Two-stage double peaks ageing and its effect on stress corrosion cracking susceptibility of Al-Zn-Mg alloy

Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking(SCC) susceptibility of Al-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η' phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6%(33.2 MPa) and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. Al-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates(MPts), coarsened and discontinuous grain boundary precipitates(GBPs), as well as the narrow precipitate free zone(PFZ) in the second peak ageing condition.     

Precipitation hardening in Al-Zn-Mg-Al2O3(p) composite

The precipitation hardening of a Al-Zn-Mg-Al₂O₃(p) composite is explored. It is found that the peak hardness achieved is almost double that of precipitation hardening of Al-Zn-Mg alloy or dispersion strengthening of Al-Zn-Mg with 5% Al₂O₃(p). Toughness is marginally improved and tensile strength is one and half times that of precipitation hardened Al-Zn-Mg alloys. The ageing time for peak hardness is reduced due to acceleration of formation of precipitate.     

Coarsening kinetics of coherent γ′ precipitates in ternary Ni–based alloys: the Ni–Al–Si system

Coarsening of coherent precipitates γ’ in alloys such as Ni-based alloys has been studied extensively not only for its practical significance in the design of engineering alloys but also in an effort to understand the phe omenon of coarsening. However, a complete understanding of the role of the multiple factors that can affect the coarsening kinetics in such systems i still lacking. Although some advances have been made through computer simulations, studying experimentally the influence o the volume fraction of the second phase and that of coherency strains on the kinetics of coarsening has been particularly challenging.

This paper will highlight some of the issues that are relevant to the study of coarsening in multi–component alloy systems. Recent results obtained for the kinetics of coarsening of γ’ precipitates in different alloys within the Ni-Al-Si system will be presented. Compositions of these alloys have been chosen so as to vary both the magnitude and the sign of coherency strains between the precipitate and the matrix. Some anomalies related to the composition dependence of the kinetics of coarsening will be highlighted. This paper will conclude with a discussion on the role of the volume fraction of the second phase and coherency strains in contributing to these anomalies and identify directions for future work.     

Coarsening kinetics of coherent γ′ precipitates in ternary Ni-based alloys: the Ni–Al–Si system

Coarsening of coherent precipitates γ' in alloys such as Ni-based alloys has been studied extensively not only for its practical significance in the design of engineering alloys but also in an effort to understand the phenomenon of coarsening. However, a complete understanding of the role of the multiple factors that can affect the coarsening kinetics in such systems is still lacking. Although some advances have been made through computer simulations, studying experimentally the influence of the volume fraction of the second phase and that of coherency strains on the kinetics of coarsening has been particularly challenging.This paper will highlight some of the issues that are relevant to the study of coarsening in multi-component alloy systems. Recent results obtained for the kinetics of coarsening of γ' precipitates in different alloys within the Ni-Al-Si system will be presented. Compositions of these alloys have been chosen so as to vary both the magnitude and the sign of coherency strains between the precipitate and the matrix. Some anomalies related to the composition dependence of the kinetics of coarsening will be highlighted. This paper will conclude with a discussion on the role of the volume fraction of the second phase and coherency strains in contributing to these anomalies and identify directions for future work.     

Initial clustering after quenching in AlZnMg alloys

After solution treatment and quenching a fast increase in hardness takes place in Al-Zn-Mg alloys in the first few minutes of ageing at or near room temperature. The kinetics of this process have been studied on a number of different compostition alloys by microhardness measurements between 246 and 295 K and by resistivity measurements between 263 and 323 K. On the basis of the observed microhardness and resistivity kinetics the activation energy of the process was determined. According to this the ageing process starts with the diffusion of zinc atoms to magnesium atoms and Guinier-Preston zones develop from the so formed Mg-Zn clusters. The average Mg/Zn ratio in the clusters is 1/4. In the alloys investigated the microhardness after 15 min ageing at room temperature can be composed simply of the contributions to the hardness of pure aluminium, MgZn₄ clusters and of magnesium atoms remaining in solid solution.     

时效对Mg-Y-Nd合金的影响

研究了Mg-Y-Nd合金时效过程中的硬化软化现象．在时效初期(-473K／2h左右)出现了一次明显的时效硬化峰，之后硬度急剧下降；然后合金的硬度缓慢增加，出现小幅二次硬化现象；523K／600h后，合金出现软化现象微观结构的分析结果表明，初次时效峰是合金中析出了5nm大小的MgY弥散分布沉淀相所致；随后弥散相溶解消失，材料出现软化；在时效过程中β沉淀相的析出、增多和板条状组织的形成导致材料二次硬化．长时间时效后β沉淀相的聚集长大，以及在α／β界面产生纳米晶MgO区导致材料再次软化。     

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.     

人工时效对激光选区熔化AlMg4.5Sc0.55Mn0.5Zr0.2合金显微组织和力学性能的影响

采用激光选区熔化制备AlMg_4.5Sc_0.55Mn_0.5Zr_0.2合金,研究人工时效工艺参数对合金维氏硬度的影响规律,分析沉积态和优选时效态合金的室温拉伸性能和显微组织。结果表明：人工时效使该合金的维氏硬度由102HV提升至140HV以上。随着时效温度升高（305~335℃）或时效时间延长（1.5~48 h）,维氏硬度呈现先增加、再降低、最后逐渐趋于稳定的规律。在315℃时效3 h或12 h后,合金的室温拉伸性能基本相当,无明显的各向异性;抗拉强度和屈服强度分别达到470 MPa和410 MPa,断后伸长率保持在15.0%。力学性能的提升得益于人工时效过程中弥散析出且与基体共格的纳米增强颗粒Al₃（Sc,Zr）。     

微量Sc对Al-Zn-Mg合金焊接接头组织和性能的影响

使用光学显微镜、扫描电镜和透射电镜分析了Al-Zn-Mg合金和含微量钪的Al-Zn-Mg合金钨极氩弧焊接头的微观组织,并对其力学性能和耐应力腐蚀性能进行了对比。结果表明：在传统Al-Zn-Mg合金板材熔合线附近的热影响区出现再结晶和晶粒异常长大,而含钪Al-Zn-Mg合金基体中热稳定性优良的纳米Al₃(Sc, Zr, Ti)相在焊接过程中能阻碍晶界迁移,抑制再结晶晶粒的形核和长大,进而细化熔合线附近的组织。同时,含微量钪的Al-Zn-Mg合金焊接接头的强度明显比传统合金的高,其强化效果主要来源于熔合线附近区域的细晶强化和二次Al₃(Sc, Zr, Ti)相的弥散强化。     

A study on the nickel-rich ternary Ti–Ni–Al shape memory alloys

The transformation sequence and hardening effects of 400 °C aged Ti47.5Ni50.65Al1.85 and Ti49.5Ni50.13Al0.37 shape memory alloys have been investigated by electrical resistivity tests, internal friction measurements, hardness tests and TEM observations. Both solution hardening and precipitation hardening are found to occur in these alloys. The hardening effects of Ti47.5Ni50.65Al1.85 alloy are obvious and much higher than those of Ti49.5Ni50.13Al0.37 alloy due to the former having the larger Ni/Ti ratio and a higher Al solute content in its matrix. The transformation sequence of 400 °C aged Ti47.5Ni50.65Al1.85 alloy shows B2↔R-phase only for an ageing time of more than 10 h and that of 400°C aged Ti49.5Ni50.13Al0.37 alloy shows the sequence B2↔R-phase↔B19′ or B2↔R-phase with different ageing times. All of these characteristics are associated with Ti11Ni14 precipitates during the ageing process. These aged Ti–Ni–Al alloys exhibit very good shape memory effects, in which the maximal shape recovery occurs at the peak of hardness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Role of diffusional coherency strain theory in the discontinuous precipitation in Mg-Al alloy

Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. It is called discontinuous precipitation because precipitation occurs on prior matrix grain boundaries followed by grain boundary movement. The DP nodule consists of alternate lamellae of the precipitate and the matrix respectively. The chemical driving force for DP is one of solute supersaturation. Although solute supersaturation is responsible for precipitation, it has to be coupled with another driving force to explain grain boundary migration. This coupling driving force has been identified to be diffusional coherency strain which has been verified to be active in diffusion induced grain boundary migration and liquid film migration. To test diffusional coherency strain theory for discontinuous precipitation Mg-7Al and Mg-7Al-1Pb alloys were studied. While the fraction transformed was high at 6% in Mg-7Al alloy, dit was significantly low at 2% in Mg-7Al-1Pb alloy. The velocity of DP nodules decreased by half in alloy with Pb as compared to the alloy without Pb. Theoretical calculations also predict that the misfit parameter δ_th decreases with the addition of Pb. These observations are an evidence to the fact that diffusional coherency strain is the most active driving force for the movement of the grain boundaries of the DP nodules during discontinuous precipitation in Mg-Al alloy.     

Studies on age-hardenable aluminium alloys by atom-probe field-ion microscope

Field-ion microscopy connected with the successive field-evaporation technique and the atom-probe analysis has been applied to studying precipitation processes, especially the early stage of precipitation, in age-hardenable aluminium alloys, such as Al-Cu, Al-Ag and Al-Sc alloys. In Al-Cu alloys, coexistence of the single-layer G.P.(1) zones and the multilayer G.P.(1) zones has been confirmed and the difference between G.P.(1) zones and G.P.(2) zones has been clearly recognized. In Al-Ag alloys, the octahedral shape of η-G.P. zones has been confirmed. In Al-Sc alloys, it has been found that the equilibrium Al₃Sc phase particles precipitate homogeneously from the beginning without any preprecipitation stage and coherency between the precipitate and the matrix is maintained even in an alloy aged at relatively higher temperature for prolonged periods of time.     

Age-hardening characteristics of aluminium–chromium alloys

The age-hardening behaviour of Al–Cr alloys containing varying amounts of chromium has been studied, and the effect of silicon on the ageing response of the alloy system was also investigated. Hardness measurement and tensile properties evaluation were used to describe the ageing response of the alloys. With the aid of microstructural observation, the ageing mechanism has been explained. Results of isothermal ageing were used to describe the kinetics of precipitation in both the binary and ternary alloys. It was noticed that the age hardening in aluminium–chromium alloys was mainly due to the precipitation of intermetallics on to the dislocations. The ageing response of the binary alloy containing chromium in excess of its solubility limit was rather poor. However, addition of silicon helped to improve the ageing response of the alloys. Silicon was found to stimulate nucleation of ageing precipitates in the investigated alloys. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile properties and thermal expansion of discontinuously reinforced aluminium composites at subambient temperatures

The effects of temperature on the mechanical properties and thermal expansion of two discontinuously reinforced aluminium composites have been determined over the range 300–100 K. Silicon carbide particulate-reinforced 2009 and 6092 aluminium alloys were studied by tensile testing, in which both longitudinal and transverse strains were recorded, and by thermal expansion measurements. The test results clearly show that cooling to 100 K induces plastic flow in the aluminium alloy matrices due to the thermal expansion difference between aluminium and silicon carbide. At very low temperatures, the linear region of the stress-strain curve is greatly reduced or eliminated and the Poisson’s ratio, ν, increases. For the higher yield strength 2009 matrix composite, ν increases from a room-temperature value of 0.28 to 0.35 at 100 K. For the lower-strength 6029 matrix composite, ν increases from a room-temperature value of 0.33 to a value of 0.5 at 100 K. A Poisson’s ratio of 0.5 is the value characteristic of plastic flow in an incompressable material. Changes in yield strength, Young’s modulus and thermal expansion with decreasing temperature are also consistent with thermally induced plastic flow in the composite matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

Yield stress of SiC reinforced aluminum alloy composites

This article develops a constitutive model for the yield stress of SiC reinforced aluminum alloy composites based on the modified shear lag model, Eshelby’s equivalent inclusion approach, and Weibull statistics. The SiC particle debonding and cracking during deformation have been incorporated into the model. It has been shown that the yield stress of the composites increases as the volume fraction and aspect ratio of the SiC particles increase, while it decreases as the size of the SiC particles increases. Four types of aluminum alloys, including pure aluminum, Al–Mg–Si alloy, Al–Cu–Mg alloy, and Al–Zn–Mg alloy, have been chosen as the matrix materials to verify the model accuracy. The comparisons between the model predictions and the experimental counterparts indicate that the present model predictions agree much better with the experimental data than the traditional modified shear lag model predictions. The present model indicates that particle failure has important effect on the yield stress of the SiC reinforced aluminum alloy composites.     

Work-hardening behaviour of a heat-treatable AA7108 aluminium alloy deformed to intermediate strains by compression

This work is an experimental study of ageing and work-hardening considering various heat treatments of an AA7108 aluminium alloy in the as-cast and homogenized condition. Specimens have been exposed first to a solution heat treatment and a then to a two-step age-hardening to obtain different tempers. These tempers have been tested in an upsetting test applying state-of-the-art cold lubrication allowing intermediate strains to be reached before the onset of barrelling. The measured work-hardening response has been analyzed by fitting an extended Voce equation to the experimental stress–strain curves. With increasing overageing the stage III part of the stress–strain curve shrinks, and for the most overaged condition studied here, the stress–strain curve has reached a linear stage IV already at a strain of about 0.3. Interestingly, the slope of the stage IV curve then is lower than commonly reported for Al alloys. The microstructure and texture of the deformed material have been further investigated for a selection of tempers, and their influence on the work-hardening behaviour has been discussed.     

Discontinuous precipitation in copper base alloys

Discontinuous precipitation (DP) is associated with grain boundary migration in the wake of which alternate plates of the precipitate and the depleted matrix form. Some copper base alloys show DP while others do not. In this paper the misfit strain parameter, η, has been calculated and predicted that if 100 η > ± 0·1, DP is observed. This criterion points to diffusional coherency strain theory to be the operative mechanism for DP.     

Al-Zn-Mg系铝合金的微合金化研究进展EI北大核心CSCD

Al-Zn-Mg系铝合金作为一种轻质高强合金在航空航天和交通等领域有着重要的应用。获得更高的力学性能以及更优的耐腐蚀性能是Al-Zn-Mg系合金的发展方向,因此需要进一步优化其微观组织。在合金成分和热处理制度调控空间有限的情况下,微合金化成为该合金性能改善的一种重要手段。本文简要总结了微合金化元素对Al-Zn-Mg系铝合金力学性能、热加工行为及耐腐蚀性能的影响,重点关注了微合金化元素在不同工艺阶段下形成的第二相颗粒能有效细化晶粒并强烈阻碍位错运动;讨论了热加工变形过程中钉扎晶界及亚晶界、抑制回复再结晶的作用;阐述了提高合金耐腐蚀性能方法的内在机理。最后对Al-Zn-Mg系铝合金微合金化的研究方向进行展望,深入理解微合金化元素间、主微合金元素间的相互作用机理,实现微合金化元素的精准、精确投放将是未来主要的研究内容之一。明确微合金化元素在热加工过程中对变形组织及位错组态的调控作用将对提高合金耐腐蚀性能提供借鉴。     

Ageing behaviour of SiCp-reinforced AA 7075 composites

The precipitation behaviour in 7075 aluminium alloy matrix composites reinforced with 0–40 vol% particulate SiCp (12.5 μm) was studied using macrohardness (HV) measurements and differential scanning calorimetry (DSC). In the low volume percentage (5,10) SiCp composites, the hardness-ageing curves and DSC scans are similar to those of the unreinforced alloy. However, the age-hardening quantities and DSC Gurnier-Preston (GP) zone peak size are smaller than those of the unreinforced alloy. Additionally, the high-temperature peaks (ageing curves at 200 °C or DSC scanning curves) are broader. In the high volume percentage (20, 30, 40) SiCp composites, the hardness-ageing curves and DSC scans are very different from those of the unreinforced alloys. Only a high-temperature broad peak was observed during the DSC scanning. On the hardness-ageing curves no hardening phenomena were detected, but rather a softening phenomenon occurred in the 30% or 40% SiCp composites, suggesting that during ageing an exothermic dislocation recovery softening process coexists with precipitation hardening. A model was introduced by dividing the matrix of the composite into Region I (normal precipitation) and Region II (particular precipitation). The precipitation of GP zones is completely suppressed and the precipitation of η′ phase is accelerated in Region II. The matrix of the low volume fraction SiCp composite comprises Regions I and II, whereas that of the high volume fraction SiCp composite comprises only Region II. The ageing behaviour and DSC scans of the composites can be fully explained by this model. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hydrogen bubbles in embrittled Al-Zn-Mg alloys

Al-Zn-Mg alloys become embrittled during exposure to moist environments due to hydrogen penetration of grain boundaries. The result of this hydrogen penetration due to surface reaction with water vapour of both bulk specimens and electron-transparent thin foils, has been studied at high resolution in the JEM 100 C transmission electron microscope as a function of alloy composition and ageing treatment. In bulk specimens of alloys solution-heated, water-quenched, and aged in water-vapour-saturated air at 70° C, the hydrogen is in the form of a mobile atomic species which is transformed to bubbles of molecular hydrogen under the action of the electron beam. However, in electron-transparent specimens of aged alloys after exposure to water vapour the accumulated hydrogen is observed directly as bubbles. These bubbles take the form of hexagonal lenses bounded by {111} planes, and are associated with grain-boundary precipitates, particularly in over-aged microstructures, and with primary intermetallic particles in alloys containing sparingly soluble transition elements. The consequence of the observed hydrogen penetration of grain boundaries in promoting environmental debilitation of mechanical properties and stress-corrosion cracking of Al-Zn-Mg alloys is discussed.