TOSI - erry . | . . 
® bi Studies and microstructural researches about the strengthening mechanism of 
USE Operation Al-Li based alloys 
WE sel, Ty 
Lg . . 
emg: M Vasilescu, C. Dumitrescu, E. Cazimirovici, I. Vasilescu, University Politehnica Bucharest (Ro); 
C. Herwartz Dumitrescu, RWTH Aachen (D) 
~ Microcracks, 
it Wo i ; 
ot deter + by Introduction 
- J . . . . Ce 
Exe pe The hardening process of Al-Li based alloys take place owing to massive coherent precipitation of 
wh sphere ordered phases AlsLi. The germination and growing of ALi precipitates still remain the 
subject of a lot of investigations, the main results being published in specialty review, (1), (2), (3). 
In the last time, a special attention is accorded to the study of interaction between dislocations and 
8 precipitation with a diameter of 20-40 10° m, and of the fundamentals hardening mechanism in 
the binary alloys of high purity. 
+ Ny . 
i Experimental work 
The studies and microstructural research about strengthening mechanism has been effectuated over 
Al-Li binary alloy with 2%Li, Al-Li-Cu alloy with 2%Li and 2%Cu, respectively Al-Li-Cu-Ti alloy 
with 2%Li, 2%Cu and 0,5%Ti. 
The successive stage of precipitation processes has been analyse through DSC, TEM, HREM and 
CRSS methods. 
The initial stage of forming of AlCus ordered phase, respectively of Al Li (8°) are shown on the Al- 
Li samples quenched and natural aging. The initial process is a rapid ordering followed by a lent 
decomposition of aa; oversaturation solid solution. It was observed that in ordered matrix take place 
the process of homogeneous germination of 5’ phase, which influence the mechanical 
characteristics. The ordered matrix and the ALLi (8°) precipitate allow the development of the 
process of planar slight which start after the quenching. 
ICrMoV121 steel From the study of microstructures results that ALi (8°) uncoherent precipitates have diameter of 
the DIN IN 40 -10° m and are black, Fig. 1 a. From the dark field image (Fig. 1 b) results that in ay solid 
is having a stable solution are presented very fine globular precipitates with diameter of 10” m. 
out 30.000 hours 
tions. The results of the studies and experimental researches present that the precipitation process take 
he oreep strength | place as follows: a, sol.sol. — G-P zone — 8’ (ALLi) — 7 (AlL1). 
The precipitation process of 8 phase take place in oversaturation ou solid solution immediately 
after quenching through holding the sample at room temperature. 
In the alloys with higher content of Li. the oversaturation and nonordered os solid solution suffer a 
continuous process of ordering, without germination process At the same time take place a spinodal 
Vomstrien® decomposition which leads to a lower concentration of Li and of spontaneous disorder process. 
io 
The exposed model are based on a substantial number of experimental results which confirm the 
fact that the processes of apparition of zone with ordering atomic arrangement take place 
bil 71 (1990) immediately after quenching and holding the alloys with 7-9%At Li at room temperature. 
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