Precipitation Strengthening Mechanisms Research Articles

Effect of Heat Treatments on Microstructures and Tensile Properties of Cu–3 wt%Ag–0.5 wt%Zr Alloy

The microstructures and tensile properties of Cu–3 wt%Ag–0.5 wt%Zr alloy sheets under different aging treatments are investigated in this research. As one kind of precipitate, Ag nanoparticles with coherent orientation relationship with matrix precipitate. However, after the peak-age point, most of Ag nanoparticles grow into short rod shape with the interface translating to semi-coherent, which leads to the lower strength of over-aging sample. The yield strength is estimated by considering solid solute, grain boundary and precipitation strengthening mechanisms. The result shows that the Ag precipitates provide the main strengthening role. Then a constitutive equation representing the evolution of dislocation density with plastic strain is built by considering work-hardening behavior coming from shearable and non-shearable precipitates which is mainly the particles containing Zr. The flow stress contributed by shearable particle hardening is higher than that of non-shearable one. Due to the coarsening of grain boundary precipitates and low rate of damage accumulation of these non-shearable particles, the micro-cracks nucleate easily at grain boundary which leads to intergranular fracture.

Precipitation Strengthening by Induction Treatment in High Strength Low Carbon Microalloyed Hot-Rolled Plates

The use of microalloyed steels in the production of thick plates is expanding due to the possibility of achieving attractive combinations of strength and toughness. As market requirements for high strength plates are increasing and new applications require reduced weight and innovative designs, novel approaches to attaining cost-effective grades are being developed. The mechanism of precipitation strengthening has been widely used in thin strip products, since the optimization of the coiling strategy offers interesting combinations in terms of final properties and microalloying additions. Precipitation strengthening in thick plates, however, is less widespread due to the limitation of interphase precipitation during continuous cooling after hot rolling. With the main objective of exploring the limits of this strengthening mechanism, laboratory thermomechanical simulations that reproduced plate hot rolling mill conditions were performed using low carbon steels microalloyed with Nb, NbMo, and TiMo additions. After continuous cooling to room temperature, a set of heat treatments using fast heating rates were applied simulating the conditions of induction heat treatments. An important increase of both yield and tensile strengths was measured after induction treatment without any important impairment in toughness properties. A significant precipitation hardening is observed in Mo-containing grades under specific heat treatment parameters.

Effects of multiple trace alloying elements on the microstructure and properties of Cu-4 wt% Ti alloys

Effects of multiple trace alloying element Fe and Cr addition on the solid solution-treated Cu-4wt% Ti alloys were studied with respect to microstructure and mechanical properties. It was elucidated that the addition of trace alloying elements results in the formation of intermetallic phase CuTi, lowering the titanium content of Cu solid solution phase, and refining the microstructure. The contributions of different strengthening mechanisms were calculated, which are in good agreement on experimental results. It turns out that the solute solution strengthening in Cu- Ti alloys plays a crucial role, even though the fine grain size and precipitation strengthening mechanism are contributed to the strength of Cu-Ti-Cr -Fe alloys. Therefore, the Cu-Ti-Cr-Fe alloys exhibits lower strength and higher elongation compared with Cu-4wt% Ti alloys. However, Cu-Ti-Cr-Fe alloys show a great potential to balance the mechanical properties and electrical conductivity during aging treatment as the titanium content decreases in the Cu matrix, and the strength is higher than that of others in previous study.

Evolution of nano-size precipitation and mechanical properties in a high strength-ductility low alloy steel through intercritical treatment

A two-step intercritical heat treatment was designed to obtain a multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and stable retained austenite in a low carbon and copper alloyed steel, characterized by high strength and high ductility combination. The evolution of copper precipitation during intercritical tempering was studied by transmission electron microscopy (TEM). Electron microscopy studies indicated that the precipitation of copper during tempering followed the sequence (as a function of time): twinned 9R-Cu (0.5h) → de-twinned 9R-Cu (1h) → ε-Cu (greater than 3h), which was accompanied by increase in the size of precipitates from ~ 11nm to ~ 30nm. Considering the cutting mechanism of precipitation strengthening, ε-Cu precipitation contributed to ~ 248MPa and ~ 207MPa toward yield strength for 3h and 5h tempering, respectively. The average size of niobium-containing carbides varied marginally from ~ 11–16nm and had a Baker–Nutting (B-N) orientation relationship with the ferrite matrix. The combination of transformation induced plasticity (TRIP) effect and nano-sized precipitation strengthening contributed to excellent mechanical properties (yield strength > 700MPa, tensile strength > 800MPa, the uniform elongation > 16% and the total elongation > 30%).

Precipitation structure and strengthening mechanisms in an Al-Cu-Mg-Ag alloy

The evolution of the Ω-phase dispersion during aging in an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr-0.1Ge-0.08Ti-0.02Fe-0.01Si (wt%) alloy was examined at the temperatures, T, of 200 and 250°C with dwelt times, τ, of 1.8–86.4 and 0.6–25.2ks, respectively. The precipitation sequence during aging can be written as SSSS→Ag-Mg clusters→Ω-phase+θ′-phase→θ-phase+Ag,Mg,Cu-enriched phases. Only a minor portion of Cu is consumed for the precipitation of the Ω-phase. Approximately 3.3wt% of Cu is retained in the solid solution after peak-aging. Therefore, the Ω-phase is a transition phase with a high free energy. The Ω-phase plates exhibit superior coarsening resistance. The kinetics of the coarsening is successfully predicted by the equation in a form of Ctt×τ0.2, where Ctt is a coefficient depending on aging temperature. Effect of aging on yield stress (YS) was analyzed in terms of additive sum of the contribution arising from the grain boundary strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening mechanisms. The contribution of the precipitate strengthening can be predicted satisfactorily by Nie and Muddle’s model of dislocation shearing of the {111}α plates. It was shown that the Ω-phase is a most efficient strengthening agent in the Al-Cu-Mg(-Ag) alloys and gives the major contribution to the overall YS. It is attributed to high efficiency of interfacial strengthening and high number density of the Ω-phase plates.

Influence of Initial Microstructure on Microstructural Stability and Mechanical Behavior of Cryorolled A356 Alloy Subjected to Annealing

In the present work, various heat treatment cycles are imposed on an A356 aluminum alloy to develop two different base microstructures, one with supersaturated aluminum matrix and the other with coarse and uniformly distributed precipitates. Both the developed materials are subjected to cryorolling and then isochronally annealed for 1 hour at various temperatures ranging from 373 K to 673 K (100 °C to 400 °C). The overall strength is maximum for the cryorolled material with supersaturated base microstructure due to the dominant dislocation strengthening and precipitation strengthening mechanisms. However, the cryorolled material with precipitated base microstructure is found to have superior microstructural stability with retained ultrafine-grained (UFG) microstructure up to the annealing temperature of 473 K (200 °C) due to effective grain boundary pinning by the precipitates. Also, the material retains about 85 pct of as-cryorolled strength with enhanced ductility even after annealing at 473 K (200 °C). A detailed investigation on the microstructural evolution of the material at various annealing temperatures along with their mechanical behavior is presented in this article.

Effect of solution heat treatment on the precipitation behavior and strengthening mechanisms of electron beam smelted Inconel 718 superalloy

Inconel 718 superalloy was fabricated by electron beam smelting (EBS) technique. The effect of solution heat treatment on the precipitation behavior and mechanical properties of EBS 718 superalloys were studied, the strengthening mechanisms were analyzed and related to the mechanical properties. The results indicate that the optimized microstructures can be acquired by means of EBS, which is attributed to the rapid cooling rate of approximately 280℃/min. The solution heat treatment shows a great impact on the microstructures, precipitation behavior and mechanical properties of EBS 718 superalloy. The γ`` phase shows an apt to precipitate at relatively lower solution temperatures followed by aging, while the γ` precipitates are prone to precipitate at higher temperatures. When solution treated at 1150℃, the γ` precipitates are dispersively distributed in the matrix with size and volume fraction of 8.43nm and 21.66%, respectively, a Vickers hardness of approximately 489 HV0.1 is observed for the aged superalloy. The precipitation strengthening effect of EBS 718 superalloy could be elucidated by considering the interaction between the dislocations and γ``/γ` precipitates. The shearing of γ` is resisted by the coherency strengthening and formation of antiphase boundary (APB), which shows equal effect as weakly coupled dislocation (WCD) model. And for γ``, the strengthening effect is much more prominent with the primary strengthening mechanism of ordering. Moreover, it is interestingly found that the strengthening mechanism of stacking fault (SF) shearing coexists with APB shearing, and SF shearing plays a major role in strengthening of EBS 718 superalloy.

Effects of warm laser peening on the elevated temperature tensile properties and fracture behavior of IN718 nickel-based superalloy

IN718 nickel-based superalloy specimens were treated by laser peening (LP) and warm laser peening (WLP) respectively. Tensile tests were performed on the matrix and treated specimens at both room (25°C) and elevated temperature (700°C). Tensile properties were examined by engineering stress-strain curves. The sectional micro-structural observations were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in sequence to investigate the micro-structural evolution and precipitation strengthening mechanism. The fracture morphologies of untreated, LP and WLP treated specimens after tensile deformation at elevated temperature were also analyzed. The results show that LP process, in particular WLP process were found to have a remarkable effect on improving tensile properties of IN718 superalloy at both room and elevated temperatures. WLP treated specimens exhibited more stable high-temperature tensile properties. A serrated flow phenomenon was found in the stress-strain curves at elevated temperature. More δ precipitates dispersed in the grain boundaries after WLP treatment. Detailed TEM microstructural analysis showed that various typical microstructures including dislocation lines, dislocation tangles, dislocation arrays, mechanical twins and precipitated particles tangled together, forming the strengthening phase. A new formed substructure of submicron rhombic blocks with lengths in the range of 400–600nm was observed in the WLP treated specimens. In addition, the WLP treated specimens exhibited a ductile fracture mode, which was totally different with the ductile-brittle mixed fracture mode in untreated and LP treated specimens. WLP process is verified as an effective method to enhance the high-temperature properties of IN718 nickel-based superalloy.

Influence of Microalloying Elements Ti and Nb on Recrystallization during Annealing of Advanced High-Strength Steels

Microalloying elements Ti and Nb are commonly added to high-strength Dual Phase steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as fine carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing and especially on recrystallization kinetics. The present work investigates the influence of microalloying elements Ti and Nb on recrystallization in various cold-rolled Dual Phase steel grades with the same initial microstructure but different microalloying contents. Using complementary experimental and modeling approaches makes it possible to give some clarifications regarding both the nature of this effect and the comparative efficiency of Ti and Nb on delaying recrystallization. It is shown that niobium is the most efficient micro-alloying element to impede recrystallization and that the predominant effect is solute drag.

Alloying Mg with Gd and Y: Increasing both plasticity and strength

The addition of rare elements to Mg enhances mechanical behavior via solution and precipitation strengthening mechanisms. To provide fundamental insight into the underlying mechanisms, we apply density-functional theory (DFT) calculations to systematically study the generalized planar fault energy (GPFE) for pure Mg and its alloys with Gd, Y, and Gd–Y. Special attention is focused on the {0001}〈11¯00〉 basal and {11¯00}〈112¯0〉 prismatic slip systems. Our results show that the addition of Gd and Y in Mg significantly reduces the magnitude of GPFE, in particular for the {11¯00}〈112¯0〉 prismatic slip system. The analysis of the charge density distribution reveals that the predicted reduction in GPFE can be primarily attributed to a decrease of shear resistance between the slip planes. Based on the criterion for the anisotropy of the dislocation mobility and disembrittlement parameter, we demonstrate that alloying Mg with Gd and Y yields lower resistance to slip and hence an improvement in plasticity. Our results also suggest that the strength and plasticity of the Mg–Gd–Y system can be simultaneously enhanced due to charge transfer between Mg and alloying atoms.

Strengthening effect of nano-scale precipitates in a die-cast Mg–4Al–5.6Sm–0.3Mn alloy

In this paper we report a quantitative study of the age-hardening in the high-pressure die-cast Mg–4Al−5.6Sm−0.3Mn alloy. The results indicate that a number of nano-scale spherical precipitates identified as Al3Sm using high-angle annular dark-field scanning transmission electron microscopy, precipitated in Mg matrix after aging at 150–225 °C, with no obvious changes on grain sizes, intermetallic phases formed during solidification, and dislocation densities. From the existing strengthening theory equations in which some lacking parameters were taken from the first-principles density functional theory (DFT) calculations, a quantitative insight into the strengthening mechanisms of the nano-scale precipitate was formulated. The results are in reasonable agreement with the experimental values, and the operative mechanism of precipitation strengthening was revealed as Orowan dislocation bypassing.

Strengthening Mechanisms of Magnesium-Lithium Based Alloys and Composites

Mg-Li based alloys are widely applied in various engineering applications. The strength of these alloys is modified and enhanced by different strengthening mechanisms. The strengthening mechanisms of these alloys and their composites have been extensively studied during the past decades. Important mechanisms applied to strengthening the alloys include precipitation strengthening, solution strengthening, grain and subgrain strengthening, and dislocation density strengthening. Precipitation and solution strengthening mechanisms are strongly dependent on composition of the alloys and thermal treatment processes, whereas grain and subgrain and dislocation density strengthening mechanisms majorly depend on thermomechanical processing. In this paper, recent studies on conventional processes for the strengthening of Mg-Li based alloys are summarized as they are critical during the alloys design and processing. Main strengthening mechanisms are objectively reviewed, focusing on their advantages and drawbacks. These can contribute to enhancing, initiating, and improving future researches for alloys design and suitable processing selection.

Simultaneous improvement of strength, ductility and corrosion resistance of Al2024 alloy processed by cryoforging followed by ageing

The aim of the present study is to simultaneous improvement of strength and ductility as well as corrosion resistance of ultrafine grained 2024 Al-alloy processed by multiaxial cryoforging (MAF) and cryorolling followed by ageing. The evolution of ultrafine grained microstructure during MAF followed by ageing is investigated using optical and transmission electron microscopy. Both multiaxially forged (MAFed) and cryorolled (CRed) samples showed an improvement in yield strength (YS) with a corresponding decrease in the ductility. Aging treatment not only improved the YS, but also its ductility. Improvement in the ductility after ageing is confirmed by the fractography analysis. Corrosion resistance of the MAFed+aged samples found to be higher compared to that of the MAFed and coarse grained counterpart. The corrosion behavior has been analyzed in the light of open circuit potential (OCP), solutionizing, grain size and precipitation strengthening mechanisms. SEM images of the corroded samples also corroborated the corrosion test results.

Effects of re-ageing treatment on microstructure and tensile properties of solution treated and cold-rolled Al–Cu–Mg alloys

The effects of re-ageing (RA) at different temperatures on the tensile properties and microstructures of the Al 2024 alloy (Al–Cu–Mg series alloy) were investigated, which had been solution treated (ST) at 495°C for 1h and conventionally cold-rolled (CR) with 83% thickness reduction. The strength of the alloy could be increased markedly by the thermo-mechanical treatment while maintaining acceptable ductility. The ultimate tensile strength (UTS), yield strength (YS) and elongation to failure (Ef) of the RA sample at 100°C for 72h were 755MPa, 711MPa and 7.3%. As comparison, the UTS and YS values of the conventional peak-aged (PA) sample at 190°C for 8h were 528MPa and 426MPa, respectively, with the Ef value of 13%. The quantitative estimation indicated that the high strength of the RA samples was attributed to the combined effects of dislocation, precipitation, grain size and texture strengthening mechanisms. Microstructure observations revealed that the ductility of the RA samples could be improved by the GBP zones rather than S′ phase. Fractography observations displayed that the precipitation of S′ phase accelerated the tensile failure of the RA samples by microvoid coalescence, whereas the precipitation of GPB zones would not.

Elucidating of tool rotational speed in friction stir welding of 7020-T6 aluminum alloy

The 7020-T6 aluminum alloy plates of 4 mm thickness were friction stir welded at rotational speeds of 400, 600, 800, and 1000 rpm and constant traverse speed of 100 mm/min. The peak temperatures of the joints were recorded by precise thermocouples. Microstructure, hardness, tensile properties, and fracture surfaces of the joints were analyzed. The results showed that decreasing the tool rotational speed from 1000 to 400 rpm decreased the peak temperature from 311 to 209 °C, and hence caused a lower heat input. In addition, lower rotational speeds result in higher hardness and tensile strengths. The higher hardness and ultimate tensile strength were related to the grain boundary, precipitation, and substructure strengthening mechanisms. In addition, the fracture surfaces of the joints welded at higher heat input conditions showed more ductile mode in comparison with that of welded at lower heat input condition, which confirmed the lower tensile elongation of the joints welded at lower rotational speeds.

Effect of tool traverse speed on microstructure and mechanical performance of friction stir welded 7020 aluminum alloy

The effect of tool traverse speed on microstructure and mechanical properties of friction stir welded 7020-T6 aluminum alloy was investigated. For this purpose, 5 mm thick 7020-T6 aluminum alloy plates were friction stir welded at traverse speeds of 50, 100, 150, and 200 mm/min and constant rotational speed of 900 r/min. Also, the peak temperatures of the joints during friction stir welding were recorded by accurate thermocouples. Moreover, the microstructure, hardness, tensile properties, and fracture surfaces of the joints were examined. The results showed that increasing the tool traverse speeds from 50 to 200 mm/min decreased the peak temperature from 331 ℃ to 211 ℃, and hence caused to lower heat input during friction stir welding. Furthermore, the higher hardness and ultimate tensile strength of the joints welded at higher traverse speeds was related to the grain boundary, precipitation, and substructure strengthening mechanisms. In addition, the fracture surfaces of the joints welded at higher heat input conditions showed more ductile mode in comparison with those welded at lower heat input condition, which confirmed the lower tensile elongation of the joints welded at higher traverse speeds.

The mechanism of precipitation strengthening in Fe–Ni austenitic alloy electron beam weldment

Precipitates and their effects on deformation and mechanical performance of Fe–Ni alloy electron beam weldment after post-weld aging treatment were investigated by transmission electron microscopy and microhardness tests. Results show that precipitates in the base material and the weld are both γ′ (or Ni3(Al, Ti) ) phase. However, precipitates in the weld are much larger in size and more dispersed than that of the base metal. Atomic resolution observation revealed lots of vacancy clusters inside the γ′ precipitates and the γ′/γ interface in the weldment. In contrast to the dislocation planar slip mode in the base metal, dislocations in the weldment tended to entangle around the γ′ precipitates forming dislocation loops. Microhardness of the weldment was about 15% lower than that of the base metal. The precipitate size, distribution and vacancy clusters are correlated with the observed dislocation structure and microhardness. It is concluded that for the weld, the resistance to fracture was determined by the size and distribution of precipitates.

Effects of Mn partitioning on nanoscale precipitation and mechanical properties of ferritic steels strengthened by NiAl nanoparticles

The critical role of Mn partitioning in the formation of ordered NiAl nanoparticles in ferritic steels has been examined through a combination of atom probe tomography (APT) and thermodynamic and first-principles calculations. Our APT study reveals that Mn partitions to the NiAl nanoparticles, and dramatically increases the particle number density by more than an order of magnitude, leading to a threefold enhancement in strengthening. Atomistic structural analyses reveal that Mn is energetically favored to partition to the NiAl nanoparticles by preferentially occupying the Al sublattice, which not only increases the driving force, but also reduces the strain energy for nucleation, thereby significantly decreasing the critical energy for formation of the NiAl nanoparticles in ferritic steels. In addition, the effects of Mn on the precipitation strengthening mechanisms were quantitatively evaluated in terms of chemical strengthening, coherency strengthening, modulus strengthening and order strengthening.

Tensile yield behavior and precipitation strengthening mechanism in Super304H steel

Tensile yield behaviors of the solution- and aging-treated Super304H austenitic steel have been investigated at temperatures from 300 to 923K. The results showed that the contribution to yield stress from precipitation strengthening of Cu-rich phases is 17–23%. The higher thermal activation volume and energy imply an interaction between dislocation and Cu-rich precipitates. The precipitation strengthening mainly arises from coherency strain and partially from stacking fault strengthening. The calculated shear stress from precipitation strengthening agrees reasonably with the experimental result.

Phase Diagram Simulation and Investigation of Microstructure of Ni-Based Alloys

To develop proper Ni-based alloys for high-temperature vitriol pump which is submitted the corrosion of vitriol and rush of liquid with solid particles, the equilibrium phase diagrams of some Ni-Cr-Fe-C-Mo-Si-Cu alloys were calculated by software Thermo-Calc. Based on the simulation results and the mechanism of sigma phase precipitation strengthening, the chemical composition of a new Ni-based alloy was proposed. The alloy was casted and treated by solid solution at 1373K+2h, followed cooling to 1323K and quenching in water at room temperature, then strengthened by ageing respectively at 973K, 1023K, 1073K, 1123K for 4h. It was investigated the phases by XRD, the microstructures by OM and SEM. The proposed alloy consists of g, s and M23C6 from 1073K to 1273K. The most amount of s phase is up to 12.45 % (mass) at 1023K, it decreases with the augment of equilibrium temperature. s phase disappears above 1323K. The amount of s phase is enough in alloy to supply good precipitation hardening at a large temperature range. Experimental results were compared with the results of phase diagram simulation by Thermo-Calc. The alloy can be effectively strengthened by sigma phase precipitation at the temperature from 1023K to 1073K. Experimental results verified the validity of phase diagram simulation.