Kai Wen

Effect of Various Retrogression Regimes on Aging Behavior and Precipitates Characterization of a High Zn-Containing Al–Zn–Mg–Cu Alloy

In the present work, the influence of various retrogression treatments on hardness, electrical conductivity and mechanical properties of a high Zn-containing Al–Zn–Mg–Cu alloy is investigated and several retrogression regimes subjected to a same strength level are proposed. The precipitates are qualitatively investigated by means of transmission electron microscopy (TEM) and high-resolution transmission electron microscopy techniques. Based on the matrix precipitate observations, the distributions of precipitate size and nearest inter-precipitate distance are extracted from bright-field TEM images projected along 〈110〉Al orientation with the aid of an imaging analysis and an arithmetic method. The results show that GP zones and η′ precipitates are the major precipitates and the precipitate size and its distribution range continuously enlarge with the retrogression regime expands to an extent of high temperature. The nearest inter-precipitate distance ranges obtained are quite the same and the average distance of nearest inter-precipitates show a slight increase. The influence of precipitates on mechanical properties is discussed through the interaction relationship between precipitates and dislocations.

Single-stage aging behaviour and precipitate evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy

ABSTRACT Property evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy treated at 120°C, with the microstructure subjected to under-aging (UA), peak-aging (PA) and over-aging (OA), is investigated. Precipitate size distributions and distances between neighbouring precipitates are determined. Results indicate that a certain time is required to reach peak hardness and yield strength, and that peak values can be sustained for a relatively long time. As the aging time increases, the conductivities increase persistently. As the alloy temper varies from UA to PA to OA, the main matrix precipitates change from ‘GPI zone and GPII zone and η′ phase’ to ‘GPII zone and η′ phase’ and then to η′ phase. Meanwhile, the precipitate size distribution becomes broader, and the average precipitate size increases.

Single Stage Ageing Property and Precipitation Variation of a High-Zinc Al–Zn–Mg–Cu Alloy

In order to analyze single stage ageing behavior of a high-zinc Al–Zn–Mg–Cu alloy, the microstructure and property variations of the alloy treated by various temperatures were explored by hardness, conductivity, tensile properties, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) techniques. The results indicated that a long time was needed to reach a peak hardness value for the alloy treated at 100 and 120 °C and the peak hardness value could sustain a relatively long time interval. The alloy treated at 120 °C adopted a shorter time to reach peak hardness values and its peak hardness value was larger compared with that at 120 °C. The hardness treated at 140 °C reached the peak rapidly and then diminished continuously. With ageing time prolonging, the conductivities of the alloy treated at various temperatures increase persistently. After a peak ageing regime of 120 °C/24 h, the alloy possessed fine and dispersedly distributed matrix precipitates (MPs) and the ultimate tensile strength (UTS), yield strength (YS), elongation and electrical conductivity were 697 MPa, 655 MPa, 12.9% and 17.3 MS m−1, respectively. With the alloy temper varying from under-ageing to peak-ageing to over-ageing, the variety of MPs changed from GPI zone, GPII zone and η′ phase to GPII zone and η′ phase to η′ phase. During this process, the size distribution of MPs became broader and average precipitate size became larger.

Quantitative Investigation of Precipitates in a High-Zinc Al–9.54Zn–2.10Mg–1.69Cu Alloy with Various Typical Tempers

In order to analyze single stage ageing behavior of a high-zinc Al–9.54Zn–2.10Mg–1.69Cu alloy, the microstructure of the alloy subjected to T6, T76 and T77 states are investigated via transmission electron microscopy (TEM) combined with high-resolution transmission electron microscopy (HRTEM) attached to it. Under the premise in precipitate observations, diameter distributions and average diameter size of precipitates are deduced from Bright-Field TEM (BF TEM) images projected along \( \left\langle {110} \right\rangle_{\text{Al}} \) orientation with the help of an image processing. The results indicate that the main precipitates are GPII zone and η′ phase in the T6 and T77 alloys while η′ and η phase in the T74 alloy. The Bright field TEM observations reveal that the matrix precipitates for the T6 and T77 alloys have small size and dispersive distribution while that for the T74 alloy has big size and sparse distribution. Quantitative precipitate characteristics including diameter distribution and average diameter size have been gained by an image processing relying on BF TEM images projected along \( \left\langle {110} \right\rangle_{\text{Al}} \) orientation. The results reveal that the T6 and T77 alloys have more than a half percentage of precipitates with a size less than 2 nm while the T77 and T74 alloys have broad precipitate distribution range till 14 and 16 nm, respectively. The grain boundary precipitates (GBPs) for the T6 alloy have continuous distribution with small size while that for the T74 and T77 alloys distribute intermittently with big size.

Effect of Solution Treatment on Microstructure and Mechanical Properties in an Al-9.3Zn-2.0Mg-1.8Cu Alloy

The microstructure solution treated by various temperatures of 2h in as-extruded Al-9.3Zn-2.0Mg-1.8Cu alloy was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analysis. The mechanical properties treated at 465oC for various times were tested by room temperature tensile mechanical properties test. The results indicated that second phase of the as-extruded alloy mainly consists of Mg (Zn,Cu,Al)2 and Fe-rich phases. Mg (Zn,Cu,Al)2 phase completely dissolved into the matrix solution treated at 465oC or higher for 2h while residual phase was mainly Fe-rich phase. The mechanical properties treated at 465oC for various time were tested and optimized solution treatment parameter was chosen as 465°C/1.5h.

Research on Microstructure Evolution in Al-9.8Zn-2.0Mg-1.8Cu Alloy during Solution Treatment

The microstructure of as-extruded Al-9.8Zn-2.0Mg-1.8Cu aluminum alloy and its evolution during solution treatment were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC) analysis and electron back-scatter diffraction (EBSD). The results indicated that second phase of the as-extruded alloy mainly consisted of Mg (Zn, Cu, Al)2 and Fe-rich phases. After solution treated at 475°C for 4h, Mg (Zn, Cu, Al)2 phases were dissolved into the matrix, while Fe-rich phases still existed. Fe-rich phases cannot dissolve by prolonging solution time. The room temperature tensile strength gradually increased by prolonging solution time at 475oC. The ultimate tensile strength of the alloy reached 700MPa after both single and two-step solution treatments.