Boundary trend of α1／（α1＋α2 ） phase region at lower Cu side in Al-Zn-Cu system

The range of miscibility gap above 300℃ at low Cu side in Al-Cu-Zn ternary system was obtained by EPMA of the designed alloys and diffusion-couples treated for equilibrium. The results about the boundary trend of the α1/(α1 α2) phase region was obtained. The α1/(α1 α2) boundary moves towards the lower Zn side with the in-crease of Cu content. The results are opposite to traditional phase diagrams obtained by experiments, but consistent with recent thermodynamic calculations.     

Boundary trend of α1/(α1 +α2) phase region at lower Cu side in Al-Zn-Cu system

The range of miscibility gap above 300 ℃ at low Cu side in Al-Cu-Zn ternary system was obtained by EPMA of the designed alloys and diffusion-couples treated for equilibrium. The results about the boundary trend of the α1 / (α1 α2 ) phase region was obtained. The α1 / (α1 α2 ) boundary moves towards the lower Zn side with the increase of Cu content. The results are opposite to traditional phase diagrams obtained by experiments, but consistent with recent thermodynamic calculations.     

Effect of Solution Heat Treatment in α+β Phase Region on Shape Memory Characteristics of Cu-Al-Ni-Mn-Ti Alloys

The effect of aluminium content and solution heat treatment in α+β phase region on the shape memorycharacteristics and mechanical properties of cold wrought Cu-Al-Ni-Mn-Ti alloy are studied in this paper. Re-sults indicate that the transformation temperature (Tt) of Cu-Al-Ni- Mn-Ti alloy reduces obviously with the in-crease of the amount of α-phase. During aging at 623K, T_l increases at first up to a peak value, then decreaseswith prolongation of aging time. Life time of heat resistance of the alloy at high temperatures is improved withincrease of the amount of α-phase, this life time becomes poor with Bainite precipitation. When the amount ofα-phase is less than 5% the ratio of shape recovery brought about by the solution heat treatment in α+β phaseregion is almost not effected. However, plasticity of the alloy increases obviously as aluminium content de-creases. We believe that improving cold workability of Cu-Al-Ni-Mn-Ti alloy and keeping good heat resistantproperty and shape memory effects are possible by means of reducing the content of aluminium and solulionheat treatment in α+β phase region.     

PHASE CONSTITUTION AND MORPHOLOGIES OF Mg-Zn-Zr ALLOY(MB15)

The equal-width lath-shaped Zn_2Zr_3 phase and the irregular lump MgZn_2 phase were ob-served in the Zr-and Zn-enriched nucleus region and along the grain boundaries of theas-cast alloy.The very fine ZnZr_2 phase occurs in the grains and the superfine MgZn phasedisperses in the α-Mg solid solution.In extruded shapes,the coarse grains of Zn_2Zr_3 phaseand a great many dispersively distributed ZrH_2 phase and MgZn_2 phase particles appear inthe Zr-and Zn-enriched lath region.They play an important role in impeding the proceedingof recrystallization.The precipitation of Mg_2Zn_3 phase may be led to the non-lath region byageing at 170℃ for 10h.     

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.     

LAMELLAR MICROSTRUCTURE IN DUPLEX Ti-Al-V INTERMETALLIC COMPOUND

The morphology and crystallographic orientation of(α_2+γ)lamellar structure in duplexTi-47.5Al-2.5V intermetallic compound have been studied by means of TEM,microdiffraction as well as 180° and non-180° rotation twinning analysis.A possible atomicmodel of the interface was suggested.Thus,approach was made to a mechanism on complexnucleation of α_2+γ two phases along α-Ti interface,and growth of α_2/γ/γ_1/α_2 orα_2/γ/α_2/γ as fundamental structural unit.     

Precipitate Behavior in Fe–20Cr–30Ni–2Nb Austenitic Heat-Resistant Steel

In this study,the precipitation behavior of a new austenitic heat-resistant steel(Fe–20Cr–30Ni–2Nb,in at%)was investigated.The effects of alloying addition of boron(B) and lanthanum(La) on the microstructure of the austenitic steel were scrutinized using SEM,EPMA,TEM,and XRD.The results showed that the addition of B enhanced the precipitation of bar-type Laves phase.A small precipitate with high La concentration was observed at the grain boundary in the alloy without aging;similar precipitates without La also presented in region adjacent to the La single phase.This result indicates that La can exist independently and does not contribute to the formation of new compounds.However,in both Band La-modified alloy,B appeared in the precipitate free zone.In the alloy containing both B and La,only Fe2 Nb Lavesphase precipitates,as indicated by the XRD result.     

Turning machining induced microstructural stability of a high Nb-containing TiAl alloy during high temperature exposure

Turning machining induced microstructural instability was investigated in a fully lamellar Ti-45Al-8.5Nb-(W,B,Y) alloy during high temperature exposure.After turning machining followed by thermal exposure at900 or 1000℃ for 100,300 and 500 h,a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface.Two typical layers,a fine-grained(FG) layer with equiaxed grains and a coarse-grained(CG) layer with elongated grains,are formed in this region in transversal direction.The thickness of the two layers is up to 120 urn after thermal exposure at 1000℃ for 500 h,which is less than the depth of the hardened region(200 μm) after turning machining.Most of the new grains in FG and CG layers are constituted of γ single phase,while short α_2 segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains.Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer,respectively.The residual deformation energy stored is considered to be the main driving force of this process.     

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.     

ON THE ORDERING TRANSFORMATIONS IN Ti_3AL-Nb ALLOY

Study was made of the behaviour of ordering transformation in Ti_3AI-Nb alloy,including the ordering at high temperatures,the transformation of high temperature β-phase during cooling,and the decomposition of metastable β-phase during aging.The results show that the ordered primary α_2 and high temperature β in alloy form at 1060℃.The transformation of high temperature β-phase proceed by β→α_2+ω type during cooling,and the decomposition of metastable β and ω type proceeded by(β+ω)_(metustabte)→(α_2+β)_(stable)during aging at 700℃.     

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH_3 nanoparticles

Grain boundary diffusion technique with TbH_3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient(α) and the coercivity temperature coefficient(β) of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd_2Fe_(14)B grains and is expected to exist as(Nd,Tb)_2Fe_(14)B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH_3 nanoparticles exhibit enhanced coercivity.     

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.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.     

ROOM TEMPERATURE SECTION OF Cu-Dy(≤wt-%)-Ni TERNARY PHASE DIAGRAM

The room temperature section of Cu-Dy(≤35wt-%)-Ni phase diagram has been determinedby means of X-ray diffraction technique.The results are as follows.four single-phase regions(α,Dy_2Ni_(17),DyNi_5 and DyCu_5),five two-phase regions(α+Dy_2N_(17),Dy_2Ni_(17)+DyNi_5,DyCu_5+DyNi_5,α+DyNi_5 and DyCu_5+α)and two three-phase regions(α+Dy_2Ni_(17)+DyNi_5 and α+DyNi_5+DyCu_5)exist in the seetion.No any new phase was ob-servered.The solubility of Dy in α-phase is up to 2 wt-%.     

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.     

CREEP CRACK GROWTH BEHAVIOR OF ALLOY 718 DURING HIGH TEMPERATURE EXPOSURE

Alloy 718 is a precipitation strengthened nickel-hased superalloy based on the precipitation of γ’’-Ni3Vb(DO22 structure) and γ‘-Ni3(Al, Ti) (Ll2 structure) phases. Creep crack growth rate (CCGR) was investigated after high temperature exposure at 593, 650 and 677℃ for 2000h in Alloy 718. In addition to the coalescence of γ‘/γ‘‘ and the amount increasing of 6 phase, the existence of a bcc chromium enriched α-Cr phase was observed by SEM, and the weight fraction of α-Cr and other phases were determined by chemical phase analysis methods. The CCGR behavior and regulation have been analyzed by means of strength and structure analysis approaches. The experimental results show higher the exposure temperature and longer the exposure time, lower the CCGR. This is probably attributed to the interaction of material softening and brittling due to complex structure changes during high temperature exposure. Therefore,despite α-Cr phase formation and amount enhancement were run in this test range. It seems to us a small amount of α-Cr will be not harmful for creep crack propagation resistance, which is critical for disk application in aircraft and land-based gas turbine.     

Effects of trace Cr on as-cast microstructure and microstructural evolution of semi-solid isothermal heat treatment ZC61 magnesium alloy

The content and kind of trace elements in magnesium alloys have important effects on their ascast and semi-solid microstructures. In this research work, effects of trace Cr on as-cast and semi-solid microstructures of ZC61 magnesium alloy were investigated by metal mold casting and semi-solid isothermal heat treatment. The results show that the addition of Cr can refine the α-Mg phase without generating a new phase, noticeably change the eutectic phase, and decrease the average size of solid particles at the same isothermal heat treatment conditions. Non-dendritic microstructures of all alloys are constituted of α_1-Mg phases, α_2-Mg phases and eutectic phases after water quenching. With isothermal temperature increased or holding time prolonged, the eutectic microstructure(α-Mg+MgZn_2+CuMgZn) at the grain boundaries in as-cast alloy is melted preferentially and then turned into semi-solid non-dendritic microstructure by processes of initial coarsening, microstructure separation, spheroidizing and final coarsening. Especially when the ZC61-0.1 Cr alloy was treated at 585 ℃ for 30 min, the ideal non-dendritic microstructure can be obtained, and the corresponding solid particle size and shape factor were 37.5 μm and 1.33, respectively. The coarsening process of solid α-Mg phase at higher temperature or longer time, which is affected by both combining growth and Ostwald ripening mechanism, is refrained when Cr is added to the ZC61 alloy.     

Effects of Ti and Cu on the Microstructure Evolution of AlCoCrFeNi High-Entropy Alloy During Heat Treatment

The microstructure evolution of AlCoCrFeNiTi_(0.5) alloy and AlCoCrFeNiCu alloy during heat treatment was systematically studied,to reveal the influence rules of chemical activity of adding element on the microstructure evolution of AlCoCrFeNi system.Owing to the negative mixing enthalpy with the constituent elements,Ti element was mainly dissolved in the Al-Nirich phases,and aggravated the lattice distortion of B2 phase.The structure variation of BCC phase by adding Ti inhibited the formation of FCC phase and enhanced the precipitation of σ phase during heat treatment.Owing to the positive mixing enthalpy with constituent elements,Cu element tended to be repelled to the ID region and formed metastable Cu-rich FCC1 phase which would transform into Cu-Al-Ni-rich FCC2 phase with increasing temperature.The addition of Cu inhibited the precipitation of σ phase during heat treatment.Adding Ti maintained the stable dendritic morphology,while adding Cu reduced the thermal stability of microstructure.Two dramatic morphology changes occurred at 1000~℃ and 1100~℃ in the AlCoCrFeNiCu alloy.The lattice distortion of phase in AlCoCrFeNiTi_(0.5) alloy was aggravated with increasing temperature up to 800~℃,then relaxed together with the dissolution of σ phase when temperature was above 900~℃.The variation in lattice distortion dominated the hardness of AlCoCrFeNiTi_(0.5) alloy.With increasing heating temperature,the increasing volume fraction of region with FCC structure due to the transformation between FCC phases,and the pronounced coarsening in microstructure due to the reduced thermal stability,resulted in the mainly decreasing trend in the hardness of AlCoCrFeNiCu alloy.     

Phase equilibria of low-Y side in Mg-Zn-Y system at 400℃

Phase equilibrium relations of the Mg-Zn-Y system in the low-Y side at 400℃were investigated by alloy-equilibrated method,combined with thermal analysis.The results show that there is a liquid phase which could be in equilibrium with an a-Mg solid solution and an icosahedral quasicrystal I phase in the low-Y side of the Mg-Zn-Y system at 400℃.The liquid phase region originates from the binary Mg-Zn system and extends to 0.4 at%Y in the Mg-Zn-Y system.Besides,the hexagonal structure H phase,fee W phase and LPSO phase(X phase)are in equilibrium with a-Mg.With Y/Zn(atomic ratio,the same as follows) increasing,there exist four three-phase regions consisting of I+liquid+α-Mg,I+H+α-Mg,H+W+α-Mg and W+X+α-Mg,respectively,in the low-Y side of the isothermal section at 400℃.The twophase region a-Mg+I phase exists between I+H+aMg and I+liquid+a-Mg.In this two-phase region,the Y/Zn ratio is in the range of 0.14-0.17;and a three-phase region of a-Mg+I phase+H phase appears when Y/Zn ratio comes up to 0.17-0.27.Not I but W phase is in equilibrium with a-Mg,when Y/Zn ratio 0.27.The system is in liquid-state phase equilibrium,when Y/Zn ratio 0.14.     

Dependence of phase selection on cooling rates in as-cast A1-Si-Mg alloys

The effect of cooling rate on the solidification process of Al-2.06%Si-1.58%Mg was numerically and experimentally investigated. The solidification paths and the phase precipitation sequence were predicted based on the solute transportation analysis in the solidification process by coupling the thermodynamic calculation. Due to the different solute diffusion speeds, the solidification paths can be largely influenced by the cooling rates. Different phase precipitation sequences can be obtained through calculation under different cooling rates. And the later experiments have also proved this phenomenon. In the researched Al-2.06%Si-1.58%Mg alloy, the solidification sequences are α（Al）-α（Al）＋Si-α（Al）＋Mg2Si＋Si under low cooling rate and α（Al）- α（Al）＋Mg2Si-α（Al）＋Mg2Si＋Si under high cooling rate, respectively. The experimental results confirm the calculation predications.