Thermal stability of Al-Mg-Sc-Zr alloy
ZHAO Wei-tao(赵卫涛), YAN De-sheng(闫德胜), RONG Li-jian(戎利建)
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016，China
Received 20 April 2006; accepted 30 June 2006
Abstract: The thermal stability of Al3(Sc,Zr) precipitates in the cold worked Al-Mg-Sc-Zr alloy after elevated temperature exposure was investigated. The evaluation was conducted using room temperature tensile, Vicker’s hardness, optical metallography and scanning electron microscope (SEM) with the backscatter. The results show that the Al3(Sc,Zr) precipitates and mechanical properties have no obvious change, and the grains keep elongated along the working direction as that in cold worked sample after exposure at 300 ℃ for 1 000 h. The coarsening of Al3(Sc,Zr) precipitates occurs and is no longer effective on the recrystallization resistance, and partial recrystallization is observed after 400 ℃ exposure. In particular, after 500 ℃ exposure, the hardness decreases drastically and the alloy has fully recrystallized due to the obvious coarsening of Al3(Sc,Zr) precipitates.
Key words: Al-Mg-Sc-Zr alloy; thermal stability; Al3(Sc,Zr); recrystallization
1 Introduction
Al-Mg-Sc-Zr alloy is a potential candidate material for aerospace applications due to its low density, good weldability and excellent corrosion resistance.
In Al-Mg alloy, scandium can form Al3Sc dispersoids that impart the alloy a high degree of recrystallization resistance. Precipitation-strengthened Al-Sc alloys can serve in an environment up to 300 ℃ due to the presence of elastically hard and coherent Al3Sc precipitates with L12-type crystal structure. Zr is known to increase the strength as well as the recrystallization resistance of Al(Sc) alloys by substitution of Sc for Zr to form Al3(Sc1-xZrx) precipitates with decreased coarsening kinetics[1, 2]. Al-0.14Sc-0.12Zr and Al-0.06Sc-0.005Zr (mass fraction, %) alloys can maintain their peak hardness for aging times as long as 144 h, at 300 ℃[3].
Mechanical property retention during or after thermal exposure is a key concern in many applications, especially for aerospace. The thermal exposure of 85℃/ 1 000 h caused no appreciable change in strength and elongation values for 5XXX+Sc alloys[4]. The performance of the Al-4Mg-0.2Sc alloy C557 was evaluated to assess its potential for a broad range of aerospace application, including airframe and launch vehicle structure. The specific interest was mechanical property at anticipated service temperature and thermal stability of the alloy. Thermal stability in tensile properties was demonstrated by less than 5% variation of tensile strengths at ambient temperature after exposure at 107 ℃[5].
In the present study, to establish the service temperature applicability and evaluate the thermal stability, the mechanical property of Al-6.0Mg-0.2Sc-0.1Zr alloy at room temperature after exposure at temperature from 250 ℃ to 500 ℃ was studied.
2 Experimental
The compositions of the Al-Mg-Sc-Zr alloy is shown in Table 1. The starting materials are 99.8%Al, 99.5%Mg, Al-2%Sc, Al-10%Zr, Al-2%Cr and Al-2%Be (mass fraction) master alloys. The alloys were melted in vacuum induction furnace and then poured into an iron mode with 120 mm in diameter. The ingot was homogenized at 460 ℃ for 24 h, and then hot extruded to rods in diameter of 30 mm. Finally, the rods were warm rolled and cold drawn to bars in diameter of 10 mm with the last reduction of 45%.
Samples for mechanical and hardness tests were machined from the cold-worked rods, and they were subject to annealing at 250, 300, 400 and 500 ℃ for different time in air furnace and subsequently ooled in air.
Changes in mechanical properties or Vickers hardness due to the heat treatment were monitored to reveal the thermal stability. The hardness were measured on polished cross section perpendicular to the working direction by using a Vickers diamond pyramid indenter with a load of 50 N, and the values of hardness measurement were the average of 4 indentation values with the maximum error less than ±2HV5.
Table 1 Chemical composition of Al-Mg-Sc-Zr alloy (mass fraction, %)
Optical metallography was used to observe the microstructural development. Samples were first mechanically ground with abrasive paper and polished with 2.0 μm, 1.0 μm and 0.5 μm diamond paste, and then etched with Barker’s reagent at 25 ℃ for about 2 min. The distribution of the Al3(Sc,Zr) precipitates was observed by a scanning electron microscope (SEM) with the backscatter detector to maximize contrast.
3 Results
3.1 Mechanical property
The mechanical properties at ambient temperature for the experiment alloy after exposure at 250 ℃ for different time are shown in Fig.1. The tensile strength (σb) decreases sharply at the initial 50 h exposure and then has a slight variation until 200 h. The yield strength (σ0.2) keeps steady condition until 500 h.
Fig.1 Strength of Al-Mg-Sc-Zr alloy after thermal exposure at 250 ℃ for different time
Vickers hardness of cold worked Al-Mg-Sc-Zr alloy after exposure at 300, 400 and 500 ℃ for different time are shown in Fig.2. The hardness of the cold worked alloy is 123 HV5. Drastic changes happen in the initial 100 h, and subsequently the hardness is not significantly affected by 300 ℃ exposure until 1 000 h. After 400 ℃ exposure, the hardness falls uniformly and the loss is approximately 10% for 500 h compared to the 1 h exposure. Alloy has the poorest thermal stability after 500 ℃ keeping even heated for 5 h, and the hardness is only 83HV5 after 100 h exposure.
Fig.2 Hardness of Al-Mg-Sc-Zr alloy after thermal exposure for different time
3.2 Optical metallography
The optical micrographs of the alloy are shown in Fig.3. The grains of cold worked alloy are elongated along the working direction. After exposed at 300 ℃ for 1 000 h, the alloy still keeps unrecrystallized grains, as shown in Fig.3(b). When the temperature increases, partial recrystallization is observed in Figs.3(c) and (d).
After maintained at 500 ℃ for 100 h and 550 ℃ for 1 h, the cold worked alloy has fully recrystallized, and the equiaxed grains are found in the size of about 30 μm as shown in Figs.3(e) and (f). The similar results are found in Al-0.12Zr-0.2Sc-2Mg alloy. For Al-0.12Zr-0.2Sc-3.5Mg alloy, partial recrystallization occurs between 400 ℃ and 500 ℃. At 550 ℃, the Al-0.12Zr-0.2Sc-3.5Mg alloy is fully recrystallized within 1 h[6].
WEN et al[7,8] has shown that a low temperature heat treatment in the range of 149-204 ℃ is most effective for the precipitation of Mg atoms in the form of Mg2Al3. Fig.3(b) shows some Mg2Al3 phases distribute along the grain boundaries at 300 ℃. On the other hand, with the high temperature heat treatment, very fine particles will precipitate and distribute uniformly within the entire matrix(Figs.3(c)-(f)).
3.3 Coarsening of Al3(Sc, Zr) precipitates
Particles of Al3(Sc, Zr) phase can form during solidification process after cast and high temperature processing in the range of 400-600 ℃. For example, hot rolling or hot extrusion can give a dense distribution of Al3(Sc, Zr) particles in 20-100 nm size[2].
Fig.3 Optical micrographs of Al-Mg-Sc-Zr alloy: (a) Cold worked; (b) Exposed at 300 ℃ for 1 000 h; (c) Exposed at 400 ℃ for 500 h; (d) Exposed at 500 ℃ for 1 h; (e) Exposed at 500 ℃ for 100 h; (f) Exposed at 550 ℃ for 1 h(Rolling direction is horizontal)
The Al3(Sc, Zr) phases on the cross section of Al-Mg-Sc-Zr alloy were studied by SEM. In these images, the bright, cuboidal shapes were identified as Al3(Sc, Zr) phase by electron dispersive spectros- copy(EDS).
The Al3(Sc, Zr) precipitates produced by keeping the Al-Mg-Sc-Zr alloy at 250 ℃ for 500 h are similar to that of the cold worked alloy, as shown in Figs.4(a) and (b). The phases exhibit spheroidal or rod-like shape with average size less than 200 nm. The precipitates have a little change in dimension after long preserving at 300 ℃ for 1 000 h(Fig.4(c)). In Fig.4(d), the precipitates begin to grow after exposed at 400 ℃ for 500 h. After exposed at 500 ℃ for 100 h, the Al3(Sc, Zr) phases rapidly coarsen, the density decreases and the average size reaches to 400 nm(Fig.4(e)).
4 Discussion
The early change of structure and properties that occurs upon annealing a cold-worked metal is considered to be the beginning of the recovery. As recovery proceeds, a sequence of structural change emerges, including the annealing out of point defects and their clusters, the annihilation and rearrangement of dislocation, the formation of subgrains and their subsequent growth[9]. Therefore, the tensile stress and hardness of the cold worked alloy decrease in a large degree in the initial maintaining stage. The decrease in hardness at 400 ℃ and 500 ℃ with prolonging time is associated with recrystallization of deformed grains.
In the Al-Mg-Sc-Zr alloy, the recrystallization is influenced by Al3(Sc,Zr) dispersoids. During deformation, particles will affect the deformation microstructure and texture through an increase in dislocation density. During annealing, the primary effect of closely spaced particles is to pin grain boundaries (Zener pinning)[10].
The ability of dispersoid to counteract boundary pressure is inversely dependent on its size. It is
Fig.4 SEM images of morphology of Al3(Sc, Zr) particles (cross section) in Al-Mg-Sc-Zr alloy: (a) Cold worked; (b) Exposed at 250 ℃ for 500 h; (c) Exposed at 300 ℃ for 1 000 h; (d) Exposed at 400 ℃ for 500 h; (e) Exposed at 500 ℃ for 100 h
established the Zener-drag Pz that is imposed on an advancing grain boundary by random distributed dispersoids can be expressed in the form:
Pz=kf/r
where k is a factor that includes the interface energy between the dispersoids and the matrix, f is the volume fraction and r is the mean radius of the dispersoids. Thus the Zener drag will decrease rapidly in the case where the dispersoids are coarsening.
The coarsening behavior of Al3Sc precipitates has been investigated by Lifshitz-Slyozov-Wagner(LSW) theory in some experiments[11-13]. In addition, Mg has no significant effect on the precipitation kinetics and volume fraction of Al3Sc precipitates in Al-4.00%Mg-0.54%Sc alloy[14]. Mg in solid solution increases the lattice parameter of the Al matrix. This is in turn expected to influence the critical diameter of Al3Sc phase for Al/Al3Sc coherency loss. For example, the researchers have reported a critical diameter for transition from coherent to semi-coherent particles in excess of 116 nm in an Al-6.3%Mg-0.2%Sc alloy[15]. In this way, the critical diameter of Al3(Sc,Zr) particles would be about 100 nm in Al-6.0Mg-0.2Sc-0.1Zr alloy.
Most of Al3(Sc, Zr) dispersoids have formed during deformation and subsequent annealing. When the alloy is aged at different temperatures for long time, the precipitated dispersoids will be very limited. Coarsening of the Al3(Sc, Zr) phases is mostly expected to take place and thus the Zener drag from the particles are expected to be change. Al3(Sc, Zr) phases do not significantly coarsen at 300 ℃ exposure[16]. For alloy containing coherent particles, the Zener pinning pressure is remarkable, moreover, the larger pinning pressure of the coherent particles is shown to expand the range of stability[17]. So the Al-Mg-Sc-Zr alloy hardly softened at 250 ℃ and 300 ℃. At 500 ℃, the coarsening rate of Al3(Sc, Zr) phase increases dramatically in an Al-0.12Zr-0.2Sc alloy[6] and the Al3(Sc, Zr) phase loses coherency with matrix. As a result, the decrease of Zener drag weakens the driving force for recrystallization sharply and deteriorates hardness.
5 Conclusions
In Al-Mg-Sc-Zr alloy, the Zener pinning of thermal stability Al3(Sc, Zr) dispersoids preserve the stable microstructure and mechanical properties at ambient temperature after 250 ℃ and 300 ℃ exposure. At 400 ℃ exposure, recrystallization gradually takes place and the stability begins to degrade. The noticeable changes happen after exposed at 500 ℃ for 100 h, and the Al3(Sc, Zr) dispersoids coarsen rapidly and the alloy has fully recrystallized.
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(Edited by ZHAO Jun)
Corresponding author: RONG Li-jian; Tel: +86-24-23971979; E-mail: ljrong@imr.ac.cn
