strain was increased to a certain extent. Under heavy deformation, ferrite grains and granular cementites in vanadium microalloyed steel were finer compared with vanadium free steel.

Nanomechanical Responses of an Intermetallic Compound Layer in Transient Liquid Phase Bonding Using Indium

Low-temperature transient liquid phase bonding (TLPB) can be achieved using indium, and the joints thus formed can withstand high temperatures. Since the joints that are formed with TLPB entirely consist of intermetallic compounds (IMCs), this study investigates the phase evolution and mechanical properties of IMCs that are obtained from isothermal reactions between In and commonly used substrate metals, Cu, Ag and Au. Using nanoindentation, the relationships between the yield strain, work hardening exponent and strain rate sensitivity of In-bearing and other frequently observed IMC phases were compared. Notably, the strain-induced precipitation of Ag3In occurred in the indent of Ag9In4.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (Tnr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Control of Laves Precipitation in a FeCrAl-based Alloy Through Severe Thermomechanical Processing.

In recent years, the development of nuclear grade FeCrAl-based alloys with enhanced accident tolerance has been carried out for light water reactor (LWR) fuel cladding to serve as a substitute for zirconium-based alloys. To achieve excellent microstructure stability and mechanical properties, the control of precipitation particles is critical for application of FeCrAl-based alloys. In this paper, the effect of thermomechanical processing on the microstructure and precipitation behavior of hot-rolled FeCrAl alloy plates was examined. After hot rolling, the FeCrAl alloy plates had typical deformation textures. The rolling direction (RD) orientation gradually rotated from <100> to <110> along with increasing reduction. Shear bands and cell structures were formed in the matrix, and the former acted as preferable nucleation sites for crystallization. Improved deformation helped to produce strain-induced precipitation. The plate with 83% reduction had the most homogeneous and finest precipitation particles. Identification results by TEM indicated that the Laves precipitation was of the Fe2Nb-type crystal structure type, with impurities including Mo, Cr, and Si. The plate with uniform Laves particles displayed favorable heat stability after a long period of aging at 800 °C. The microstructure evolution of the aged sample was also observed. The deformation microstructure and the strain-induced precipitation mechanism of FeCrAl alloys are discussed.

Effect of deep surface rolling on microstructure and properties of AZ91 magnesium alloy

Abstract A solution-treated AZ91 bulk material was deep-surface-rolled at room temperature to investigate the effect of deep surface rolling on the microstructure and mechanical properties of the alloy. Microhardness and microstructure along the depth of the treated surface layer were characterized. The results show that the affected layer was up to 2.0 mm thick and consisted of three sublayers: a severe deformation layer with thickness of about 400 μm from the topmost surface, a medium deformation layer with thickness of around 600 μm and a small deformation layer up to 1000 μm thick. In addition to grain refinement in the deformation layer, strain-induced precipitation of β phase (Mg17Al12) was observed, particularly in the severe and medium deformation layers. It is believed that the cooperative effects of grain refinement, strain hardening and precipitation strengthening led to the significant increase in hardness of the AZ91 alloy after the deep surface rolling.

Effect of precipitates on high-temperature tensile strength of a high W-content cast Ni-based superalloy

Tensile tests for a high W-content (13–15.5 wt%) cast Ni-based superalloy were carried out at temperatures ranging from 20 °C to 1100 °C. Temperature was varied in increments of 100 °C after reaching 500 °C. The tensile strength of the superalloy used in this study was 150–200 MPa higher than that of other low W-content (<10 wt%) cast Ni-based superalloys at 1100 °C. Microstructural observations revealed that the tensile behavior of the alloy was controlled by the cross-slip mechanism which optionally operated in most of the γ′ precipitates and a large number of isolated stacking faults (with high densities) with two different orientations were formed as temperature reached 1000 °C. At temperatures higher than 1000 °C, the applied stress facilitated the strain-induced precipitation of tungsten-rich MC type carbide particles in the matrix and along the γ/γ′ interface. On the other hand, the low lattice misfit reduced the nucleation barrier, which resulted in the formation of nanosized γ′ phases in the matrix. The strong solid solution effect along with the formation of the tungsten-rich MC precipitates and nanosized γ′ phases improved the high-temperature (temperatures higher than 1000 °C) strength of the superalloy.

Evolution of Microstructure During Double-Sided Friction Stir Welding of Microalloyed Steel

Microstructures observed by analytical scanning and transmission electron microscopy in the overlap region of a double-sided friction-stir-welded microalloyed steel (EH46) were recorded in detail. They are compared with microstructures in the thermomechanically affected region of the weld and with the base material. The differences suggest that the overlap region has been stirred in the single-phase ferrite, and consists mainly of small equiaxed ferrite grains with strain-induced precipitates, while the thermomechanically affected zone was processed in the austenite-ferrite-phase field, resulting in a mixture of bainite lath packets and ferrite grains. The almost complete absence of pearlite or cementite in the overlap region has led to the suggestion that it dissolved during friction stir welding, providing carbon for strain-induced precipitation. Also, in the complex microstructures of the overlap region, ferrite grains containing a high density of cell-like structures were observed, some of them having precipitates nucleated on their intersections. This implies that strain-induced continuous dynamic recrystallization has occurred.

The effect of strain on interphase precipitation characteristics in a Ti-Mo steel

In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation.

Through-Scale Analysis of Strain Path Effects in Microalloyed Austenite Subjected to Reverse Rolling

Thermomechanical hot rolling processes are often realized using reverse rolling stands, where the rolled stock is fed forward and backward through the rolling gap. During those processes, material undergoes several strain reversals that significantly alter microstructure evolution of austenite with respect to continuously rolled counterparts. In Nb-microalloyed steels, where precipitation hardening is usually expected, the effects of strain reversal are especially complex. When rolling direction is reversed, both static recrystallization (SRX) kinetics and strain-induced precipitation (SIP) processes are slowed down due to decreasing dislocation density. It affects the competition between driving force for SRX and pinning pressure for SIP and, in turn, changes the non-recrystallization temperature (Tnr), compared to the case where strain path is linear. In the present paper, detailed through-scale analysis of strain path effects in microalloyed austenite will be presented. Physical simulation and detailed microstructural analysis will be employed to study global and local effects in microalloyed austenite after complex deformation histories. Conclusions regarding the influence of strain path changes on the interactions between SRX and SIP will be drawn.

Role of Precipitates in Recrystallization Mechanisms of Nb-Mo Microalloyed Steel

Recrystallization behavior of a Nb-Mo microalloyed steel has been investigated using double-hit compression tests. A suitable reheating temperature and soaking time were established for the complete dissolution of the microalloying precipitates prior to hot deformation. The influence of hot deformation conditions on the flow behavior and the effect of alloying elements, Nb in particular, on austenite recrystallization kinetics are highlighted. The strain-induced precipitation has been found to play an important role in hindering recrystallization kinetics. Post-deformation microstructural analysis indicates that strain-induced grain boundary migration (SIMB) is one of the mechanisms for the formation of recrystallized grains. Retardation of recrystallization has been explained by estimating pinning force of Nb(C, N) precipitates and recrystallization driving force at 1000 °C, which is in good agreement with the experimental observations in the present study as well as other reported data. The study brings out a better understanding on the influence of strain-induced precipitates on recrystallization behavior.

Improving Elevated-Temperature Strength of an Al–Mn–Si Alloy by Strain-Induced Precipitation

The coupled effect of strain-induced precipitation and stabilized substructure on the elevated-temperature strength of an Al–Mn–Si alloy and its thermal stability have been investigated. Prestrain significantly promotes the nucleation of nano-sized dispersoids, and strain-induced precipitation suppresses recrystallization, stabilizing substructure at elevated temperatures. Compared with the dispersoids formed during the heat treatment of as-cast alloy, substructure does not increase the coarsening rate of strain-induced precipitates. The strain-induced precipitation and stabilized substructure profoundly strengthen the aluminum alloy at the elevated temperature (300 °C).

Effect of Vanadium on the Phase Transformation Behavior of Ti–Mo Microalloyed Ultra‐High Strength Steel

Vanadium is a vital microalloying element in the precipitation hardening of hot‐rolled high strength steel. Its effects on the continuous cooling phase transformation behavior, microstructure, and properties of Ti–Mo and Ti–Mo–V microalloyed ultra‐high strength steels are studied by the thermal simulator, scanning electron microscopy (SEM), high‐resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer (EDS), and electron backscatter diffractometer (EBSD). The dynamic continuous cooling transformation curves are obtained which show that vanadium can restrain the ferrite‐pearlite and bainite transformation, refine the microstructure, and harden the steel matrix. It is also found that the strain‐induced precipitation of nano‐sized (Ti, Mo) C and (Ti, V, Mo) C particles, with NaCl type structure, exists in Ti–Mo and Ti–Mo–V steel, respectively, as identified by the HRTEM and EDS. And the lattice constants of (Ti, Mo) C and (Ti, V, Mo) C are 0.432 and 0.430 nm, respectively. The higher hardness of Ti–Mo–V steel is attributed to the grain refinement and precipitation hardening by the nano‐sized (Ti, Mo, V) C particles.

Effects of lanthanum on hot deformation behaviour of Mn-Cr-Mo bainitic rail steel

Effects of lanthanum (La) as micro-alloying element on the hot deformation behaviour of the high strength Mn-Cr-Mo bainitic rail steel were investigated under a range of deformation conditions. The results indicate that La increases the flow stress by 10–30 MPa through strengthening nanoscale strain induced precipitation (SIP) θ-(Fe,La)3C during hot deformation. The hot deformation activation energy increases by 10–40 kJ/mol due to the “Zener effect” of SIP and the dynamic recrystallization (DRX) is retarded due to the competitive behaviour between SIP and DRX. Bainite plates in the DRX domain can be refined by adding La, resulting in the improvement of hot workability. The DRX domain with peak power dissipation efficiency of 52% is determined to be the optimal processing region for Mn-Cr-Mo-La bainitic rail steel.

Interaction of precipitation, recovery and recrystallization in the Mo-Hf-C alloy MHC studied by multipass compression tests

The interaction between strain-induced precipitation, recovery and recrystallization governs the properties of the particle strengthened Mo-Hf-C alloy MHC. Complex, multi-staged thermomechanical processes have become the key approach for further improvement of the high temperature properties of MHC products within recent years. An investigation of those processes, including a wide field of parameters, usually turns out to be both difficult and time consuming. With multi-pass compression tests it is possible to analyze complex thermomechanical processes on small scale samples in a straightforward manner. The evaluation of the gathered information is demonstrated and the results are compared to microstructural investigations, conventional mechanical tests as well as relevant literature. Furthermore, the influence of different particle size distributions in MHC is interpreted. Precipitation between individual deformation steps resulted in minor additional hardening of the final material. Intermediate recrystallization during the thermomechanical processing showed high softening and ductilization of the final condition.