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Most forms of steel abruptly become brittle at temperatures below about -25°C unless they are mixed with other metals.
2019年6月六级真题（第三套）阅读 Section A
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Rather than adding other metals, they developed a complex mechanical process involving repeated heating and very severe mechanical deformation, known as temp-forming.
2019年6月六级真题（第三套）阅读 Section A
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So scientists have strived to find a solution by mixing it with expensive metals such as nickel.
2019年6月六级真题（第三套）阅读 Section A
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This is partly driven by the rising price of raw materials and metals.
2019年6月六级真题（第二套）阅读 Section B
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Together, the value of these metals is estimated to be about $52 billion.
2019年6月六级真题（第二套）阅读 Section B