Precipitation hardening of a highly oversaturated Al-4Zr solid so-lution

R. von Bargen, J. Kovac, A. von Hehl, H.-W. ZochLightweight Materials, IWT - Stiftung Institut für Werkstofftechnik, Germany

1 Introduction

Nowadays products for example in the micro electronic or the automotive sector require (microscopically) small components in high quantities. New manufacturing processes are ne-cessary to be able to fulfil the requirements related to increased functionality, higher precision and shorter product cycle times. Related to the base material new production processes for special alloys and custom heat treatment methods have to be developed. Heat treatment steps are included in most manufacturing process chains either to achieve a good formability of the material or to adjust the final mechanical properties of the component. A good cold form-ability for example can be reached by recrystallisation annealing, but the grain size evolution has to be observed carefully, especially for material thicknesses of 100 µm and below. A mean grain size of 10 to 20 µm implies a significantly decreased number of grains over the cross section compared to macro sheets with a thickness of several millimetres. The impact of single grains on the material behaviour is increased [1, 2] due to the reduced number of avail-able gliding systems for cold forming. The Collaborative Research Centre 747 “Micro Cold Forming” of the German Research Foundation already solved some of the above mentioned challenges for example by using a physical vapor deposition (PVD) Magnetron Sputtering process at low sputtering temperatures (<200 °C) to achieve high strength aluminum alloys containing Sc or Zr with improved stability at elevated temperatures [3]. In contrast to con-ventional casting techniques where Zr is normally used for dispersoid strengthening [4] be-cause solubility is very low [5] it was possible to suppress precipitation during the foil pro-duction and to ensure the required supersaturated solid solution in a Scandium containing al-loy Al-2Sc and an Al-4Zr alloy with approximately 4 mass-% Zirconium. After depositing the alloyed aluminium layer on a steel substrate it has to be removed. In most cases spontaneous delamination occurs, otherwise the steel substrate has to be dissolved in HNO3 aqueous mix-tures [6]. Artificial ageing results for the Al-2Sc alloy have already been published and show a great potential of increasing the final mechanical properties of a component for later usage. Based on these results the novel alloy Al-4Zr is examined in an equivalent way. Samples of this material are artificially aged with typical ageing parameters [4] and characterized by ultra micro hardness measurements (UMH), transmission electron microscopical (TEM) analysis.

2 Experimental

2.1 Material

Figure 1 shows an etched cross section of the Al-4Zr-foil produced by PVD sputtering with a medium thickness of 15 µm. It reveals the distinct columnar morphology typical for PVD deposition. The chemical composition of the material was determined by optical glow dis-charge spectroscopy (GDOS) (Table 1).

Figure 1: Scanning electron microscopic picture of an Al-4Zr micro sheet cross section etched with 5 vol.-% H2SO4 and 2.5 vol.-% HF in aqueous solution. Table 1. Chemical composition of the Al-4Zr micro sheet determined by optical glow discharge spectroscopy (GDOS).

Element Al Zr Fe Si Mass % balance 3.7 0.1 0.06

The mean zirconium content over the foil thickness is around 3.7 mass-% Zr which reveals that not the complete zirconium content of the Al-4Zr target (4 mass-%) could be deposited on the substrate.

2.2 Artificial Ageing

The artificial ageing parameters were chosen to match the ones for conventional alloys with a low Zr content [4] using 300 and 400 °C with durations from 10 minutes up to 50 hours. To achieve peak strength at shorter ageing durations an increased temperature of 500 °C was used as well. The ageing treatments were performed in a standard air-circulation furnace. Afterwards foil samples for each parameter combination were embedded and polished to per-form ultra-micro hardness (UMH) measurement with a test load of 5 mN that was applied in 10 s and maintained for 10 s as well. UMH was chosen as an easily measurable indicator for the material strengthening. For further analysis of the formed precipitates some samples were electrolytically polished to use transmission electron microscopy (TEM).

3 Results

3.1 Hardness measurements

Figure shows the hardness of the aged samples versus the ageing duration. The hardness in the as sputtered condition is 2156 HIT 0.005/10/10/10 [N/mm2]. For the samples aged at 300 °C the hardness increases up to 2685 HIT 0.005/10/10/10 [N/mm2] then drops down to 2298 HIT 0.005/10/10/10 [N/mm2] and finally reaches peak hardness at

2760 HIT 0.005/10/10/10 [N/mm2]. This effect is most likely caused by overlapping effects of decreasing solid solution strengthening and increasing precipitation strengthening. For the higher ageing temperatures this effect could not be detected. At 400 °C ageing temperature the hardness stays nearly constant up to an ageing duration of 5 h. Then it increases linearly up to 2659 HIT 0.005/10/10/10 [N/mm2] peak hardness after ageing for 20 h before overage-ing takes places which causes a hardness reduction below the initial state level. Ageing at 500 °C shows direct overageing effects. The hardness after ageing for 10 min is 2667 HIT 0.005/10/10/10 [N/mm2] and decreases nearly constantly for longer ageing treat-ments down to 1382 HIT 0.005/10/10/10 [N/mm2] after 50 h.

Figure 2: Ultra-micro hardness values of Al-4Zr foils in the as sputtered condition and after artificial ageing.

3.2 TEM analysis

TEM analysis of the sample in the as sputtered condition reveal very fine homogeneously distributed, but unexpected precipitates all over the grains in the bright field images (Fig-ure 3a). The diffraction pattern in the <100> direction support this observation by showing only weak but clearly detectable diffraction marks for Al3Zr in between the regular matrix ones (Figure 3b). Therefore it must be concluded, that in contrast to the previously examined Al-2Sc alloy [7] it was not possible to realize an oversaturated solid solution with the com-plete Zr content dissolved in the matrix. Due to the low sputtering temperature, it can be as-sumed that the detected precipitates are the metastable form of Al3Zr.

Figure 4 shows a comparison of the two peak aged conditions for 300 and 500 °C ageing temperature. In both cases a homogeneous distribution of fine globularly shaped Al3Zr pre-cipitates equal in size is detectable. The diffraction pattern in <100> direction for the peak aged state shows much brighter Al3Zr effects in between the aluminium matrix ones com-pared to the initial state (Figure 3b).

a) b)

Figure 3: Al-4Zr foil as sputtered: a) bright field TEM image, b) diffraction pattern in <100> direction.

a) b) c)

Figure 4: TEM bright field images of the Al-4Zr foil after peak ageing at a) 300 °C for 50 hours and b) 500 °C

for 10 min; c) diffraction pattern in <100> direction after 10 min at 500 °C.

Ageing at higher temperatures (> 400 °C) leads to precipitation of rod like shaped precipitates most likely the stable form of Al3Zr, besides the already known finely distributed globular ones (Figure 5a). In case of significant overageing the precipitates coarsen heavily and the distance between them increases, as indicated in Figure 5b. Additionally to the rod like shaped precipitates very large globular ones occur which are presumably the stable form of Al3Zr as well.

Al

Al3Zr

50 nm

50 nm 50 nm

a) b)

Figure 5: TEM bright field images of the Al-4Zr foil after ageing at a) 400 °C for 10 hours and b) 500 °C for 50 h.

4 Summary

With a PVD magnetron sputtering process it is possible to produce a highly oversaturated Al-Zr alloy with zirconium content around 3.7 mass %. Compared to the previously realized Al-2Sc alloy [7] where the complete scandium content could be kept in solid solution due to the PVD inherent features, i.e low process temperature and atom scale deposition, some of the zirconium atoms already formed Al3Zr precipitates during sputtering. Nevertheless a signifi-cant age hardening effect could be realized. The initial ultra micro hardness in the as sputtered condition of 2156 HIT 0.005/10/10/10 [N/mm2] could be increased up to 2760 HIT 0.005/10/10/10 [N/mm2] after ageing at 300 °C for 50 h. If the ageing temperature is increased the peak hardness could be realized much faster. At 400 °C it takes 20 h and at 500 °C only 10 min to reach nearly the same hardness level. TEM analyses of the aged sam-ples show a very homogeneous distribution of the strengthening metastable Al3Zr precipitates at peak hardness. Increased ageing temperatures lead to coarsening of the precipitates and transformation to different shapes that are most likely the stable equilibrium Al3Zr phase. Further work should focus on the very short ageing durations at high temperatures (> 500 °C) to use a drop-down tube furnace as a specialized heat treatment facility for micro components, that could already be used successfully for deep drawn micro cups made of Al-2Sc [8].

50 nm 50 nm

5 Acknowledgment

The authors gratefully acknowledge the financial support by DFG (German Research Founda-

tion) for Subproject A2 "Heat Treatment” within the SFB 747 (Collaborative Research

Centre) "Micro Cold Forming - Processes, Characterisation, Optimisation".

5 References

[1] Bomas, H., Linkewitz, T., Mayr, P., Extremes, 1999, 2, p. 149. [2] Köhler, B., Bomas, H., Hunkel, M., Lütjens, J., Zoch, H.-W., Scripta Mater., 2010, 62,

p. 548. [3] Uliasz, P., Knych, T., Mamala, A., Smyrak, B., Aluminium alloys - Their physical and

mechanical properties 1, Proc. ICAA 11, 2008, p. 248. [4] Knipling, K.E., Dunand, D.C., Seidman, D.N., Metallurgical and Materials Transactions,

2007, 38A, p. 2552. [5] Massalski, T.B., Binary Alloy Phase Diagrams, ASM International, 1990. [6] Eisbrecher, I., Stock, H.-R., Proc. 4th Congress „Mikroproduktion”, Bremen, Germany,

2009, p. 211. [7] von Bargen, R., von Hehl, A., Zoch, H.-W., Materials Science Forum, 2011, 690, p. 327. [8] von Bargen, R., Hu, Z., Zoch, H.-W., Vollertsen, F., Materials Science and Engineering

Technology, 2011, 42 (4), p. 1035.