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等溫過程中奧氏體晶界硼偏聚的非平衡特征

The Characteristic of Nonequilibrium Boron Segregation at Grain Boundaries During Isothermal Holding

  • 摘要: 低碳Mn—Mo鋼及Fe—Ni合金中硼的非平衡偏聚是一種動力學現象。在一定條件下進行等溫處理時,晶界上會出現比平衡偏聚高很多倍的非平衡硼偏聚,但是隨保溫時間延長,過量偏聚消失。這種偏聚量隨等溫時間出現峰值變化的現象是這類偏聚非平衡性的重要標志。
    在不同溫度等溫時,硼偏聚量隨時間變化曲線呈三種形式,其特征分別是出現峰值后偏聚逐步消失,部分消失和不消失,各種類型溫度區間的劃分主要取決于非平衡偏聚發生,發展過程與等溫時含硼相析出過程的相互關系。

     

    Abstract: The behaviour of grain boundary segregation and precipitation of boron in low carbon MO-B steels (with or without Mn) and Fe-30%Ni (B) alloys during isothermal holding have been investigated with Particle Tracking Au-toradiography (PTA).Specimens heated to 1350 or 120013 are quenched and isothermally held at 550-1000℃ for different periods.It is shown that the nonequilibrium grain boundary segregation of boron occurs as a dynamic Process and the magnitude of nonequilibrium grain boundary segregation of boron may be many times of that of equilibrium segregation.
    After cooling, the boron segregation at grain boundaries,on isothermal holding at a relatively high temperature, probably bring there by complex of boron atoms with supersaturated vacances, and increases with time to the annihilation of vacancies,then it becomes supersaturated with respect to equilibrium segregation concentration. Thus it decrease by diffusing back into the grain interior, giving a peak On the segregaton degree-isothermal holding time curve.The height and position of the peak depend on the initial heating and subsequent holding temperature.On the other hand,when the holding temperature is low, the nonequilibriumly segregated boron becomes supersaturating and will be Precipitated at the grain boundaries, owing to the difficulty in the back diffusion at low temperature. Thus a curve increasing at the first stage and then reaching a steady segregation (including Precipitated boron) is given.

     

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