doi: 10.1111/j.1460-2695.2006.01025.x Fracture toughness analysis of laser-beam-welded superalloys Inconel 718 and 625 C. YENI 1 and M. KOC¸AK 2 1 Dokuz Eylul University, Faculty of Engineering, 35100 Bornova, Izmir, Turkey (currently at GKSS Research Center), 2 GKSS Research Center, Institute for Materials Research, D-21502, Geesthacht, Germany Received in final form 31 January 2005 ABSTRACT In this study, two 3.2-mm thick Ni-base superalloys, Inconel 718 and 625, have been laser-beam-welded by a 6-kW CO 2 laser and their room temperature fracture tough- ness properties have been investigated. Fracture toughness behaviour of the base metal (BM), fusion zone (FZ) and heat affected zone (HAZ) regions was determined in terms of crack tip opening displacement (CTOD) using compact tension-type (C(T)) specimens. Laser-beam-weld regions showed no significant strength overmatching in both alloys. Ductile crack growth analysis (R-curve) also showed that both materials exhibited similar behaviour. Compared to the BM there is a slight decrease in fracture toughness of the fusion and the HAZ. Keywords CTOD; fracture toughness; laser-beam welding; R-curve; superalloy. NOMENCLATURE a 0 = initial crack length (mm) A = total elongation (%) BM = base metal C(T) = compact tension CTOD = crack tip opening displacement FZ = fusion zone F = load (kN) F max = maximum load (kN) H = half weld width (mm) HAZ = heat affected zone L 0 = initial gauge length in transverse tensile specimen (mm) L 0eff = effective gauge length (=L 0 -2H) M = mis-match ratio (=R p0.2WM /R p0.2BM ) R p0.2 = yield strength at 0.2% plastic strain (MPa) R-curve = crack resistance curve R m = ultimate tensile strength (MPa) W = width of C(T) specimen (mm) WM = weld metal δ 5 = CTOD measured over a gauge length of 5 mm (mm) δ 5max = δ 5 measured at F max (mm) a = crack extension (mm) L = increase in L 0 (mm) ε = L/L 0 Correspondence: C. Yeni. E-mail: emine.cinar@deu.edu.tr INTRODUCTION Alloy 718 of Ni-based superalloys is an age-hardenable, high-strength alloy suitable for service temperatures from 546 c  2006 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 29, 546–557