I was unable to find a definitive answer about the specific materials used for the 2014 FIFA World Cup, but here are some of the regulations I could find.
According to the official "Regulations of the 2014 FIFA World Cup":
33 Stadiums, fields of play, clocks, displaysThe Organising Association shall ensure that the stadiums and facilities in which the matches of the final competition take place fulfil FIFA requirements and comply with the FIFA Safety Regulations and other FIFA guidelines and instructions for international matches.
According to the Laws of the Game 2014/2015 (PDF):
The goalposts and crossbar must be made of wood, metal or other approved material.
And lastly, here are some technical details about the goalposts used in the World Cup from Football Stadiums: Technical recommendations and requirements - 5th Edition (PDF):...