isothermal compression tests were conducted in the temperature ranges from 300 ℃ to 500 ℃ and strain rate ranges from 0.001 s-1 to 1 s-1 using a Gleeble 3500 thermo–mechanical simulator. The maximal true strain was 0.91. The samples were first heated up to the required temperatures, and then held for 2min prior to compression. Graphite sheet was used to minimize frictions. The samples were quenched into water to preserve the microstructure after compression tests. The samples for optical microscopy were sectioned in the center parallel to the compression axis, and then cold mounted, grinding and polishing, after that the samples etched in the picric acid.