 Beryllium improves many physical properties when added as an alloying element to aluminium , copper ( notably the alloy beryllium copper ) , iron and nickel . Tools made of beryllium copper alloys are strong and hard and do not create sparks when they strike a steel surface . In structural applications , the combination of high flexural rigidity , thermal stability , thermal conductivity and low density ( 1 @.@ 85 times that of water ) make beryllium metal a desirable aerospace material for aircraft components , missiles , spacecraft , and satellites . Because of its low density and atomic mass , beryllium is relatively transparent to X @-@ rays and other forms of ionizing radiation ; therefore , it is the most common window material for X @-@ ray equipment and components of particle physics experiments . The high thermal conductivities of beryllium and beryllium oxide have led to their use in thermal management applications . 
