Effect Of Precipitation Hardening Research Articles

REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY

Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures. Heat treatment in intercritical area can not only produce multi-phase microstructure,but also lead to the redistribution of alloying elements in different phases. Multi-step intercritical heat treatment is favorable to obtain retained austenite that is stabilized by repeated enrichment of alloying elements in reversed austenite and nanometer-sized precipitate that are primarily formed during tempering. Excellent mechanical properties are contributed by transformation-induced-plasticity effect of retained austenite and precipitation hardening effect of nanometer-size precipitates. In this work, the microstructural evolution and relative mechanical properties were investigated in a low carbon low alloy steel processed by a three-step heat treatment, namely, intercritical annealing, intercritical tempering and tempering. The microstructure was a typical dual- phase microstructure consisting of intercritical ferrite and bainite/martensite after intercritical annealing, and primarily comprised of intercritical ferrite, tempered bainite/martensite and retained austenite after intercritical tempering. Retained austenite with volume fraction of 29% distributed at the ferrite/bainite(martensite) boundaries and betweent bainitic/martensitic laths. Retained austenite was stabilized by enrichment of C, Mn, Ni and Cu in reversed austenite during the reversion transformation process. Nb C precipitates with average size of 10 nm was formed in ferrite matrix and bainite/martensite, while Cu- containing particles in size range of 10~30 nm precipitated in ferrite and retained austenite during intercritical tempering and tempering process. The morphology of Nb C precipitates was spherical, elliptical and irregular, and copper precipitates were spherical. With the combination of transformation- induced- plasticity(TRIP) effect of retained austenite and precipitation hardening, the steel possessed outstanding mechanical properties: yield strength 700 MPa, tensile strength 900 MPa, uniform elongation 20%, and total elongation 30%.

Effect of diamondoids on the microstructure and mechanical behavior of nanostructured Mg-matrix nanocomposites

Nanostructured Mg–10Al nanocomposites reinforced by diamondoids were processed via cryomilling and spark plasma sintering. The effect of 1wt% of diamantane on the microstructure of the nanostructured Mg–10Al nanocomposites was studied using scanning and transmission electron microscopies, electron dispersive spectroscopy and X-ray diffraction. A detailed microstructural examination revealed that diamondoids mainly located within the Mg-matrix induce a lattice strain along the c-direction and a potential effect on the γ-Al12Mg17 precipitation. The mechanical behavior of the nanostructured Mg-based nanocomposites was investigated by micro- and macro-compression tests along with in-situ SEM nanoindentation experiments at room temperature. The compressive strength, elastic modulus and hardness of the nanocomposites were found to be significantly improved as compared with the mechanical behavior of diamondoids-free nanostructured Mg–10Al alloys. Since no change in the grain size has been found after adding diamantane, we attribute the enhanced mechanical behavior of the nanocomposites to a combined effect of precipitation hardening and the microscopic residual stress generated by diamantane–Mg matrix mismatch. Our results provide new insights into the mechanisms involved in the development of high-performance Mg-nanocomposites.

Synergistic effect of CNTs reinforcement and precipitation hardening in in-situ CNTs/Al–Cu composites

In-situ synthesized carbon nanotubes (CNTs) reinforced aluminum–copper (Al–Cu) composites have been prepared by powder metallurgy. The strengthening effect of CNTs reinforcement and precipitation hardening of Al–Cu matrix has been investigated. CNTs and Cu nanoparticles are homogeneously embedded into Al powders through the process of in-situ preparation and ball milling. XRD results show that Cu elemental phase is converted to Al2Cu phase during the procedure of sintering. Raman spectra indicate that CNTs could maintain its structural integrity during powder metallurgy. In T6 heat treatment, the aging time of peak hardness has been reduced from 9h to 3h, demonstrating accelerated aging of the composite. Besides, the ultimate tensile strength of CNTs/Al–Cu composites have a 33.9% increase compared with that of Al–Cu alloy, and a further increase could be achieved (20.3%) after T6 heat treatment. The strengthening mechanism of CNTs/Al–Cu composites combines two synergistic factors: CNTs reinforcement and precipitation hardening of Al–Cu alloy.

Development of Ti–V–Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel

A new Ti–V–Mo complex microalloyed hot-rolled high-strength steel sheet was developed by controlling a thermo-mechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. The effects of coiling temperature (CT) on various hardening mechanisms and mechanical properties of Ti–V–Mo complex microalloyed high-strength low-alloy steels were investigated. The results revealed that the steels are mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in simulated coiling temperature causes a significant difference in strength, which is mainly attributed to different precipitation hardening increment contributions. When the CT is 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Moreover, about 82 wt% of the total precipitates are nano-sized carbide particles with diameter of 1–10 nm, which is randomly dispersed in the ferrite matrix. The nano-sized carbide particles led to a strong precipitation hardening increment up to 310 MPa.

Nanolaminate transformation-induced plasticity–twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance

Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe–9Mn–3Ni–1.4Al–0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite–martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility.

Effect of nanoprecipitates on the transformation behavior and functional properties of a Ti–50.8 at.% Ni alloy with micron-sized grains

In order to take advantage of both grain refinement and precipitation hardening effects, nanoscaled Ni4Ti3 precipitates are introduced in a Ti–50.8at.% Ni alloy with micron-sized grains (average grain size of 1.7μm). Calorimetry, electrical resistance studies and thermomechanical tests were employed to study the transformation behavior and functional properties in relation to the obtained microstructure. A significant suppression of martensite transformation by the obtained microstructure is observed. The thermomechanical tests show that the advantageous properties of both grain refinement and precipitation hardening are combined in the developed materials, resulting in superior shape memory characteristics and stability of pseudoelasticity. It is concluded that introducing nanoscaled Ni4Ti3 precipitates into small grains is a new approach to improve the functional properties of NiTi shape memory alloys.

The Influence of Carbon Addition on Microstructure and Mechanical Properties of Fe-22.0Al-5.0Ti Alloy

The effect of carbon addition on Fe-22.0Al-5.0Ti alloy on structure and properties has been investigated. Microstructural and phase analysis have been investigated by using optical microscopy, scanning electron microscope (SEM) equipped with EDAX. For low carbon addition (0.1 wt.%), two-phase microstructure consisting of precipitates of TiC in B2 matrix. The presence of large amount of carbon (1.0 or 1.5 wt.%) resulted formation of Fe3AlC0.5 and TiC precipitates in B2 matrix. The results show that the mechanical properties of Fe-22.0Al-5.0Ti increased with increase in the carbon content and strongly depend upon nature and volume fraction of different precipitates. The volume fraction of precipitates increased with increase in the content of carbon. The behavior of Fe-22.0Al-5.0Ti alloy was explained by the combined effect of precipitation hardening and solid solution strengthening. The main effect of addition of carbon related to improvement in the compressive strength without loss in the ductility. The decrease in the wear rate is mainly attributed to the high hardness of the composites and as well hard TiC play a role of load carrying.

Effect of precipitation hardening and thermomechanical training on microstructure and shape memory properties of Ti50Ni15Pd25Cu10 high temperature shape memory alloys

Microstructural analysis, evolution of phase transformation temperatures and shape memory properties for Ti50Ni15Pd25Cu10 high temperature shape memory alloys were investigated in solution treated and aged conditions. It is shown that aging at 600°C for 3h resulted in the formation of two types of fine precipitates i.e. TiPdCu and Ti2Pd. It is observed that due to the formation of these precipitates, martensite start temperature under stress free and constrained (500MPa) conditions are decreased by 32°C and 29°C respectively. Recovery ratio of 76% obtained for solution treated sample increased to 94% at 500MPa for the 600°C aged sample; resultantly the irrecoverable strain is decreased by 18%. Training for 5 thermal cycles was carried out under stress free condition resulted in the decrease of thermal hysteresis by 5°C for the solution treated sample, whereas thermal hysteresis of the aged sample remained stable. Similarly thermomechanical training cycles at 500MPa demonstrated an improvement in the recovery ratio by 14% and 5% for solution treated and 600°C aged samples respectively. Aging at 600°C for 3h and thermomechanical training greatly improved the cyclic stability and shape memory properties of Ti50Ni15Pd25Cu10 high temperature shape memory alloys.

Influence of AlCuSc Ternary Phase on the Microstructure and Properties of 1469 Alloy

Microstructure evolution and mechanical properties of a 1469 alloy and a Sc-free1469 type alloy were examined. SEM observation indicates that AlCuSc ternary phases (W) are formed after homogenization annealing, and cannot be dissolved during the following heat treatments. These coarse particles consume abundant Cu atoms from the Al matrix that are available for solutioning, which results in the decrease of precipitation of the T1 phase during aging treatment. The W phase has negative effects on the examined alloy’s mechanical properties. The tensile strength of Sc-added alloy is 40MPa lower than that of the Sc-free alloy. The formation of the W phase has a close relationship with Cu/Sc ratio, which shows the importance of controlling the concentration of Cu and addition of Sc. Formation of W phase suppress the effect of precipitation hardening of the T1 phase in high strength Al-Cu-Li alloys

The Influence of Nickel and Tin Additives on the Microstructural and Mechanical Properties of Al-Zn-Mg-Cu Alloys

The effects of nickel and nickel combined tin additions on mechanical properties and microstructural evolutions of aluminum-zinc-magnesium-copper alloys were investigated. Aluminum alloys containing Ni and Sn additives were homogenized at different temperatures conditions and then aged at 120°C for 24 h (T6) and retrogressed at 180°C for 30 min and then reaged at 120°C for 24 h (RRA). Comparison of the ultimate tensile strength (UTS) of as-quenched Al-Zn-Mg-Cu-Ni and Al-Zn-Mg-Cu-Ni-Sn alloys with that of similar alloys which underwent aging treatment at T6 temper showed that gains in tensile strengths by 385 MPa and 370 MPa were attained, respectively. These improvements are attributed to the precipitation hardening effects of the alloying element within the base alloy and the formation of nickel/tin-rich dispersoid compounds. These intermetallic compounds retard the grain growth, lead to grain refinement, and result in further strengthening effects. The outcomes of the retrogression and reaging processes which were carried on aluminum alloys indicate that the mechanical strength and Vickers hardness have been enhanced much better than under the aging at T6 temper.

Investigation on grain refinement and precipitation strengthening applied in high speed wire rod containing vanadium

To obtain necessary information for the simulation of high speed wire production process, the effect of grain refinement and precipitation strengthening on two high speed wire rod steels with different vanadium and nitrogen contents was investigated by continuous cooling transformation (CCT) characteristics. CCT curves were constructed by the dilatometer test and microscopic observation. Results showed that the formation of intra-granular ferrite (IGF) could refine grain remarkably and accelerate the ferrite transformation. Schedules for high speed wire production process focused on the effect of cooling rate. Ferrite grain was refined by increasing cooling rate and the formation of IGF. The microhardness calculation revealed that the steels were strengthened mostly by a combined effect of grain refinement and precipitation hardening. Degenerated pearlite was observed at lower transformation temperature and the fracture morphology changed from cementite lamellar to nanoscale cementite particle with increasing cooling rate. Based on the analysis above, an optimal schedule was applied and the microstructure and microhardness were improved.

Modeling of aging process for supersaturated solution treated of Al–3wt%Mg alloy

The age-hardening curves of micro-hardness measurements obtained for sheets of Al–3wt%Mg alloy under different temperatures, applied loads and dwell times showed leveling and pronounced oscillations, indicating instability and reflecting a competition between the effect of dynamic recovery or sub-structure coarsening and the effect of solute drag and precipitation hardening. An artificial neural network (ANN) and the Rprop training algorithm were used to model the nonlinear relationship between the parameters of the aging process and the corresponding micro-hardness measurements. The predicted values of the ANN are in accordance with the experimental data. A basic repository on the domain knowledge of the age-hardening process verified the expected effect of micro-hardness decrease by increasing any of the applied parameters.

Development of Ti and Mo micro-alloyed hot-rolled high strength sheet steel by controlling thermomechanical controlled processing schedule

A new Ti and Mo micro-alloyed hot-rolled high strength sheet steel was developed by controlling thermomechanical controlled processing (TMCP) schedule, in particular focusing on rolling and coiling temperatures. The results revealed that the steels developed were strengthened mostly by a combined effect of ferrite (α) grain refinement and precipitation hardening. The rolling at austenite (γ) non-recrystallization region provided desirable features by introducing an accumulation of local strains (i.e., dislocations), which served as nucleation sites for γ→α transformation during coiling process, making ferrite grain more fine and homogeneous, although the precipitation hardening was somewhat weakened due to the precipitation of large (Ti, Mo)C carbides (i.e., γ region precipitation). Moreover, these fine and homogeneous ferrite grain, and well developed dislocation structure improved impact properties. The TMCP schedule of the rolling at γ non-recrystallization region (final rolling temperature of 880°C) and coiling at 620°C was found to provide an attractive combination of tensile and impact properties.

Effects of on-line solution and off-line heat treatment on microstructure and hardness of die-cast AZ91D alloy

The effects of on-line solution, off-line solution and aging heat treatment on the microstructure and hardness of the die-cast AZ91D alloys were investigated. Brinell hardness of die-cast AZ91D alloy increases through on-line solution and off-line aging treatment but decreases after off-line solution treatment. By X-ray diffractometry, optical microscopy, differential thermal analysis, scanning electron microscopy and X-ray energy dispersive spectroscopy, it is found that the microstructures of the die-cast AZ91D magnesium alloy before and after on-line solution and off-line aging are similar, consisting of α-Mg and β-Al12Mg17. The precipitation of Al element is prevented by on-line solution so that the effect of solid solution strengthening is enhanced. The β-Al12Mg17 phases precipitate from supersaturated Mg solid solution after off-line aging treatment, and lead to microstructure refinement of AZ91D alloy, so the effect of precipitation hardening is enhanced. The β-Al12Mg17 phases dissolve in the substructure after off-line solution treatment, which leads to that the grain boundary strengthening phase is reduced significantly and the hardness of die cast AZ91D is reduced.

Precipitates and precipitation behavior in Al–Zr–Yb–Cr alloys

The precipitates and precipitation behavior in Al–0.16wt.%Zr–0.30 wt.%Yb–0.18 wt.%Cr alloy at 500°C is studied. By adding Zr, Yb and Cr in combination, a relatively high density of coherent and well-dispersed L12-structured (Al, Cr)3(Zr, Yb) dispersoids are uniformly precipitated during homogenization at 500°C. These dispersoids are coherent with the sizes remaining essentially 15–25nm even after being exposed for 400h. The (Al, Cr)3(Zr, Yb) dispersoid is finer and more coarsening-resistant than L12-structured Al3Zr and Al3(Zr, Yb) dispersoids around 500°C. The uniform and high density precipitation leads to an improvement of precipitation-hardening effect by adding Zr, Yb and Cr in combination.

Correlation between microstructure and tensile properties in powder metallurgy AZ91 alloys

The ultrafine-grained (0.3–1.3 μm) AZ91 alloys, which were fabricated by powder extrusion in the range of 200 to 350 °C and subsequent aging at 100 °C for 8 h, exhibit a remarkable yield stress of 360–478 MPa and moderate tensile elongations of 6–8%. A composite structure was developed after extrusion with uniform β (Mg 17Al 12) particles dispersed in magnesium matrix. The extrusion temperature has an indirect role on yield stress since partial dissolution of β particles induced by high extrusion temperature fails to retard grain growth. Moreover, the strength was further enhanced by the formation of nano-scale precipitates during artificial aging. The high strength could be attributed to a combination effect of grain refinement, particle reinforcement and precipitation hardening.

Effect of Ageing on Strength and Ductility of Ultrafine Grained Al 6061 Alloy

An effect of ageing on mechanical properties of ultrafine grained Al 6061 alloys has been investigated in the present work. The solution treated bulk Al 6061 alloy was subjected to cryorolling to produce ultrafine grain structures and subsequently ageing treatment to improve its both strength and ductility. The hardness and tensile properties of solution treated, cryorolled, cryorolled and aged Al alloys were measured and explained by using their corresponding microstructural morphologies. The pre-cryorolled solid solution treatment combined with post-CR ageing treatment (1300C-30h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with improved tensile strength (362MPa) and good tensile ductility (10.7%) in the Al 6061 alloy. The combined effect of precipitation hardening and recovery are responsible for the simultaneous improvement of both strength and ductility observed in the present work.

A study of rapidly hardened aluminum alloys with iron and vanadium

Alloys of the Al - V, Al - Fe, and Al - Fe - V systems obtained by spinning are studied with the use of methods of physicochemical analysis. The ranges of homogeneity of the formed Al-base supersaturated solid solutions are determined. The stages of decomposition of the supersaturated solid solutions and the effect of vanadium additives on the microstructure and mechanical properties of the rapidly hardened alloys are studied, including their action on the effect of precipitation hardening.

A comparative study on mechanical properties of ultrafine-grained Al 6061 and Al 6063 alloys processed by cryorolling

The present work has been focused to investigate the mechanical behavior and microstructural characteristics of cryorolled Al 6063 and Al 6061 alloys. Hardness and tensile tests of the cryorolled Al alloys were carried out to understand its deformation behavior. SEM/EBSD was used to characterise the microstructures of cryorolled Al alloys and observed the formation of ultrafine-grained microstructures in the materials due to severe plastic strain induced during cryorolling. XRD was used to analyse the formation of different phases during cryorolling of the Al alloys. It is evident from the present study that UFG Al alloys exhibit higher hardness and strength when compared to the bulk Al alloys due to the grain size, higher dislocation density and precipitation hardening effect. The cryorolled Al 6061 alloys exhibit higher tensile strength (346 MPa) and hardness (120 Hv) as compared to Al 6063 alloys (Tensile strength: 240MPa and Hardness: 96.5 Hv) in the present investigation. The deformation mechanisms of UFG Al alloys contributing to their enhanced strength are discussed.

Effect of Precipitations on Microstructures and Mechanical Properties of Nanostructured Al-Zn-Mg-Cu Alloy

A bulk nanostructured Al-Zn-Mg-Cu alloy with high contents of alloy elements was fabricated by cryomilling followed by spark plasma sintering (SPS) techniques. Higher strain and lower strength were obtained in the SPS-processed Al-Zn-Mg-Cu alloy samples. This is due to the agglomeration of MgZn(2) precipitated on the contact zones of powders during the SPS process. The strength of the Al-Zn-Mg-Cu alloy can be improved further by an optimize heat treatment technique, after which the precipitation-hardening effect of MgZn(2), particles precipitated homogeneously in grain interiors was enhanced. The experimental results have shown that modifying the MgZn(2), size and distribution is beneficial to improve the mechanical property of the nanostructured Al-Zn-Mg-Cu alloy. [doi: 10.2320/matertrans.MRP2008303]