根據相(xiang)(xiang)圖，多數(shu)合(he)金(jin)元(yuan)(yuan)(yuan)素在(zai)固(gu)相(xiang)(xiang)中(zhong)的(de)溶(rong)(rong)(rong)解(jie)度要低(di)于液(ye)相(xiang)(xiang)，因(yin)此(ci)在(zai)凝固(gu)過(guo)(guo)程中(zhong)溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)原子不(bu)(bu)斷被(bei)排出(chu)到(dao)液(ye)相(xiang)(xiang)，這種固(gu)液(ye)界面(mian)兩側溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)濃度的(de)差異導致(zhi)合(he)金(jin)凝固(gu)后溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)(yuan)(yuan)素成(cheng)分(fen)(fen)不(bu)(bu)均(jun)勻(yun)(yun)(yun)性(xing)(xing)，稱作偏(pian)析(xi)。溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)(yuan)(yuan)素分(fen)(fen)布不(bu)(bu)均(jun)勻(yun)(yun)(yun)性(xing)(xing)發生(sheng)在(zai)微觀(guan)結構形(xing)成(cheng)范(fan)圍內（有(you)10~100μm的(de)樹狀枝晶(jing)），此(ci)時為(wei)微觀(guan)偏(pian)析(xi)。溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)(yuan)(yuan)素通(tong)過(guo)(guo)對流傳質(zhi)(zhi)(zhi)等(deng)(deng)質(zhi)(zhi)(zhi)量傳輸，將導致(zhi)大范(fan)圍內成(cheng)分(fen)(fen)不(bu)(bu)均(jun)勻(yun)(yun)(yun)性(xing)(xing)，即(ji)形(xing)成(cheng)了宏(hong)觀(guan)偏(pian)析(xi)。宏(hong)觀(guan)偏(pian)析(xi)可以(yi)認(ren)為(wei)是(shi)由凝固(gu)過(guo)(guo)程中(zhong)液(ye)體和固(gu)體相(xiang)(xiang)對運動和溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)再分(fen)(fen)配過(guo)(guo)程共同導致(zhi)的(de)。此(ci)外(wai)，在(zai)凝固(gu)早期所形(xing)成(cheng)的(de)固(gu)體相(xiang)(xiang)或非金(jin)屬夾(jia)雜的(de)漂浮和下沉也會(hui)造成(cheng)宏(hong)觀(guan)偏(pian)析(xi)。一般認(ren)為(wei)在(zai)合(he)金(jin)鑄(zhu)件或鑄(zhu)錠內，從幾(ji)毫(hao)米到(dao)幾(ji)厘(li)米甚至(zhi)幾(ji)米范(fan)圍內濃度變化為(wei)宏(hong)觀(guan)偏(pian)析(xi)。因(yin)為(wei)溶(rong)(rong)(rong)質(zhi)(zhi)(zhi)在(zai)固(gu)態(tai)中(zhong)的(de)擴散系數(shu)很(hen)低(di)，而(er)成(cheng)分(fen)(fen)不(bu)(bu)均(jun)勻(yun)(yun)(yun)性(xing)(xing)范(fan)圍又很(hen)大，所以(yi)在(zai)凝固(gu)完成(cheng)后，宏(hong)觀(guan)偏(pian)析(xi)很(hen)難(nan)通(tong)過(guo)(guo)加(jia)工處理來消除，因(yin)此(ci)抑制宏(hong)觀(guan)偏(pian)析(xi)的(de)產生(sheng)主要是(shi)對工藝參數(shu)進行優化，如控制合(he)金(jin)成(cheng)分(fen)(fen)、施(shi)加(jia)外(wai)力場（磁場等(deng)(deng)）、優化鑄(zhu)錠幾(ji)何形(xing)狀、適當加(jia)大冷卻速率等(deng)(deng)。

  宏觀偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是(shi)大(da)范圍內(nei)的(de)(de)(de)成(cheng)分不均勻現象，按其(qi)表現形(xing)(xing)式可分為(wei)正(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)、反偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)和比重偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)等(deng)(deng)。①. 正(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：對平衡分配系數o<1的(de)(de)(de)合(he)金系鑄(zhu)(zhu)(zhu)錠先凝(ning)固的(de)(de)(de)部(bu)(bu)分，其(qi)溶(rong)(rong)質(zhi)含(han)量低于后(hou)凝(ning)固的(de)(de)(de)部(bu)(bu)分。對ko>1的(de)(de)(de)合(he)金系則正(zheng)好相(xiang)反，其(qi)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)程度與(yu)(yu)凝(ning)固速(su)率、液體對流(liu)以及溶(rong)(rong)質(zhi)擴散等(deng)(deng)條件有關。②. 反偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：在(zai)(zai)ko<1的(de)(de)(de)合(he)金鑄(zhu)(zhu)(zhu)錠中(zhong)，其(qi)外(wai)層溶(rong)(rong)質(zhi)元(yuan)素高于內(nei)部(bu)(bu)，和正(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)相(xiang)反，故稱(cheng)為(wei)反偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)。③. 比重偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：是(shi)由合(he)金凝(ning)固時形(xing)(xing)成(cheng)的(de)(de)(de)初晶(jing)相(xiang)和溶(rong)(rong)液之間(jian)的(de)(de)(de)比重顯(xian)著差(cha)別(bie)引起的(de)(de)(de)一(yi)種宏觀偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)，主(zhu)要存(cun)在(zai)(zai)于共晶(jing)系和偏(pian)(pian)(pian)(pian)(pian)(pian)晶(jing)系合(he)金中(zhong)。如圖(tu)2-49所示，由于溶(rong)(rong)質(zhi)元(yuan)素濃度相(xiang)對低的(de)(de)(de)等(deng)(deng)軸晶(jing)沉積導(dao)(dao)致在(zai)(zai)鑄(zhu)(zhu)(zhu)錠的(de)(de)(de)底部(bu)(bu)出現負(fu)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)；由于浮力(li)和在(zai)(zai)凝(ning)固的(de)(de)(de)最后(hou)階段收縮所引起的(de)(de)(de)晶(jing)間(jian)流(liu)動，在(zai)(zai)頂部(bu)(bu)會出現很嚴重的(de)(de)(de)正(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)（頂部(bu)(bu)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)）。A型(xing)(xing)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是(shi)溶(rong)(rong)質(zhi)富集(ji)的(de)(de)(de)等(deng)(deng)軸晶(jing)帶，由溶(rong)(rong)質(zhi)受(shou)浮力(li)作(zuo)用流(liu)動穿過柱狀(zhuang)晶(jing)區，其(qi)方(fang)(fang)向與(yu)(yu)等(deng)(deng)溫線移動速(su)度方(fang)(fang)向一(yi)致但速(su)率更(geng)快所導(dao)(dao)致。A型(xing)(xing)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)形(xing)(xing)狀(zhuang)與(yu)(yu)流(liu)動類型(xing)(xing)有關。V型(xing)(xing)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)位于鑄(zhu)(zhu)(zhu)錠中(zhong)心，源(yuan)于中(zhong)心形(xing)(xing)成(cheng)等(deng)(deng)軸晶(jing)區和容易(yi)斷裂(lie)的(de)(de)(de)連接疏松的(de)(de)(de)網狀(zhuang)物的(de)(de)(de)形(xing)(xing)成(cheng)，之后(hou)裂(lie)紋沿切應力(li)面展開為(wei)V型(xing)(xing)，并且充(chong)滿(man)了(le)富集(ji)元(yuan)素的(de)(de)(de)液相(xiang)。而(er)沿鑄(zhu)(zhu)(zhu)錠側壁分布的(de)(de)(de)帶狀(zhuang)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)則是(shi)由凝(ning)固過程初期的(de)(de)(de)不穩定傳熱和流(liu)動導(dao)(dao)致的(de)(de)(de)。

  對于宏(hong)(hong)觀(guan)偏析(xi)(xi)的(de)(de)研究主要(yao)有實驗(yan)檢測和模擬計(ji)算(suan)兩種(zhong)手段(duan)。實驗(yan)檢測包括硫印(yin)檢驗(yan)法(fa)、原(yuan)位(wei)分(fen)析(xi)(xi)法(fa)、火花放電原(yuan)子發射光譜法(fa)、鉆孔取樣法(fa)以(yi)及(ji)化學分(fen)析(xi)(xi)法(fa)等(deng)(deng)。模擬計(ji)算(suan)是通(tong)過數值(zhi)求(qiu)解能量(liang)、動量(liang)以(yi)及(ji)溶質(zhi)傳輸等(deng)(deng)數學模型，進而探討元(yuan)素成分(fen)不(bu)均勻性的(de)(de)方法(fa)；進入20世紀(ji)后，人們對凝固過程中的(de)(de)宏(hong)(hong)觀(guan)偏析(xi)(xi)現象進行(xing)了大量(liang)系統的(de)(de)研究。Flemings研究表明鑄錠中多種(zhong)不(bu)同的(de)(de)宏(hong)(hong)觀(guan)偏析(xi)(xi)都(dou)可(ke)由凝固時的(de)(de)傳熱、流動和傳質(zhi)過程來定(ding)量(liang)描述，從而為(wei)宏(hong)(hong)觀(guan)偏析(xi)(xi)的(de)(de)定(ding)量(liang)計(ji)算(suan)提(ti)供可(ke)能性，隨著計(ji)算(suan)機計(ji)算(suan)能力迅(xun)(xun)猛提(ti)升，宏(hong)(hong)觀(guan)偏析(xi)(xi)的(de)(de)模擬計(ji)算(suan)得到了迅(xun)(xun)速發展，主要(yao)分(fen)為(wei)多區(qu)域法(fa)和連(lian)續介(jie)質(zhi)法(fa)等(deng)(deng)。

  對于高氮不(bu)銹鋼，改善氮偏析以及消除氣孔等凝固缺陷，優化制備工藝制度，是高氮奧氏體不銹鋼制備技術中亟待解決的難題之一。氮作為重要合金元素之一，其偏析程度對材料強度、韌性、抗蠕變性、耐磨性和耐腐蝕等性能的均勻性至關重要，直接影響材料的服役壽命。與高氮不銹鋼中鉻、錳等其他元素相比，氮的分配系數較小，氮偏析嚴重，易形成氮氣泡，凝固末了殘留在鑄錠中形成氮氣孔等凝固缺陷，甚至導致材料直接報廢，因此氮偏析的控制對高氮不銹鋼制備而言至關重要。不同壓力和不同初始氮含量下21.5Cr5Mn1.5Ni0.25N含氮雙相鋼中氮偏析導致氮氣孔的形貌如圖2-50所示，其中D1、D3和D5分別在0.04MPa、0.1MPa和0.13MPa下完成凝固，不同氮質量分數的D2(0.25%N)、D3(0.26%N)和D4(0.29%N)均在0.1MPa下凝固。