Hydrogen and porosity

 It is widely known that there are micropores in cast materials. Recent 3D imaging by X-ray CT has demonstrated that there are also many spherical micropores, which are sometimes larger than 10μm, as shown in figure 1, even in sufficiently flattened materials(Paper 1). The presence of a number of micropores in aluminum alloy was a surprising discovery for us, who have researched and investigated materials engineering and have observed many materials surfaces and sections. The knowledge is industrially significant because it throws doubt on various processing and heat treatment conditions, which have been determined without any consideration of the presence of micropores.

Fig. 1 3D images of various aluminum alloys by projection CT. Micropores are present at high densities in all materials except in pure aluminum.

Fig. 2 Occupancy of each trap site in an Al-Mg alloy (HH, MH and LH correspond to the high, medium and low hydrogen concentrations, respectively) (top), and the hydrogen share ratio of each trap site

 Aluminum contains hydrogen at concentrations far exceeding the solid solubility limit, even after processing and heat treatment. Micropores have been shown to develop on dispersed grains through heterogeneous nucleation(Paper 2), and almost no micropores develop in pure aluminum of the same hydrogen concentration because there are few nucleation sites, as shown in figure 1 (Paper 2). Figure 2 shows the occupancy of trap sites determined by assuming the heat equilibrium in the trap sites and interstitial lattice, and the distribution of hydrogen in each site(Paper 2). Micropores are formed by hydrogen that exists in ordinary solid aluminum at supersaturation levels, being trapped mainly at points of dislocation and in the interstitial lattice, and being discharged within the material in the form of hydrogen molecules. The sizes of the micropores are determined by the equilibrium between the internal pressure of hydrogen and the surface tension of the micropore (Paper 2). As shown on the left of Fig. 3, the micropores grow in size through exposure to high temperature. The mechanism involved has been shown to be Ostwald ripening(Paper 2).

Fig. 3 Homogenization of Al-Mg alloy, and changes in the micropores during hot rolling, cold rolling and annealing

 

Fig. 4 Analysis of the local displacement field using the radial basis function (top), and application to Al-Mg alloy (table). Of the 381 micropores in the specimen, 69 remained even after 60% compression, and 72 disappeared once but recovered at the same sites after reheating.

 
 The healing behaviors of micropores have been analyzed through measuring the local displacement fields by using the radial basis function and dispersed particles as shown in Fig. 4(Paper 3).According to the analysis, micropores behave complicatedly, and even those that seemed to disappear recover when reheated. Micropores that remained even after rolling are visualized on the right of Fig. 3(Paper 3). The mechanical conditions that heal micropores have been determined by 4D mapping of internal distortions achieved by tracking dispersed particles(Paper 3,5).

Fig. 5 Internal distortion distribution measurements in an Al-Mg alloy during unconfined compression

 As shown in Fig. 5, local distortions within a material are highly heterogeneous, even under a simple unconfined load. As shown in Fig. 6, the micropores disappeared at local effective strains of about 0.4 to 0.5; and the disappearance of the micropores does not depend on hydrostatic stress, which has been used in conventional assessments.

Fig. 6 Disappearance of micropores by compression and surface cold working, and local effective strain distribution measured by 4D imaging

 Statistical analysis was also conducted on the effect of micropores on the strength and ductility of aluminum alloy die-castings(Paper 6), and on fatigue fracture(Paper 8).We also clarified that micropores grow and amalgamate rapidly when exposed to external loads, and induce ductile fracture(Paper 9)。

Review paper

  1. H. Toda, T. Kobayashi, T. Ohgaki, Advanced in Visualization Techniques by High-resolution X-ray CT: Application to in-situ Measurement of Internal Local Mechanical Quantities of Materials, Zairyo-shiken-gijyutsu, Vol.48,No.1,2004,5-10.
  2. H. Toda, M. Kobayashi, Y. Suzuki, A. Takeuchi, K. Uesugi, 3D・4D Materials Science: Its Current State and Prospects, Hihakaikensa, Vol.58,No.10,2009,433-438

Research paper

  1. 増田翔太郎,戸田裕之,青山俊三,折井晋,植田将志,小林正和,熱処理したアルミニウム合金ダイカストで新たに見つかった鋳肌欠陥とその疲労特性への影響,鋳造工学,Vol.81,No.10,2009,475-481(日本鋳造工学会論文賞受賞)
  2. H. Toda, T. Hidaka, M. Kobayashi, K. Uesugi, A. Takeuchi, K. Horikawa, Growth behavior of hydrogen micropores in aluminum alloys during high-temperature exposure, Acta Materialia, Vol.57, 2009, 2277-2290
  3. H. Toda, K. Minami, K. Koyama, K. Ichitani, M. Kobayashi, K. Uesugi, Y. Suzuki, Healing behavior of preexisting hydrogen micropores, in aluminum alloys during plastic deformation, Acta Materialia, Vol.57, 2009, 4391-4403
  4. 小林正和, 戸田裕之, 南恵介, 森豊和, 上杉健太朗, 竹内晃久, 鈴木芳生, 軽金属,Vol.59,No.1,2009,30-34.
  5. H. Toda, T. Yamaguchi, M. Nakawaza, Y. Aoki, K. Uesugi, Y. Suzuki and M. Kobayashi, Four-dimensional annihilation behaviors of micro pores during surface cold working, Materials Transactions, Vol.51, No.07, 2010, 1288-1295
  6. 戸田裕之,小林正和,伊藤真也,中澤満,青木義満,堀川宏,鈴木聡,ミクロポアがアルミニウム合金ダイカストの強度・延性に及ぼす影響の統計的解析,鋳造工学,Vol.82,No.7,2010,427-432
  7. 大語英之,戸田裕之,上杉健太朗,鈴木芳生,小林正和,軽金属,Vol.60,No.8,2010,409-410.
  8. H. Toda, S. Masuda, R. Batres, M Kobayashi, S. Aoyama, M. Onodera, R. Furusawa, K. Uesugi, A. Takeuchi, Y. Suzuki, Statistical assessment of fatigue crack initiation from sub-surface hydrogen micropores in high-quality die-cast aluminum, Acta Materialia, Vol. 59, No.12, 4990-4998, 2011
  9. H. Toda, H. Oogo, K. Horikawa, K. Uesugi, Y. Suzuki, Y. Aoki, M. Nakazawa and M. Kobayashi, The true origin of fractures in structural metals, Acta Materialia, to be submitted.