Malcolm Turnbull, a former investment banker and lawyer, was poised to become the prime minister of Australia on Monday night after defeating Tony Abbott in a vote of Liberal Party lawmakers. Turnbull, 60, is a moderate Liberal whose views, most recently on same-sex marriage, had conflicted with those of Mr. Abbott, 57. The Liberals, despite their name, are the more conservative of Australia’s two major parties. The new prime minister is likely to be more open to the outside world and less conservative in approach than Mr. Abbott.