An airfoil (400; 500; 600; 700; 900) for a gas turbine engine, the airfoil comprising: an airfoil body radially extending from a first end to a second end and axially extending from a leading edge (410; 502; 908) to a trailing edge (412; 504; 910), the airfoil body defining a single unitary airfoil cavity (404; 506; 602; 708, 710; 902) therein, the airfoil cavity defined by a cavity wall (422; 508; 604; 704; 912) having at least one smooth portion (918a, 918b); and at least one expansion seal (402; 510, 512, 514; 706; 904, 906) installed within the airfoil cavity, the at least one expansion seal dividing the airfoil cavity into at least a first sub-cavity (424; 516; 708) and a second sub-cavity (426; 518; 710), wherein the at least one expansion seal (402; 510, 512, 514; 706; 904, 906) engages with the smooth portion (918a, 918b) of the cavity wall (422; 508; 604; 704; 912) by an expansion force (F E) that biases ends (914a, 914b, 916a, 916b) of the at least one expansion seal into contact with the smooth portion of the cavity wall, wherein the at least one smooth portion (918a, 918b) of the cavity wall (422; 508; 604; 704; 912) is a portion of an interior cavity wall of the airfoil body that is uninterrupted and does not include structure or features arranged to engage with the at least one expansion seal (402; 510, 512, 514; 706; 904, 906), and wherein the at least one expansion seal (402; 706; 904, 906) is arranged to divide the airfoil cavity (404; 902) extending from the first end to the second end such that the first sub-cavity (424; 708) is fluidly separated from the second sub-cavity (426; 710), characterised in that the at least one expansion seal (402; 510, 512, 514; 706; 904, 906) is formed from sheet metal that has a thickness of between 0.25 mm (0.010 inches) and 0.51 mm (0.020 inches). an airfoil body radially extending from a first end to a second end and axially extending from a leading edge (410; 502; 908) to a trailing edge (412; 504; 910), the airfoil body defining a single unitary airfoil cavity (404; 506; 602; 708, 710; 902) therein, the airfoil cavity defined by a cavity wall (422; 508; 604; 704; 912) having at least one smooth portion (918a, 918b); and at least one expansion seal (402; 510, 512, 514; 706; 904, 906) installed within the airfoil cavity, the at least one expansion seal dividing the airfoil cavity into at least a first sub-cavity (424; 516; 708) and a second sub-cavity (426; 518; 710), wherein the at least one expansion seal (402; 510, 512, 514; 706; 904, 906) engages with the smooth portion (918a, 918b) of the cavity wall (422; 508; 604; 704; 912) by an expansion force (F E) that biases ends (914a, 914b, 916a, 916b) of the at least one expansion seal into contact with the smooth portion of the cavity wall, wherein the at least one smooth portion (918a, 918b) of the cavity wall (422; 508; 604; 704; 912) is a portion of an interior cavity wall of the airfoil body that is uninterrupted and does not include structure or features arranged to engage with the at least one expansion seal (402; 510, 512, 514; 706; 904, 906), and wherein the at least one expansion seal (402; 706; 904, 906) is arranged to divide the airfoil cavity (404; 902) extending from the first end to the second end such that the first sub-cavity (424; 708) is fluidly separated from the second sub-cavity (426; 710), characterised in that the at least one expansion seal (402; 510, 512, 514; 706; 904, 906) is formed from sheet metal that has a thickness of between 0.25 mm (0.010 inches) and 0.51 mm (0.020 inches).