Obtaining Ultra-High Strength and Ductility in a Mg–Gd–Er–Zn–Zr Alloy via Extrusion,Pre-deformation and Two-Stage Aging

The Mg–12Gd–1Er–1Zn–0.9 Zr(wt%) alloy with ultra-high strength and ductility was developed via hot extrusion combined with pre-deformation and two-stage aging treatment.The age-hardening behavior and microstructure evolution were investigated.Pre-deformation introduced a large number of dislocations,resulting in strain hardening and higher precipitation strengthening in the subsequent two-stage aging.As a result,the alloy showed a superior strength–ductility balance with a yield strength of 506 MPa,an ultimate tensile strength of 549 MPa and an elongation of 8.2% at room temperature.The finer and denser β' precipitates significantly enhanced the strength,and the bimodal structure,small β-Mg_5RE phase as well as dense γ' precipitates ensured the good ductility of the alloy.It is suggested that the combination of pre-deformation and two-stage aging treatment is an eff ective method to further improve the mechanical properties of wrought Mg alloys.     

Precipitation behavior of 14H-LPSO structure in Mg–12Gd–2Er–1Zn–0.6Zr Alloy

The microstructures of as-cast and as-solution Mg–12Gd–2Er–1Zn–0.6Zr alloys were investigated by optical microscopy(OM), scanning electron microscopy(SEM), transmission electron microscopy(TEM), highresolution transmission electron microscopy(HRTEM)X-ray diffraction(XRD) and selected-area electron diffraction(SAED) in the present investigation. The results show that the primary eutectic phase Mg_5(Gd, Er, Zn) and some flocculent features are found in the as-cast alloy; the SAED pattern indicates that these flocculent features are the dense areas of stacking faults. The 14H-LPSO structure precipitates in the temperature range of 673–793 K, and the volume fraction of 14H-LPSO structure increases with the extension of heating time; however, there is no precipitation of 14H-LPSO structure when the temperature reaches up to 803 K. In addition, the Mg_5(Gd, Er, Zn) phase dissolves gradually along with the precipitation of 14H-LPSO structure.     

Microstructure and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.     

MICROSTRUCTURES AND THE STRUCTURE STABILITY OF INCONEL 725,A NEW AGE—HARDENABLE CORROSION RESISTANT SUPERALLOY

INCONEL725 is a highly corrosion resistant nickel based alloy capable of being age-hardened to high strength levels. The micro structure observations and the phase identification after a standard heat treatment were investigated. The results show that the precipitation phases include the strengthening phasesγ', γ", and 8 phase, primary carbide phase TiC, as well as M6 C carbide and a little extent MC (mainly TiC) precipitates which nucleate mainly at grain boundaries. An isothermal aging study was carried out on this alloy for up to 10 000 hours at 593℃. This additional aging did not affect the tensile strength. However, micro structures show that the thermal exposure has a little additional effect. With increasing the exposure time, the size of γ" phase slightly increases, almost no change for γ' phase, while M6C carbides precipitated at grain boundaries have an increased and complex tendency on a few grain boundaries. The experimental results illustrate the excellent structure stability of the ag     

Precipitation process and its effects on properties of aging Cu–Ni–Be alloy

The precipitation process of aged Cu-Ni-Be alloy was investigated by X-ray diffraction （XRD）, trans- mission electron microscopy （TEM）, and high-resolution transmission electron microscopy （HRTEM）. The tensile strength, yield strength, and electronic conductivity of this alloy after aging were also studied. The precipitation sequence of the C17510 alloy aged at 525 ℃ is supersat-urated solid solution→G.P zones→ γ″-γ′→ γ. This transformation can be achieved by the accumulation of Be-atom layers. The G.P zones are composed of disk-shaped monolayers of Be atoms, which are formed on （001） matrix planes. The intermediate γ″ precipitate is nucleated in the G.P zones. The γ″ and γ′ precipitates have the same orientation relationship with matrix, e.g., （110）p｜｜（100）M,[001]p｜｜[001]M. The tensile strength of specimen shows a maximum during the aging process and then continuously decreases if the specimen is over aged. The strengthening effect of γ′ phase precipitated in aging at 525 ℃ for 4 h is calculated to be 436 MPa according to the Orowan strengthening, which is quite consistent with the experimental data.     

Phase precipitation behavior and tensile property of a Ti-Al-Sn-Zr-Mo-Nb-W-Si titanium alloy

The characteristic of precipitation behavior of a2 phase and silicide, and the tensile properties at room temperature and 650℃after heat treatments in anovel TiAl-Sn-Zr-Mo-Nb-W-Si titanium alloy(BTi-6431 S) were investigated by microstructure analysis and mechanics performance testing. The results show that no second phase precipitates after solution treatment(980 ℃/2 h, air cooling(AC)). However, when the solution-treated specimens are aged at 600 ℃(600 ℃/2 h,AC),α_2 phase precipitates in the primary α phase, and the size of α_2 phase increases with the aging temperature increasing to 750 ℃. Meanwhile, 50-100-nm S2-type silicide particles precipitate along lamellar phase boundaries of transformed β phase after aging at 750 ℃. BTi-6431 S alloy shows the best650 ℃ ultimate tensile strength(UTS) and yield strength(YS) when treated in solution treatment. However, aging treatment results in a decline in 650 ℃ ultimate tensile strength. This may be attributed to the loss of solution strengthening due to the depletion of Al, Si and Zr of the matrix caused by the precipitation of Ti_3 Al and(TiZr)_6 Si_3.Silicide is a brittle phase; therefore, its precipitation causes a sharp decrease in the room-temperature ductility of BTi-6431 S alloy.     

Effects of aging on the microstructures and mechanical properties of extruded AM50 ＋ xCa magnesium alloys

The effects of aging treatment on the microstructures and mechanical properties of extruded AM50 ＋ xCa alloys （x=0, 1, 2 wt.%） were studied. The results indicated the secondary phase Mgl7Al12 precipitated from the saturated α-Mg solid solution while Al2Ca changed slightly when the aging time was increased. The hardness of extruded AM50 ＋ xCa al- loys increased initially to its peak, and then dropped to reach its original hardness with the increase in aging time. With the increase in aging temperature, the hardness of the AM50 ＋ 2Ca ahoy decreased, whereas the hardness of AM50 and AM50 ＋ 1Ca alloys decreased in the initial stages of aging treatment and increased in the later stages of aging treatment. The tensile strengths of AM50 and AM50 ＋ 1Ca alloys increased after aging treatment for the precipitation of Mg17Al12 phase, which increases the resistance against dislocation movement at the grain boundary; with increase in aging temperature, their tensile strengths increased. For AM50 ＋ 2Ca alloy, the tensile strength declined after aging at 150℃ and 175℃, while it increased slightly at 200℃. The ductility of AM50 ＋ xCa alloys （x = 0, 1, 2 wt.%） declined after aging treatment.     

Single-aging characteristics of 7055 aluminum alloy

The microstructures and properties of 7055 aluminum alloy were studied at different single-aging for up to 48 h using hardness test, tensile test, electrical conductivity measurement, XRD and TEM microstructure analysis. The results show that at the early stage of aging, the hardness and strength of the alloy increase rapidly, the peak hardness and strength are approached after 120 ℃ aging for 4 h, then maintained at a high level for a long time. The suitable single-aging treatment of 7055 alloy is 480 ℃, 1 h solution treatment and water quenching, then aging at 120 ℃ for 24 h. Under those condition, the tensile strength, yield strength, elongation and electrical conductivity of the studied alloy are 513 MPa, 462 MPa, 9.5% and 29%（IACS）, respectively. During aging, the solid solution decomposes and precipitation occurs. At the early aging stage of 120 ℃, GP zones form and then grow up gradually with increasing ageing time. η′ phase forms after ageing for 4 h and η phase starts to occur after 24 h aging.     

Microstructure and mechanical properties of a compound reinforced Mg95Y2.5Zn2.5 alloy with long period stacking ordered phase and W phase

The microstructure evolution of Mg100-2xYxZnx(x=2, 2.5, 3, 3.5) alloys was investigated. Results show that the Mg100-2xYxZnx alloys are composed of α-Mg, long period stacking ordered(LPSO) phase and eutectic structure phase(W phase), and the Mg95Y2.5Zn2.5 alloy has the best comprehensive mechanical properties. Subsequently, the microstructure evolution of the optimized alloy Mg95Y2.5Zn2.5 during solidification and heat treatment processes was analyzed and discussed by means of OM, SEM, TEM, XRD and DTA. After heat treatment, the lamellar phase 14H-LPSO precipitated in α-Mg and W phase transforms into particle phase(MgYZn2). Due to the compound reinforcement effect of the particle phase and LPSO phase(18R+14H), the mechanical properties of the alloy are enhanced. The tensile strength and elongation of the Mg95Y2.5Zn2.5 alloy is improved by 9.1% and 31.3% to 215 MPa and 10.5%, respectively, after solid-solution treatment.     

Influence of aging modes on microstructure and mechanical properties of AZ80 magnesium alloy

The microstructure and mechanical properties of AZ80 magnesium alloy after solid solution and aging treatments were studied by using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy(SEM) as well as tensile testing. The results indicated that β-Mg17Al12 phase was getting to distribute discontinuously along the grain boundary after treated at 395℃ ageing for 12 h followed by water-cooling, but it did not dissolve into α-Mg completely. The residual β-Mg17Al12 phase distributed along the grain boundary and had block-like or island shapes. The size of α-Mg was getting to be coarsening but not significantly. The β-Mg17Al12 precipitates appeared in discontinuous and continuous patterns from supersaturated α-Mg solid solution after aged at 200℃. The precipitation patterns were associated with the aging time essentially. The tensile strength and elongation of the alloy increased significantly but the hardness and yield strength decreased after solid solution treatment. However, with the prolonging of aging time, the hardness and strength of alloy increased while the ductility decreased.     

Relationship between microstructure and tensile properties on a near-β titanium alloy after multidirectional forging and heat treatment

In this study, a new near-beta titanium alloy, Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr, was prepared by induction skull melting(ISM) and multidirectional forging. The effect of aging heat treatment on microstructure and tensile properties of the alloy after solution treatment in the twophase(α + β) region was investigated. The micros tructure results show that the globular primary α phase(α_p) and the needle-like secondary α phase(α_s) are precipitated in the β matrix. The size of α_s increases with the increase in aging temperature,while the content of α_s goes up to a peak value and then decreases. The tensile testing results show that the strength increases first and then decreases with the increase in temperature. The variation of ductility presents the opposite way compared with the trend of strength level.When aged at 500 ℃, the alloy exhibits an excellent balance of tensile strength(1529 MPa) and elongation(9.22%). And the relative mechanism of strengthening and toughening was analyzed and discussed.     

Microstructure and mechanical properties of new metastable β type titanium alloy

A new metastable β type titanium alloy called TB-13 with the combination of excellent strength and ductility was developed successfully.In order to develop a perspective on this new alloy,the influence of several commonly used heat treatments on the microstructure and properties was studied.In solution-treated and quenched samples,a low-temperature aging at 480°C results in the precipitation of finerαphase.The precipitation of coarserαphase plate at higher aging temperature(560°C)leads to the increase of tensile ductility but reduction of strength.During low-temperature aging at 300°C,quite homogeneous distribution of fine isothermalωphase particles was found.The isothermalωphase provides nucleation sites forαphase during two-step aging process and makesαphase extremely fine and disperse uniformly in β matrix.Thus,TB-13 alloy is strengthened and its mechanical properties are improved.     

Microstructure and properties characteristic during interrupted multi-step aging in Al-Cu-Mg-Ag-Zr alloy

The effects of interrupted multi-step aging on the microstructure and properties of Al-Cu-Mg-Ag-Zr alloy were studied by tensile,hardness,electrical conductivity tests and transmission electron microscopy(TEM).Interrupted multi-step aging delayed the peak aging time compared to one-step aging and kept the same levels of hardness,electrical conductivity,ultimate tensile strength(UTS),yield strength(YS) and elongation as those of the T6 temper alloy while increased the fracture toughness notably.Ω phase and a little θ’ phase precipitated and grew simultaneously in the process of one-step aging at 160℃.During the second-step aging at 65℃ of interrupted multi-step aging,no TEM characteristic of Ω precipitates could be found.During the third step of interrupted multi-step aging,Ω began to dominate the microstructure like what happened in the process of one-step aging.The difference of properties between the T6 temper and the interrupted multi-step aged alloys might be related to the different precipitation sequences in the process of the two heat treatment technologies.     

Mechanical properties and energy absorption of extruded Mg–2.0Zn–0.3Zr alloy with Y addition

The effects of the rare earth element Y addition on mechanical properties and energy absorption of a low Zn content Mg–Zn–Zr system alloy and the deformation temperature of optimized alloy were investigated by room tensile test, optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscope(TEM). The results show that,after homogenization at 420 °C for 12 h for the as-cast alloys, Mg Zn phase forms, which decreases the strength of Mg–2.0Zn–0.3Zr alloy with Y content of 0.9 wt%. The tensile strength and elongation of the alloy with a Y addition of 5.8 wt% reach the max value(281 ± 2) MPa and(30.1 ± 0.7) %, respectively; the strength and elongation of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reach(321 ± 1) MPa and(21.9 ± 0.7) %, respectively. The energy absorption increases with the increase of Y content, the max value reached 0.79 MJ m-3with Y content of 5.8 wt%, and the energy absorption of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reaches0.75 MJ m-3.     

Effect of Precipitation on Hot Deformation Behavior and Processing Maps of Nickel-Base UNS N10276 Alloy

The hot deformation characteristics and processing maps of aged nickel-base UNS N10276 alloy were investigated and compared with those of solution-treated UNS N10276 alloy at temperatures of 950–1250°C and strain rates between 0.01 and 10 s~(-1).The dominant precipitated phase in the aged alloy was identified as topologically close-packed(TCP)l phase enriched in Mo and Ni.The precipitates present in the UNS N10276 alloy could significantly facilitate flow softening after peak stress at temperatures lower than 1150°C and strain rates higher than 0.01 s~(-1).Processing maps at true strains of 0.1–0.9 were developed using the dynamic materials model and experimental flow stress data.Although aging treatment slightly shrank the suitable hot working window of this alloy,the aged alloy showed higher peak efficiencies of power dissipation and smaller unstable regions in comparison with solution-treated alloy.Furthermore,aging treatment eliminated the instability region of processing maps at true strains of 0.2–0.5.The precipitated phase promoted dynamic recrystallization(DRX)by the particle-stimulated nucleation(PSN)mechanism,which resulted in the larger fraction of DRX as well as finer and more uniform grain structure in the aged alloy specimens compared to the solution-treated alloy.     

Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel

In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic(RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600 °C for 500,1000, and 3000 h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M_(23)C_6 and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M_(23)C_6 is more obvious than that of MX with increase in aging time. During the long-term thermal exposure, the Fe_2 W Laves phase precipitates adjacent to M_(23)C_6 along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000 h.     

Aging Precipitation Behavior and Mechanical Properties of Inconel 617 Superalloy

Aging precipitation behavior and mechanical properties of Inconel 617 superalloy aged at 760 ℃ for up to 10000 h were investigated. The results showed that the precipitates of the aged alloy are M23C6 and M6C carbides and γ phase. The carbide particles precipitated both at the grain boundaries and within grains, and the γ phase particles were situated at intragranular sites in the process of aging. The carbide particles were discontinuously dispersed at grain boundaries after aging for 3000 h, while after aged for 5000 h the carbide particles are merged. The precipitates inside grains remained stable even after aging for 10000 h. The hardness was increased for the alloy aged for 300 h up to 3000 h, which was resulted primarily from the precipitation of carbides as discrete particles both at the grain boundaries and inside grains. Small quantity γ precipitates were formed inside grains, to some extent, which contributed to an enhanced hardness. However, a decrease of the hardness was observed after aging for 5000 h. A significant drop in toughness of the alloy aged for 300 h was attributed to the reduction of the bonding interface strength when carbides precipitated at grain boundaries. Thereafter, the toughness decreased slowly with the prolonged aging time. The high temperature tensile properties of the aged alloy are rather stable even aged for 300-3000 h.     

Effect of Solution Treatment on Microstructure and Corrosion Properties of Mg–4Gd–1Y–1Zn–0.5Ca–1Zr Alloy

The as-cast multi-element Mg–4Gd–1Y–1Zn–0.5Ca–1Zr alloy with low rare earth additions was prepared, and the solution treatment was applied at different temperatures. The microstructural evolution of the alloy was characterized by optical microscopy and scanning electron microscopy, and corrosion properties of the alloy in 3.5% NaCl solution were evaluated by immersion and electrochemical tests. The results indicate that the as-cast alloy is composed of the a-Mg matrix,lamellar long-period stacking-ordered(LPSO) structure and eutectic phase. The LPSO structure exists with more volume fraction in the alloy solution-treated at 440 °C, but disappears with the increase in the solution temperature. For all the solution-treated alloys, the precipitated phases are detected. The corrosion rates of the alloys decrease first and then increase slightly with the increase in the solution temperature, and the corrosion resistance of the solution-treated alloys is more than four times as good as that of the as-cast alloy. In addition, the alloy solution-treated at 480 °C for 6 h shows the best corrosion property.     

Microstructures of Ti–Ni–Fe wire after severe cold-drawing and annealing

The microstructures and mechanical properties of Ti–47 at%Ni–3 at%Fe shape memory alloy wire under the condition of severe cold-drawing at room temperature and different postdeformation annealing processes were intensively investigated using transmission electron microscope(TEM),X-ray diffraction(XRD),Vickers microhardness tester and electron tensile tester.It is indicated that the structure of the alloy evolves into a predominant amorphous structure with a trace of nanocrystalline B2 phase after the cold-drawing of 76%areal reduction.Postdeformation annealing process exerted significant influence on the microstructure and mechanical properties.Crystallization occurs when the cold-drawn wire was annealed at 300℃ for 30 min.The ultimate tensile strength and ductility as well as the superelasticity of the wire are improved significantly by cold-drawing plus postdeformation annealing.     

Microstructure Evolution of Extruded Mg-6Gd Alloy Under 175 ℃ and 150 MPa

The tensile creep behavior of extruded Mg-6 Gd alloy,having the tensile yield strength of～ 110 MPa at 175 ℃,has been investigated under 175 ℃ and 150 MPa. In this study, the extruded Mg-6 Gd sample exhibits the total tensile strain of ～10.5% after the creep time of 1100 h,and the fast plastic strain of ～4.6% at the beginning of the creep test. The microstructure result suggests that the dislocation deformation is the main deformation mode during creep, and the grains with orientation close to(0001) II ED disappear after creep. The creep process containing a low creep strain has no effective promotion for the precipitation compared with the aging process without strain. The origination of creep crack is related to the formation of precipitate-free zone during creep. The work offers an important implication to research the microstructure evolution under an applied stress in a weak aging response Mg alloy.