2019年6月英语六级（第3套 · 含听力）选词填空

Steel is valued for its reliability, but not when it gets cold. Most forms of steel __26__ become brittle (脆的) at temperatures below about -25℃ unless they are mixed with other metals. Now, though, a novel type of steel has been developed that resists __27__ at much lower temperatures, while retaining its strength and toughness - without the need for expensive __28__.
Steel's fragility at low temperatures first became a major concern during the Second World War. After German U-boats torpedoed (用鱼雷攻击) numerous British ships, a 2700-strong fleet of cheap-and-cheerful "Liberty ships" was introduced to replace the lost vessels, providing a lifeline for the __29__ British. But the steel shells of hundreds of the ships __30__ in the icy north Atlantic, and 12 broke in half and sank.
Brittleness remains a problem when building steel structures in cold conditions, such as oil rigs in the Arctic. So scientists have __31__ to find a solution by mixing it with expensive metals such as nickel.
Yuuji Kimura and colleagues in Japan tried a more physical __32__. Rather than adding other metals, they developed a complex mechanical process involving repeated heating and very severe mechanical deformation, known as tempforming.
The resulting steel appears to achieve a combination of strength and toughness that is __33__ to that of modern steels that are very rich in alloy content and, therefore, very expensive.
Kimura's team intends to use its tempformed steel to make ultra-high strength parts, such as bolts. They hope to reduce both the number of __34__ needed in a construction job and their weight by replacing solid supports with __35__ tubes, for example. This could reduce the amount of steel needed to make everything from automobiles to buildings and bridges.