By T. Pejot, J. Barlier, L. Bourgeon, F. De Geuser, A. Deschamps
In recent times, there has been a burgeoning interest in the development of compositionally graded alloys using diffusion couples. These innovative specimens serve as crucial tools for implementing combinatorial approaches in metallurgy, particularly in quantifying the impact of alloying elements on precipitation kinetics and mechanical properties during the ageing process. In the context of this study, such specimens have been meticulously crafted within the realm of 7xxx alloys. Moreover, graded samples have been fashioned to encompass variations in pre-straining and quench rates, notably through the Jominy test. These diverse gradients, coupled with judicious high throughput characterization methods, present an invaluable opportunity to evaluate and prognosticate the microstructure and mechanical properties of 7xxx alloys across a broad spectrum of composition and processing parameters.
The strengthening mechanism in 7xxx alloys hinges significantly on the precipitation of the nanoscale 𝜂-𝑀𝑔(𝑍𝑛, 𝐶𝑢)2 phase and its precursors. Small-Angle X-ray Scattering (SAXS) emerges as a pivotal technique, offering precise quantification of their volume fraction and radius. What sets this investigation apart is the spatially and time-resolved SAXS characterization performed on these graded samples, enabling an in-depth understanding of microstructural evolution.
Introduction
This article sheds light on the results gleaned from diffusion couples subjected to hot compression, followed by an interdiffusion heat treatment. Noteworthy specimens with a gradient in quench rate were crafted using the Jominy test, facilitating a continuous quenching rate from 100 to 1°C/s. Similarly, specimens with a gradient of pre-strain were fashioned through tensile samples, yielding a continuous variation of pre-strain from 0 to 15%. Post-solution treatment, these graded samples underwent ageing to several conditions and were meticulously characterized through spatially and time-resolved SAXS.
Experimental Procedures
The composition gradient was established using two base materials with bulk compositions of 2.8at% and 4.5at% of zinc, respectively, both augmented with 0.7at% of Copper and 2.5at% of Magnesium. These materials were meticulously cut into cubes and polished down to 1μm. Subsequently, the two specimens were subjected to moderate compression heating up to 450°C in a secondary vacuum to avert oxidation. A deformation of 30% ensued, followed by cooling down to room temperature. The specimen was then heat-treated at 490°C to activate diffusion across the interface. Following a 16-day period, the specimen was water-quenched to trap alloying elements in solid solution. Post-diffusion treatment, Zinc content was meticulously measured using EPMA.
The gradient of quench rates was achieved using the Jominy test equipped with thermocouples. A simulation of the test was carried out, showcasing the average cooling rate between 400 and 200°C.
The gradient of pre-strain was achieved using a specimen with a trapezoidal section. The strain is measured using 5millions pixels camera and GOM ARAMIS software.
Results and Discussion
To maintain brevity, we shall focus on the evolution of precipitates during ageing to attain the peak aged temper (24h 120°C) concerning quenching rate.
The evolution of mean radius during ageing at 120°C for various quenching rates depicts interesting trends. While the quenching rate does have a marginal effect on precipitate size, with low quenching rates yielding larger radii, the variation between the extremes of the gradient remains less than 1Å.
Conversely, volume fraction emerges as significantly dependent on the quenching rate, resulting in a marked effect on alloy hardness.
These findings, while in line with prior knowledge, present a novel perspective owing to the continuous measurements obtained. For instance, the hardness measurements on the quench-graded specimen for various ageing times highlight a critical cooling rate of 8°C per second for quench-induced precipitation.
In the quest for a deeper understanding, the link between microstructure and hardness for the quench-graded specimen was meticulously examined. The extensive dataset underscores the need for a multi-parametric approach to predict hardness accurately, with the size of the precipitate-free zone emerging as a potentially influential factor.
In conclusion, the study of graded specimens holds immense promise in characterizing precipitation in aluminium alloys. The continuous measurements provide an avenue to observe changes in mechanism with a continuous processing variable. When combined with high throughput characterization techniques, these graded specimens pave the way for robust models for microstructure evolution and strengthening, thus offering a holistic view applicable to a diverse alloy design space.
