Early Stages of Precipitation Process in Al-(Mn-)Sc-Zr Alloy Characterized by Positron Annihilation

Abstract

Thermal effects on the precipitation stages in as-cast Al-0.70 at. pct Mn-0.15 at. pct Sc-0.05 at. pct Zr alloy were studied. The role of lattice defects was elucidated by positron annihilation spectroscopy (lifetime and coincidence Doppler broadening) enabling investigation of solutes clustering at the atomic scale. This technique has never been used in the Al-Sc-and/or Al-Zr-based alloys so far. Studies by positron annihilation were combined with resistometry, hardness measurements, and microstructure observations. Positrons trapped at defects are preferentially annihilated by Sc electrons. Lifetime of trapped positrons indicates that Sc atoms segregate at dislocations. Maximum fraction of positrons annihilated by Sc electrons occurring at 453 K (180°C) suggests that clustering of Sc bound with vacancies takes place. It is followed by peak of this fraction at 573 K (300°C). A rise of the contribution of trapped positrons annihilated by Zr electrons starting at 513 K (240°C) and attaining maximum also at 573 K (300°C) confirms that Zr participates in precipitation of the Al 3 Sc particles already at these temperatures. The pronounced hardening at 573 K (300°C) has its nature in the precipitation of the Al 3 Sc particles with a Zr-rich shell. The contribution of trapped positrons annihilated by Mn electrons was found to be negligible.

Characterization of phase development in commercial Al-Zn-Mg(-Mn,Fe) alloy with and without Sc,Zr-addition

“…One can conclude that this result is probably connected with the Sc,Zr-addition. The hardening effect at this temperature range is typical for the Al-Sc-Zr-based alloys [1,[11][12][13]18]. However, the existence of the Al 3 (Sc,Zr)-phase particles was not observed directly (by TEM) as the consequence of the small amount of these additions in the matrix and/or their inhomogeneous distribution such as randomly localized matrix regions with the higher Sc,Zr-content.…”

“…The measurements were performed using 22 Na positron source sealed in titanium foil. Detailed information about LT measurements is described in [18].…”

Early Stages of Precipitation in Mould-Cast, Cold-Rolled and Heat-Treated Aluminium Alloy AA7075 with Sc,Zr-Addition

“…Solutes of Sc and Zr are, thereby, not incorporated into solute clusters formed during NA or cooling after casting. Note that Al 3 (Sc,Zr) particles do not represent positron trapping sites [14]. There is, however, a difference between ratio curves for alloys in the MC state and in the HT state after NA.…”

“…Detailed information about measurements is described in Ref. [14]. The microstructure evolution was characterized by transmission (TEM) and scanning (SEM) electron microscopy.…”

Precipitation Evolution and Modeling of Growth Kinetics of L12-structured Al3Zr Particles in Al-0.22Zr and Al-0.32Zr (wt.%) Alloys Isothermally Aged

“…It was observed that coarsening and creep resistance at 673 K increased with the presence of Er intermetallics. 20 studied an Al-Mn-Sc-Zr aged at 573 K. 3 Zr particles were performed, although the authors were the first to study the shearing to bypassing transition of the particles by dislocations.…”