In materials, Alcoa ( NYSE:AA ) slumped 4.9 percent to $13.00 after the largest U.S. aluminum producer forecast a global oversupply of the metal for 2015 while reporting first-quarter sales that trailed market expectations. Demand will increase 6.5 percent this year, compared with 9 percent in 2014. Alcoa also said worldwide output will exceed consumption by 326,000 metric tons this year, compared with its January projection for a 38,000-ton deficit. In consumer-discretionary shares, Costco Wholesale (NASDAQ:COST) fell 1.5 percent to $149.71 as the biggest U.S. warehouse-club chain posted a larger-than-expected drop in comparable sales in March, hurt by the timing of an earlier Easter. Comparable sales, which include sales from stores open more than a year, fell 2 percent in the five weeks ended April 5. The result missed analysts' expectation for a 1.2 percent drop, according to Thomson Reuters. The closely watched metric was flat in the U.S., while international sales fell 9 percent.