Sc、Zr对Al-Mg-Mn合金再结晶行为及内耗性能的影响

为了研究微量Sc、Zr在Al-Mg-Mn合金中的作用,采用铸锭冶金方法制备了Al-6.0Mg-0.5Mn-(Sc、Zr)合金,通过光学显微镜、显微硬度、透射电镜组织观察和低频扭摆法测量内耗方法研究了微量Sc、Zr对Al-6.0Mg-0.5Mn的组织、再结晶行为及内耗性能的影响.研究表明:添加质量分数为0.21%Sc和0.15%Zr可显著细化Al-6.0Mg-0.5Mn合金铸态组织;粒状Al3Sc1-xZrx相对位错、晶界有强烈钉扎作用,抑制合金再结晶;冷变形后的Al-6.0Mg-0.5Mn-0.21Sc-0.15Zr合金的内耗表现出非线性特征,频率越低或温度越高,合金内耗Q-1越大.在频率为1Hz、应变振幅为4.6×10-5下,冷变形Al-6.0Mg-0.5Mn-0.21Sc-0.15Zr合金升温Q-1-T曲线上在326℃时产生内耗峰,该峰可由Al3Sc1-xZrx沉淀粒子与位错脱钉机制解释.微量Sc、Zr可以细化Al-Mg-Mn合金组织,抑制合金的再结晶,导致合金在升温Q-1-T曲线上产生内耗峰.     

Microstructure and internal friction of spray deposited Al-3.3Fe-10.7Si alloy

Al-3.3Fe-10.7Si alloy has been experimentally made with spray deposition technology. The internal friction of the alloy which was directly associated with the microstructures under spray deposited, extruted and heat treated conditions has been investigated using a low frequency inverted torsion pendulum over the temperature region of 10–300 °C. An internal friction peak was observed in the temperature range 50–250 °C in the present alloy. The Q^-1 peak decreased after extruted and in subsequent to the earliness of isothermal annealing, which was found to be directly attributed to the precipitation of FeAl₂ and Al– Fe– Si intermetallics from the supersaturated aluminium alloy matrix. We suggest that the internal friction peak in the alloy originates from grain boundary relaxation, but the grain boundary relaxation can also be affected by FeAl₂ and Al– Fe– Si intermetallics at the grain boundaries, which will impede grain boundary sliding.     

Effect of substituting cerium-rich mischmetal with lanthanum on high temperature properties of die-cast Mg–Zn–Al–Ca–RE alloys

Mg–Zn–Al–Ca–RE alloys have been found to be promising materials for substituting aluminum alloys used for automatic transmission case applications in the automobile industry. Particularly, Mg–0.5%Zn–6%Al–1%Ca–3%RE (ZAXE05613) alloy exhibits comparable creep resistance as ADC12 die-casting aluminum alloy that is currently used for automatic transmission case applications. Changing the rare earth (RE) content of the alloy from mischmetal to lanthanum gives a further improvement in the creep properties of the alloy. Lanthanum addition results in the crystallization of a large amount of acicular Al₁₁RE₃ (Al₁₁La₃) compound along the grain boundaries as well as across the grain boundaries and this effectively controls grain boundary sliding and dislocation motion in the vicinity of the grain boundaries. As a result, die-cast ZAXLa05613 alloy exhibits a higher creep resistance than that of ZAXE05613 alloy.     

Research and Improvement on structure stability and corrosion resistance of nickel-base superalloy INCONEL alloy 740

INCONEL alloy 740 is a newly developed Ni–Cr–Co–Mo–Nb–Ti–Al superalloy in the application to ultra-supercritical boilers with steam temperatures up to 700 °C. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), micro-chemical phase analyses, and corrosion-resisting test, this paper investigates the structure stability of the alloy at elevated temperature and concentrates on coal ash corrosion performance of the alloy under the simulated coal ash/flue gas condition. Experimental results show that the most important structure instabilities of the alloy during prolonged aging are γ′ coarsening, γ′ to η transformation and G phase formation at grain boundary. The performance of corrosion resistance of the alloy would meet the requirement of ultra-supercritical boiler tubes. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level, and also in maintenance of Cr content of the alloy were designed and melted for experimental investigation. These two modified alloys exhibit more stable microstructure during 760 °C long time aging.     

Interface phenomena in the Y2O3/(Al–Cu) system

Wetting behavior and the interface reaction in the Y₂O₃/(Cu–Al) system were investigated at 1423 K. A contact angle of about 130° was measured in the Y₂O₃/Cu system. Aluminum addition to copper improves wetting and the transition from non-wetting to wetting (θ ≤ 90°) was observed for the alloy with 50 at.% Al. The microstructure examination of the interface indicates that Al reacts with yttria, yttrium dissolves in the melt and a crater of AlYO₃ is formed at the substrate. The interface interaction in the Y₂O₃/(Cu–Al) system is in a good agreement with the results of a thermodynamic analysis in the Y–Al–Cu–O system. The crater depth and the macroscopic final contact angles are correlated with the Y and Al activities in the melt.     

Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy

Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of magnesium matrix and Mg₁₂Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO₁₉ structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.     

High temperature structural evolution of a CuAlNi alloy studied by in situ X-ray diffraction and isothermal mechanical spectroscopy

The purpose of this work is to study the microstructural mechanisms associated with the eutectoid transition in a ternary Cu–12 wt.% Al–3 wt.% Ni alloy. The samples have been initially annealed at 850 °C, then slowly cooled down to room temperature. The experiments have been carried out both on cooling and on heating above 500 °C using isothermal mechanical spectroscopy and X-ray diffraction (fitted with a temperature camera). On heating, a relaxation peak with a high intensity rises up above 600 °C, then on cooling, the peak totally disappears below 580 °C, the effect being reproducible. The structural analysis, undertaken in the same temperature domain, has clearly evidenced each step of the evolution, particularly the eutectoid transformation. Consequently, the damping effect seems to be associated to the presence of the high temperature β phase.     

Evaluation of stress corrosion resistance and corrosion fatigue fracture behavior of ultra-high-strength P/M Al–Zn–Mg alloy

Quasi-static tensile tests in air and slow strain rate tests (SSRTs) in a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al–Zn–Mg alloy fabricated through powder metallurgy. Attention is also paid to fatigue strength and fatigue crack growth behavior in laboratory air and in a 3.5% NaCl solution. The alloy has extremely high strength of about 800 MPa. However, elongation at break remains small, at about 1.3%. The final fracture occurs by a macroscopically flat crack normal to the tensile axis, with little reduction in area and little shear lip on the periphery of a smooth sample. However, it fails microscopically in a ductile manner, with dimples. Dimple size is less than 1 μm, because the grain size of the alloy is extremely small. Strengthening mechanisms operating in the alloy are: small grains, sufficient metastable η′ phase in a matrix, and intermetallic compound acting as a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution decreases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sample surface acts as a stress concentrator. Crack initiation site of quasi-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improvement of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al–Zn–Mg alloys fabricated by ingot metallurgy, because the fatigue fracture toughness, or ductility, of the alloy is inferior to other Al alloys, and intergranular cracking promotes crack growth. However, no influence of 3.5% NaCl solution on corrosion fatigue crack growth is observed, although an investigation is required into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are discussed.     

Development of new Al-based nanocomposites by mechanical alloying

Al-based binary (Al–Mg) and ternary (Al–Mg–Zr) elemental powder mixtures were mechanically alloyed to develop new Al–Mg–Zr nanocomposite materials. The phase evolution was studied in the as-milled and heat-treated powders by XRD and TEM/EDS analyses. For the binary Al–Mg alloy, the predominant phase was an Al(Mg) solid solution (SS) and an amorphous phase was not possible to be synthesized. Upon adding 5 at.% Zr to the Al–10Mg blended powder, some free Mg was present in addition to the formation of an Al(Zr,Mg)SS, which transformed to the Al₃Zr intermetallic after annealing. When the Zr content was increased a nanocomposite of a solid solution and an intermetallic was obtained with considerable improvement in terms of structural stability and hardness. The presence of an oxide phase at 35% Zr might be responsible for the increased hardness in this particular alloy.     

The effect of heat treatment on the microstructure stability of modified 316LN stainless steel

As a conduit-sheath material for Cu–Nb–Sn wires, chemically modified 316LN steel is subjected to the same reaction heat treatment (100 h at 700 °C) used in transforming the wires into superconducting composite wires. In spite of the long annealing time at 700 °C, there was little or no change in the strength of the steel. A systematic study of the material annealed for 1, 10, 20, 50 and 100 h using orientation imaging microscopy (OIM) showed that with the exception of grain boundary precipitation at t = 100 h, the grain size and grain boundary character were stable. Our results show that twin boundaries (Σ3, Σ9, and Σ27) accounted for about 50% of the total boundaries in all the material conditions studied, suggestive that the density of twins had reached a limit. The stability of the material in spite of the prolonged heat treatment was attributed to the attainment of this maximum twin density in the as-received condition. In view of the high percentage of the twin boundaries in the microstructure, a comprehensive Hall–Petch relationship, which incorporates the contribution of the chemistry, grain size as well as twin boundaries to strengthening was developed. This upper bound theoretical strength compared favorably with the experimental value at 4 K.     

Plastic deformation of Fe–Al polycrystals strengthened with Zr-containing Laves phases: I. Microstructure of undeformed materials

The microstructures of several Fe-rich Fe–Al–Zr alloys have been studied as a basis of investigating the mechanical behaviour, which is subject of Part II. The alloys with only low Zr contents show microstructures with a relatively soft matrix and a hard skeleton along the grain boundaries, the latter being residual eutectics containing the matrix phase and the Zr(Fe,Al)₂ Laves phase. Scanning electron microscopy, orientation imaging microscopy as well as transmission electron microscopy and diffraction are used to study the grain sizes, the orientation relationships between the grains and the phases and the crystallography of the Laves phase. With higher Zr contents above about 10 at.%, the matrix is formed by the Zr(Fe,Al)₂ Laves phase.     

Mechanical spectroscopy in commercial Fe–6 wt.% Si alloys between 400 and 1000 K

Mechanical spectroscopy, neutron diffraction and differential scanning calorimetry (DSC) were performed on commercial Fe–6 wt.% Si alloy after quenching from high temperature. The damping spectrum shows a peak at around 800 K and an associated modulus defect. The modulus shows an increase during the second and subsequent heating runs. In addition, an anomaly in the modulus behavior has been found at around 400 K. Different thermal treatments allows to obtain two different recovery degrees of the quenched-in defects. The influence of the recovery degree on the 800 K internal friction peak and on the anelastic modulus has been evaluated and confirm the validity of the grain boundary mechanism associated to this peak. Experimental results are discussed on the basis of recovery and ordering processes.     

The effect of chemical grain refinement and low superheat pouring on the structure of NRC castings of aluminium alloy Al–7Si–0.4Mg

The effect of the addition of Al–5Ti–1B (wt.%) chemical grain refiners on the nuclei generation for a range of superheats during pouring in new rheocasting (NRC) of aluminium alloy Al–7Si–0.4Mg (wt.%) has been investigated. The contributions to the grain density by the grain refiner additions and impurity particles were quantified and it was found that the addition of grain refiner provides increasing number of nucleation sites as the superheat is decreased from 105 to 35 °C. However, at superheats of 15 °C, which are more typical of NRC, the grain density is similar in the alloy both with and without grain refiner additions. At this superheat, the equiaxed grain morphology is globular rather than dendritic and it is postulated that the grain size is dependant upon grain coarsening mechanisms rather than the number of heterogeneous nucleation events. In agreement with previous studies on semi-solid processing, it was found that the achievement of a fine globular grain structure led to a more homogeneous casting being produced. The mechanism of the macrosegregation observed in these castings is discussed and explained by the ‘sponge effect’.     

Effect of heat treatment on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy

In this study, effect of various aging tempers (T6, T73 and RRA treatment) on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy was studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII, are major precipitates for the alloy under T6 condition. No clear precipitation free zone was observed, and the grain boundary precipitates were continuous. Under two-step aging condition, the GP zones and η′ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa (about 7%) compared to the peak-aged condition. After retrogression and re-aging treatment, the grain boundary precipitates are discontinuous, which is closed to that resulting from T73 temper. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.     

Morphology of sputter deposited Al alloy films

Morphology of Al–2.0at%Ta and Al–2.0 at.% Nd alloy films before and after annealing was investigated for applications of interconnections for liquid crystal displays. It was found that the morphology and the microstructure of Al–2.0 at.% Nd alloy films changed markedly by annealing at the temperature region from 200°C to 300°C, while the morphology of Al–2.0 at.% Ta alloy films did not change by annealing up to 400°C. For the case of Al–2.0 at.% Nd alloy films, the incline of the <111> fiber texture to the substrate normal was observed during annealing. Structural characteristics of the Al films were investigated by TEM, SAD and XRD to determine the influence of alloying elements on the morphology and the fiber texture. From these results, it was concluded that the microstructures strongly influence the morphology and the grain orientation of Al alloy films.     

The influence of impurity level and tin addition on the ageing heat treatment of the 356 class alloy

The influence of the addition of 0.5 wt.% Sn to Al–7Si–0.3 Mg alloys (356 and A356) on their ageing behaviour and mechanical properties was evaluated. Adding Sn led to a reduction of the iron rich intermetallics volume fraction, and of hardness. During solution heat treatment, Mg went into the solid solution, and Sn particles grew by competitive growth, concentrating at phase boundaries and interfaces. During aging β″ and Si precipitated. In the alloys with Sn, the β″ precipitation was accelerated and its hardening effect was greater, whereas the Si precipitation did not changed significantly. The mechanical properties of the A356 alloy were compatible with the hardening achieved during the heat treatment and to the amount of defects (pores) present in the microstructure. The yield strength and elongation of the A356 + 0.5% Sn alloy decreased after solution heat treatment and with increasing ageing temperature. These detrimental effects were minimized by treating this alloy in the T5 condition at 150 °C.     

Grain boundary statistics in nano-structured iron produced by high pressure torsion

The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130 nm using a shear strain of γ = 210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries (Σ3–Σ45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes.     

Preferred orientations of primary Al4Sr dendrites in a rapidly solidified Al–Sr alloy

Preferred crystallographic orientations of primary Al₄Sr dendrites in a rapidly solidified Al–23 Sr (wt.%) alloy have been investigated using transmission electron microscopy (TEM). The Al₄Sr dendrites with 90° branches are dominant in the Al–23 Sr alloy melt-spun at 500 rpm and the dendrite orientation is the 110 direction. Wheel speed has a significant effect on the morphology and preferred orientation of the Al₄Sr dendrites in the melt-spun Al–23 Sr alloy.     

Microstructures in Advanced Materials Characterized by HREM and Nanoscale Analysis

Segregation of yttrium induces the formation of Y_0.25Zr_0.75O_2-x and Y_0.5Zr_0.5O_2-y microdomains, with L1₂- and L1₀-like ordered structures, in ZrO₂–6mol%Y₂O₃ ceramics in both the sintered and annealed states. The compositions of precipitates such as χ_L, χ_S, χ_SS, and small precipitates formed inside χ_L, in Cu–11.88Al–5.06Ni–1.63Mn–0.96Ti (wt.%) shape memory alloys have been determined. Under electron beam irradiation, four types of dynamic behavior of the G.P. zones were observed in the Al–6.58Zn–2.33Mg–2.40Cu (wt.%) alloy. The G.P. zone and “G.P. zone-like” defect structures were also distinguished. Lattice distortion profile in the GaAs/In_xGa_1-xAs superlattice and two-dimensional lattice distortion around a 60° dislocation core in the InAs_xP_1-x/InP superlattice were determined.     

Investigation of hydrogen induced cracking in 2205 duplex stainless steel in wet H2S environments after isothermal treatment at 675, 750 and 900 °C

The effect of isothermal treatment (at 675, 750 and 900 °C) on HIC (hydrogen induced cracking) in sour environments containing hydrogen sulphide of a 2205 duplex stainless steel has been investigated. The performance and microstructure of failed material were characterized by optical microscopy, scanning electron microscopy and also X-ray diffraction. Two kinds of Cr-, Mo-enriched intermetallic phases, σ and χ, were found to precipitate preferentially at /γ interfaces and within grains after different times of aging in the temperature range of 650–900 °C. After performing tests according to the NACE Standard TM 0284 (1987) the specimens were investigated by using quantitative metallography methods. The volume fraction of σ phase was changed with the time of aging and σ phase developed into coarse particles due to the high diffusibility of solute atoms at high temperatures. The variation of size and shape of σ phase particles was obtained by applying different heat treatment conditions to 2205 steel specimens. The results showed that 2205 duplex stainless steel containing nearly 12 vol.% of σ phase in dispersed conditions was resistant to step cracking in wet environments containing hydrogen sulphide. It was highly possible that a crack would propagate faster along the embrittled σ phase. However, very small cracks were found at austenite–ferrite boundaries where o phase particles were also present.