Commercially available A7075 and A7050 alloys showed environmental degradation in ductility accompanied by intergranular fracture when tested at a slow strain rate of 10 Ϫ7 /s, compared to the ductility obtained by testing at a conventional strain rate of 10 Ϫ4 /s in a laboratory air. The degradation or embrittlement became severe with the increase in relative humidity of the testing environment. In every humidity, the embrittlement of the A7050 alloy was more serious than that of the A7075 alloy. Laboratory-made X7075 alloy which had the same amount of major solutes with the A7075 alloy but containing Zr in place of Cr revealed that additive Zr brought about more serious environmental degradation compared to the Cr-bearing alloy. The reasons for these phenomena were discussed in detail.

Effects of alloy compositions on hydrogen embrittlement was studied by means of slow strain rate testing, using Al-Mg-Si based alloys containing different amounts of excess Si, Mg 2 Si and Cu. An Al-1.12 mass%Mg 2 Si-0.35 mass%Si alloy hardly showed degradation in ductility when tested at the strain rate of 10 Ϫ7 /s in a high humidity atmosphere of 90% R.H, compared to the ductility obtained by testing at the strain rate of 10 Ϫ4 /s. In contrast, the ductility of an Al-1.11%Mg 2 Si-0.81%Si alloy obtained by testing at 10 Ϫ7 /s was lower than that at 10 Ϫ4 /s in the humid air. By means of SEM/EDS analysis, Si-rich precipitates were observed on intergranular fracture surfaces of the embrittled alloy. These results suggested that hydrogen embrittlement was clearly observed with an increase in excess Si content. The ductility of an Al-1.81%Mg 2 Si-0.80%Si alloy obtained by testing at 10 Ϫ7 /s, however, was larger than that at 10 Ϫ4 /s in the humid air. The same tendency was observed in commercially available 6070, 6013 and 6066 aluminum alloys. These results meant that the Al-Mg-Si based alloys with excess silicon contents did not always show hydrogen embrittlement. Discussion was made on the above phenomena.

Environmental embrittlement was studied by use of round tensile specimens of two kinds of alloys: Al-1.12 mass%Mg 2 Si-0.35Si S-alloy and Al-1.47%Mg 2 Si-0.34%Cu BC-alloy. They were solution treated at 540°C for 1 h, quenched in water and aged at 175°C for indicated times: 30 min under aging , 10 3 min peak aging , and 10 4 min over aging. After heat treating, they were tested at an initial strain rate of 1.7ϫ10 Ϫ4 /s or 1.7ϫ10 Ϫ7 /s in a laboratory air with relative humidity of about 100%. When tested at the strain rate of 1.7ϫ10 Ϫ7 /s, the peak aged S-alloy showed embrittlement with lower tensile elongation and reduction in area compared with a strain rate of 1.7ϫ10 Ϫ4 /s. The embrittlement also appeared on the other aging conditions under aging, over aging , and it was most apparent on over-aged alloy despite their lower tensile strength. By means of EDS analysis, Si-rich precipitates were detected on intergranular fracture surface of over-aged alloy. In contrasted to foregoing experimental results on S alloy, no embrittlement appeared in the BC-alloy irrespective of aging condition. Thus, the embrittlement is thought to be stimulated by the Si-rich precipitates which are increased with the progress of aging.

A 22-year-old woman had been treated with a Ross operation for aortic root aneurysm and aortic regurgitation 10 years previously. In the initial Ross operation, a handmade tri-leaflet conduit was used for the right ventricle outflow tract(RVOT)reconstruction. The conduit was prepared preoperatively, by sewing a folded 0.1 mm expanded polytetrafluoroethylene(ePTFE)membrane onto the luminal cavity of the 24 mm woven double velour vascular graft, thereby creating a tri-leaflet valve. During ambulatory follow up after discharge, dilation of the pulmonary autograft had been observed, and its maximal diameter reached 60 mm. Furthermore, preoperative a pressure study revealed a 25 mmHg pressure gradient between the right ventricle and the pulmonary artery. At the time of reoperation, we performed an aortic root replacement combined with RVOT conduit replacement. A 24-mm woven double velour vascular graft integrating a 21-mm On-X mechanical prosthesis was used for aortic root replacement. A handmade ePTFE tri-leaflet conduit, 26 mm in size, was used to replace the previous RVOT conduit. The operation was successful, and the postoperative course was uneventful. The explanted conduit was sent for microscopic examination, which revealed that the graft was covered by a fibrocollagenous membrane. On the contrary, no surface membrane was found on the ePTFE valve. Moreover the microscopic examination showed cystic medionecrosis of the pulmonary autograft. Both dilatation of the pulmonary autograft and RVOT conduit failure were successfully treated at the second operation. However this young patient will require follow-up of the mechanical prosthesis and RVOT conduit for the rest of her life. Jpn. J. Cardiovasc. (