Effect of ceramic reinforcement on the ageing behaviour of an aluminium alloy

The effect of a 20vol % alumina microsphere particulate on the age-hardening characteristics of a 6061 Al matrix composite was investigated — based on microhardness, electrical resistivity and X-ray mapping — and the composite is compared to the unreinforced 6061 Al alloy. It is shown that this ceramic reinforcement can affect the age-hardening behaviour of the matrix alloy by significantly accelerating the kinetics of precipitation. This acceleration is related to a decrease in nucleation time and to an increase in the precipitate-growth rate. The relative amounts of age-hardened precipitates are also observed to be affected by reinforcement addition.     

Long-term ageing characteristics of Hastelloy alloy X

Transmission electron microscopy and diffraction were used to characterize the microstructural changes which occur in Hastelloy alloy X after long-term ageing (up to 16 000 h) at various temperatures in the range 540 to 870° C. The corresponding effects on mechanical properties were determined. It was found that the alloy age hardens at temperatures in the range of 650 to 870° C. Overageing occurred at 760 and 870° C. Marked reductions in room-temperature tensile elongation were observed after ageing at all the temperatures investigated. However, in no case was the elongation reduced to less than 15 to 30%. Also, the tensile elongation at temperatures corresponding to the respective ageing temperatures was unaffected by 16 000 h ageing at a temperature. It was concluded that the above effects were associated with precipitation of various phases, such as carbides, sigma- and mu-phases, in the matrix and at the grain boundaries.     

Substitutional dynamic strain ageing in an iron — 1.1 at. % niobium alloy

The mechanical properties of an Fe-1.1 at. % Nb alloy have been studied in compression over the temperature range 300 to 1100°K. The substitutional niobium atoms were responsible for dynamic strain ageing which resulted in a small peak in the temperature dependence of the flow stress, negative strain rate sensitivity, and serrated stress strain curves.The serrations were preceded by a strain rate and temperature dependent critical strain. These dependencies were analysed using theories that have been successfully applied to substitutional strain ageing in fcc structures. The analysis showed that, unlike in fcc structures, the apparent activation energy for the onset of serrated flow increased at the faster strain rates; this was attributed to the vacancies produced by the plastic deformation rapidly annealing out due to the high temperatures involved at the faster strain rates.     

The ageing characteristics of an Fe-19 wt% Ni-9 wt% Co-5 wt% W alloy

In the present paper, we compare the behaviour during ageing in the temperature range 380 to 530° C of the two martensitic alloys Fe-18.65 Ni-8.99 Co-4.87 Mo and Fe-18.65 Ni-8.99 Co-4.87 W (wt %). The kinetics of precipitation were followed by measurements of hardness, by resistometric measurements and electron microscopy. In the case of the alloy with Mo, Mo at first concentrates in local clusters, then an phase is formed, and above 450° C, finally a Ni₃Mo or Laves phase (Fe, Ni, Co)₂ Mo is formed. In the case of the alloy with W, phase formation was not observed but an intermetallic precipitate isomorphous with Ni₃Mo, (Fe, Ni, Co)₃ W = A₃W was found. The precipitates are rod-shaped, of several nm in length. The preferred direction of growth of these precipitates is 1 1 1 of the martensitic matrix and the orientation relationship with the martensitic matrix is (0 1 1)_M (0 1 0)A₃W and [1 1 1]_M [1 0 0]A₃W, identical to that found for the Ni₃Mo precipitate in the (Fe, Ni, Co, Mo, Ti) industrial maraging steel.     

On the extraordinary low quench sensitivity of an AlZnMg alloy

The scope of this work was to investigate the quench sensitivity of a high-purity wrought aluminum alloy Al6Zn0.75 Mg (in this work called 7003_pure). This is compared to a similar alloy with the additions of Fe, Si, and Zr at a sum less than 0.3 at.% (in this work called 7003_Fe,Si,Zr). Differential scanning calorimetry (DSC) was used for an in situ analysis of quench induced precipitation in a wide range of cooling rates varying between 0.0003 and 3 K/s. In 7003_pure, three main precipitation reactions were observed during cooling, a medium temperature reaction with a distinct double peak between 325 and 175 °C and a very low temperature reaction starting at about 100 °C. An additional high temperature reaction related to the precipitation of Mg₂Si starting at 425 °C has been observed for 7003_Fe,Si,Zr. In terms of hardness after natural as well as artificial aging, alloy 7003_pure shows a very low quench sensitivity. Hardness values on the saturation level of about 120 HV1 are seen down to cooling rates of 0.003 K/s. The as-quenched hardness (5 min of natural aging) shows a maximum at a cooling rate of 0.003 K/s, while slower and faster cooling results in a lower hardness. In terms of hardness after aging, 0.003 K/s could be defined as the technological critical cooling rate, which is much higher for 7003_Fe,Si,Zr (0.3–1 K/s). The physical critical cooling rates for the suppression of any precipitation during cooling were found to be about 10 K/s for both variants.

     

Natural ageing behaviour of cast alumina particle-reinforced 2618 aluminium alloy

The effect of the presence of 10 and 15 vol% alumina particles on the natural ageing behaviour of cast 2618 aluminium alloy was investigated using microhardness measurements and differential scanning calorimetry. It was found that the addition of the alumina particles does not alter the ageing sequence of 2618 AI although certain aspects of the precipitation reactions are changed. In particular, the relative quantities of the various phases were changed by reinforcement addition. Increasing the alumina content decreased the volume fractions of the Guinier-Preston-Bagaryatskii (GPB) I phases. Also, the peak reaction temperature, (T _p), for the GPBII and S phases decreased with increasing volume fraction of alumina.     

Li含量对Al-Mg-Si合金时效析出行为的影响

采用透射电镜研究Li含量对Al-3.0Mg-0.6Si合金时效析出行为的影响,分析不同Li含量合金在170℃人工时效过程中析出相的演化与分布.结果表明:Li元素的添加改变了Al-3.0Mg-0.6Si合金的时效析出行为.添加2.15％(质量分数,下同)Li的合金,球状的δ'-Al3Li相成为晶内优先析出相,而针状的β″-Mg2Si相在时效后期阶段才逐渐析出,并在时效192 h后形成了明显的球状δ'相加针状β″相的双相析出.而添加3.12％Li的合金在所观察的时效阶段内均未发现有β″相析出.另外,未发现含Li合金晶界位置有析出相存在,晶界附近是无沉淀析出带(δ'-PFZs)和δ'相.在时效100 h后发现添加2.15％Li的合金在δ'-PFZ区域有针状β″相析出.添加2.15％Li的合金δ'-PFZ的半宽尺寸和增长速率均大于2.68％,3.12％Li添加量的合金.     

Influence of thermomechanical ageing on tensile properties of 2014 aluminium alloy

2014 aluminium alloy was subjected to various thermomechanical ageing (TMA) treatments which included partial peak ageing (25% and 50%), warm rolling (10% and 20%) and further ageing to peak hardness level at 160 ° C. The tensile tests reveal that TMA treatments cause a substantial improvement in tensile properties and thermal stability. The electron microscopic studies reveal that the TMA treatments affect substantially the ageing characteristics. The TMA Ib treatment yields the finest needles having longitudinal dimensions of 40nm. The TMA treatments also lead to precipitate-dislocation networks of different densities. It is observed that TMA IIb treatment results in the densest precipitate-dislocation tangles of all the TMA treatments. As a result, a significant improvement in the tensile properties of 2014 aluminium alloy has been observed.     

Effect of ageing on martensitic transformation in NiTi shape memory alloy

The shape memory effect in Ni-49 Ti alloy after solution treatment at 800° C for 0.5 h and ageing at 400, 500, 600and 700° C for 1 h has been investigated by means of transmission electron microscopy and differential thermal analysis (DTA) measurement. Ageing at 400 and 500° C for 1 h causes the precipitation of characteristic temperatures towards higher values. In sample aged at 500° C three distinct DTA peaks arise giving evidence of intermediate stages of the martensite transformation.     

Microstructure and properties of supersaturated ZA27 alloy during natural ageing

The decomposition of the supersaturated solid solution of ZA27 alloy at room temperature has been investigated by X-ray diffraction, TEM and mechanical properties testing. Based on the results obtained, both continuous precipitation and cellular reaction occur during the decomposition process. The continuous precipitation follows the sequence: ₁ + spherical GP zones ₂ + elliptical GP zones ₃ + R + . The cellular reaction can be written: + + . The properties of the alloy depend on the microstructure. After 1 month of ageing, a series of changes of microstructure have taken place. The properties of the alloy are: _b=500 MPa, =13%,H _v=148.     

Effect of dynamic strain ageing on the deformation behavior of Incoloy alloy MA956

Incoloy alloy MA956 is an oxide dispersion hardened ferritic stainless steel produced by powder metallurgy. It is used as a candidate material for the high temperature components of gas turbines. This material underwent dynamic strain ageing at 300–400°C and strain rate of 1.2 × 10⁻³ s⁻¹. The following features of dynamic strain aging were observed: serrated flow at 300 and 400°C, a peak in the ultimate tensile strength normalized by the elastic modules versus temperature curve at 400°C, a plateau in the 0.2% offset yield stress-temperature curve at 300–400°C, a peak in the deformation rate-temperature curve at 300°C and the elongation-temperature plot showed a minimum at 400°C associated with shear fracture and with a minimum in the reduction in area-temperature plot. These features of dynamic strain ageing were discussed in the view of the recent models of dynamic strain ageing. The effect of dynamic strain ageing on the deformation and fracture behavior of this material was discussed.     

Comparison of the ageing behaviour of PM 2124 Al alloy and Al-SiCp metal-matrix composite

The ageing response of 2124 Al-SiC particulate metal-matrix composite (MMC) and unreinforced alloy has been examined using hardness measurements and Arrhenius analysis. The formation of phases during precipitation has been studied using differential scanning calorimetry (DSC). The MMC exhibits accelerated ageing compared to unreinforced alloy, due to enhanced S formation. The activation energy for diffusion is lower in the MMC than in the unreinforced alloy. DSC scans show Guinier-Preston B (GPB) zone nucleation to occur at a lower temperature in the MMC, whilst the total volume of GPB zones formed is smaller than in the unreinforced alloy. A model has been proposed to explain the GPB zone formation behaviour, in which ease of GPB zone nucleation varies within the MMC, as a function of ageing time and of position within the matrix. S formation is enhanced in the MMC because of improved diffusion and a large increase in density of heterogeneous nucleation sites compared to the unreinforced alloy.     

高强度Fe-Ni膨胀合金丝材时效工艺研究

采用性能测试和显微组织分析手段，探讨了高强度低膨胀Fe—Ni合金丝材时效处理对合金膨胀系数及强度的影响规律。试验结果表明，采用较高温度时效（475℃），能够使碳化物能够呈弥散状析出，将有利于合金膨胀系数的控制，及合金强度的提高；而较低的时效温度（425℃）下，碳化物呈不均匀状析出，合金强化效果不明显，同时不利于膨胀系数的控制。     

The metallography of a cobalt-based implant alloy after solution treatment and ageing

The ageing characteristics of a commercial Co-Cr-Mo-C alloy after solution treatment were investigated using optical and electron microscopy. An M₂₃C₆-type carbide was identified by X-ray and electron diffraction after ageing treatments between 650 and 1150° C. Nucleation and growth of this carbide took place on intrinsic stacking faults by Suzuki segregation in the cobalt matrix. High stacking-fault densities gave rise to intragranular striations which were visible after etching once precipitation had occurred. Ageing temperatures of 925° C and above increased the stability of the f c c cobalt matrix and led to precipitation on undissociated dislocations. Grain-boundary carbides were evident at all ageing temperatures.