Careful analysis of the composition of the titanium alloys is a very critical part of the overall quality control of all components in modern jet aircraft engines. The engine manufacturing industry is seriously striving for a zero defect rate, which is a very difficult goal indeed. This control is made more difficult by the very reactive nature of titanium when it is heated to its melting point. Titanium reacts with the nitrogen and oxygen in the air to form brittle refractory compounds, so it must be melted and refined in a vacuum. Once these refractories are formed they are very difficult to find or remove and they form the basis for brittle zones in the alloy that can be the initiation site for cracks under the high stress loads found in a jet engine. The presence of nitrogen, in particular, causes a defect called a "hard-aipha", which is an area of nitrogen enrichment containing three to eight percent nitrogen and having a hardness three times that of the alloy.