MECHANICAL AND ELECTRICAL PROPERTIES OF CAST Al–Er–Zr ALLOY

Mechanical and electrical properties of an Al–Er–Zr alloy were investigated. Microhardness and resistometry measurements were supplemented by transmission electron microscopy and scanning electron microscopy. Resistivity decreases during isochronal annealing take place within the following temperature intervals: 150– 270 °C, 300–420 °C and 450–570 °C. The isochronal microhardness curve reflects hardening effects at corresponding temperatures. However, no thermal effect was observed in the DSC curves ranging from 25 °C to 600 °C. A transmission electron microscopy observation of the alloy studied isochronally annealed up to 390° C revealed a high density of L1 2 -structured particles, some of which had nucleated on dislocations. The distinct electrical resistivity, as well as microhardness, changes are in all cases connected with the precipitation and evolution of the Al 3 Er, Al 3 Zr and/or Al 3 (Er,Zr) particles.


INTRODUCTION
An addition of Sc to Al alloys increases drastically the strength of an Al alloy due to the formation of a high number density of L12-structured Al3Sc precipitates coherent with the aluminum matrix [1,2]. Further addition of a trace amount of Zr leads to formation of ternary L12-Al3(Sc,Zr) precipitates. These ternary particles have both thermodynamic and kinetic tendencies to form a core-shell structure [3][4][5][6]. The resulting Zr-rich shell can retard the fast diffusing Sc and therefore impede the coarsening of the precipitates [3,5,7].
However, the applications of Sc are restricted due to its high cost. Similarly to Sc, the heavy rare-earth element Er exhibits an Al-Al3Er eutectic reaction. Al3Er also exhibits a stable L12 structure [8]. Er has a larger diffusivity than Sc [9,10], and thus Al3Er precipitates nucleate and grow at low temperatures and therefore Al-Er alloys suffer from an early loss of strength [11]. The high matrix/precipitate interfacial energy in case of Al3Er [12] also indicates low coarsening resistance. Fortunately, it was confirmed that the co-alloying of Er and Zr in Al leads to the precipitation of L12-phase Al3(Er,Zr) particles. Li et al. [13] reported the core-shelled structure of Al3(Er,Zr) in a peak-aged Al-Er-Zr alloy. Just like in case of Al3(Sc,Zr), the shell rich in Zr can act as diffusional barrier enhancing the coarsening resistance of the precipitates [13].
Despite these findings, the Al-Er-Zr system has not been thoroughly studied and the description of nonhomogenised cast Al-Er-Zr alloys is missing. This article reports on the evolution of mechanical and electrical properties of a cast Al-Er-Zr alloy during isochronal annealing.

EXPERIMENTAL PROCEDURES
A ternary as-cast alloy with nominal composition of Al-0.25 Er-0.15 Zr (wt%) was studied. The samples were isochronally annealed from room temperature (RT) up to 570 °C in steps of 30 °C/30 min. Heat treatment was performed in a stirred silicone oil bath up to 240 °C (these steps were followed by quenching into liquid nitrogen) and in a furnace with an argon protective atmosphere above 240 °C (followed by quenching into water at RT).
Relative resistivity changes during the isochronal annealing were measured by DC four-point method with a dummy specimen in series at 77 K in liquid nitrogen bath.
Differential scanning calorimetry (DSC) was performed at heating rates of 5, 10 and 20 °C/min in Netzsch DSC 204 Phoenix apparatus. A specimen of mass ~10 mg was placed in Al2O3 crucibles. Nitrogen flowed at the rate of 40 ml/min as a protective atmosphere.
The microstructural development was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using TESCAN MIRA I LMH, equipped with an X-ray BRUKER microanalyser for energy-dispersive spectroscopy (EDS) measurements, and JEOL 2200FS, respectively. TEM foils were cut from the aged specimens, ground to about 100 μm and then electropolished with an electrolyte solution consisting of 30% nitric acid and 70% methanol at −25 °C. Figure 1 displays the response of the microhardness HV0.3 and relative resistivity changes of the Al-0.25 Er-0.15 Zr alloy to step-by-step isochronal annealing. The initial absolute value of resistivity was determined as 5.06±0.08 nΩ•m. Three distinct decreases of resistivity can be seen in the annealing curve. These occur within the following temperature intervals: 150-270 °C, 300-420 °C and 450-570 °C, the last decrease being the sharpest. The microhardness increases slightly during the first drop of resistivity and peaks at 360 °C (during the second one). The microhardness then drops at 420 °C before peaking again at 480 °C after which it reaches a plateau. The DSC measurements in heating rates of 5, 10 and 20 °C/min were done in the alloys in addition to resistivity and microhardness measurements. However, no thermal effect was observed in the DSC curves ranging from 25 °C to 600 °C.

RESULTS AND DISCUSSION
SEM micrograph of the alloy in the as-cast state (Figure 2) shows a high number density of Er-rich primary precipitates, as analyzed by EDS, indicating that the Er supersaturation in the Al matrix might be relatively low.
The eutectic is enriched in Er only as Al-Zr alloys undergo a peritectic transformation [14]. Only small density of dislocations in grain interiors in the as-state of the alloy was observed by TEM. Considering that the number of grain boundaries (millimetre-size grains, roughly estimated from SEM observations) is also very low and do not change significantly during the aging process, the dominating contributing factor to the evolution of electrical resistivity is the change of solute concentrations in the matrix.

Figure 2 SEM micrograph of the as-cast microstructure
In the available literature focusing on Al-Er-Zr ternary and Al-Er-Sc-Zr quaternary systems, Er is generally believed to precipitate first [12,13,[15][16][17][18], owing to its high diffusivity [10], despite its high critical nucleation radius and critical work of nucleation [12]. The sluggishly diffusing Zr was reported to first precipitate above 400 °C even at lower effective heating rates of isochronal annealing [15,16]. Therefore, the second microhardness peak and the resistivity changes above 450 °C can be ascribed to Zr.

Figure 3
shows dark-field TEM micrograph of the alloy isochronally annealed up to 390 °C taken with an L12 superlattice reflection. Based on the above, the displayed particles are most probably Al3Er. The diffraction pattern shown in Figure 4 confirms the L12 structure of the precipitates. The arrows in Figure 3 point at arrays of heterogeneously precipitated particles. Such arrayed precipitation was also observed in an Al-Sc alloy [19] and Al-Sc-Ti alloy [20] aged at small supersaturation. Low supersaturation means low thermodynamic driving force for nucleation and the precipitates nucleate predominately heterogeneously on dislocations to form an array [20]. The Er supersaturation in the present alloy has probably been considerably lowered by formation of the primary particles (see Figure 2). Moreover, the matrix/precipitate interfacial energy is higher for Al3Er (compared to Al3Sc) [12] and so its precipitation can be expected to be even more affected by presence of preferential nucleation sites.
May 26 -28, 2021, Brno, Czech Republic, EU So far, little effort has been made to describe the early stages of evolution of Al3Er particles. Vlach et al. [21] investigated phase transformations in an Al-0.2 Sc-0.15 Zr alloy during isochronal annealing and found that prior to Al3Sc precipitation there was a slight decrease and increase in resistivity and microhardness, respectively. The suggested explanation -Sc clusteringwas later proved by positron annihilation spectroscopy [22]. Analogically it can be proposed that the slight resistivity and microhardness changes between 150 °C and 270 °C (see Figure 1) are connected to clustering of Er. This statement, however, stems from the analogy to the similar Al-Sc system and is to be proved. Subsequently Al3Er are formed, resulting in drop of resistivity and peak hardness at about 360 °C. The decline of microhardness at 420 °C is probably Nieet al. [15] have reported that the number density of precipitates of Al-Er-Zr alloy is higher than that of Al-Er alloy and there are core-shell Al3(Er, Zr) precipitates in the Al-Er-Zr alloy with core enriched in Er and shell enriched in Zr. First-principles calculations showed that this core-shell structure is the most stable among all possible precipitation structures in Al-Er-Zr and thus shall account for majority of precipitates [12]. At temperatures greater than 420 °C, the diffusivity of Zr becomes sufficiently large to permit significant diffusion and precipitation of this solute element on previously formed Al3Er precipitates, causing the second microhardness peak and subsequent microhardness plateau.

CONCLUSIONS
The properties of cast Al-Er-Zr alloy were investigated utilizing microhardness, electrical resistivity measurements, differential scanning calorimetry and observations by electron microscopy. Based on the obtained results, following conclusions can be made: • The slight resistivity and microhardness changes between 150 °C and 270 °C might stem from clustering of Er.

•
The precipitation of Al3Er particles results in a pronounced hardening effect. Some of these particles nucleated on dislocations.

•
No thermal effect was observed in the DSC curves ranging from 25 °C to 600 °C.

•
Above 420 °C, Zr most likely precipitates on previously formed Al3Er nuclei, probably forming a Zr-rich shell.

•
In order to describe the decomposition sequence of the Al-Er-Zr system thoroughly, further investigations are required.