根據相圖，多數(shu)合(he)金(jin)元素(su)在固(gu)(gu)(gu)(gu)(gu)(gu)相中(zhong)(zhong)的溶(rong)(rong)解度(du)要(yao)低(di)于液(ye)(ye)相，因(yin)此(ci)(ci)在凝(ning)(ning)固(gu)(gu)(gu)(gu)(gu)(gu)過(guo)(guo)程(cheng)中(zhong)(zhong)溶(rong)(rong)質(zhi)原子不斷(duan)被(bei)排出到液(ye)(ye)相，這種固(gu)(gu)(gu)(gu)(gu)(gu)液(ye)(ye)界(jie)面兩側溶(rong)(rong)質(zhi)濃度(du)的差異導(dao)致(zhi)(zhi)合(he)金(jin)凝(ning)(ning)固(gu)(gu)(gu)(gu)(gu)(gu)后溶(rong)(rong)質(zhi)元素(su)成(cheng)分(fen)(fen)不均(jun)勻性，稱(cheng)作偏(pian)析(xi)(xi)(xi)。溶(rong)(rong)質(zhi)元素(su)分(fen)(fen)布不均(jun)勻性發生在微(wei)觀(guan)結構形(xing)成(cheng)范(fan)圍(wei)內(nei)(nei)（有10~100μm的樹狀枝晶），此(ci)(ci)時(shi)為(wei)(wei)微(wei)觀(guan)偏(pian)析(xi)(xi)(xi)。溶(rong)(rong)質(zhi)元素(su)通(tong)過(guo)(guo)對(dui)流傳質(zhi)等(deng)質(zhi)量傳輸，將(jiang)導(dao)致(zhi)(zhi)大(da)(da)范(fan)圍(wei)內(nei)(nei)成(cheng)分(fen)(fen)不均(jun)勻性，即形(xing)成(cheng)了(le)宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)。宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)可(ke)以(yi)認為(wei)(wei)是由凝(ning)(ning)固(gu)(gu)(gu)(gu)(gu)(gu)過(guo)(guo)程(cheng)中(zhong)(zhong)液(ye)(ye)體(ti)(ti)和(he)固(gu)(gu)(gu)(gu)(gu)(gu)體(ti)(ti)相對(dui)運動和(he)溶(rong)(rong)質(zhi)再分(fen)(fen)配過(guo)(guo)程(cheng)共同導(dao)致(zhi)(zhi)的。此(ci)(ci)外，在凝(ning)(ning)固(gu)(gu)(gu)(gu)(gu)(gu)早期所形(xing)成(cheng)的固(gu)(gu)(gu)(gu)(gu)(gu)體(ti)(ti)相或非金(jin)屬夾雜的漂浮和(he)下沉也(ye)會(hui)造成(cheng)宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)。一般認為(wei)(wei)在合(he)金(jin)鑄件或鑄錠內(nei)(nei)，從幾毫米到幾厘米甚(shen)至(zhi)幾米范(fan)圍(wei)內(nei)(nei)濃度(du)變化(hua)為(wei)(wei)宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)。因(yin)為(wei)(wei)溶(rong)(rong)質(zhi)在固(gu)(gu)(gu)(gu)(gu)(gu)態中(zhong)(zhong)的擴散系(xi)數(shu)很(hen)低(di)，而成(cheng)分(fen)(fen)不均(jun)勻性范(fan)圍(wei)又很(hen)大(da)(da)，所以(yi)在凝(ning)(ning)固(gu)(gu)(gu)(gu)(gu)(gu)完(wan)成(cheng)后，宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)很(hen)難通(tong)過(guo)(guo)加工處理來消除，因(yin)此(ci)(ci)抑制(zhi)宏(hong)(hong)(hong)(hong)觀(guan)偏(pian)析(xi)(xi)(xi)的產生主要(yao)是對(dui)工藝(yi)參數(shu)進行優化(hua)，如控制(zhi)合(he)金(jin)成(cheng)分(fen)(fen)、施(shi)加外力場(chang)(chang)（磁(ci)場(chang)(chang)等(deng)）、優化(hua)鑄錠幾何形(xing)狀、適當加大(da)(da)冷卻速率等(deng)。

  宏觀(guan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)是大范圍內的(de)(de)(de)(de)成(cheng)分(fen)不均勻現(xian)象，按其(qi)表現(xian)形(xing)式(shi)可分(fen)為正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)、反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)和(he)(he)(he)比重(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)等(deng)。①. 正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)：對(dui)(dui)平衡分(fen)配系(xi)數o<1的(de)(de)(de)(de)合(he)(he)金(jin)系(xi)鑄錠先凝(ning)固(gu)的(de)(de)(de)(de)部(bu)(bu)分(fen)，其(qi)溶(rong)質(zhi)含量低于(yu)后(hou)凝(ning)固(gu)的(de)(de)(de)(de)部(bu)(bu)分(fen)。對(dui)(dui)ko>1的(de)(de)(de)(de)合(he)(he)金(jin)系(xi)則正(zheng)好相反(fan)，其(qi)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)程度(du)與凝(ning)固(gu)速率(lv)、液體對(dui)(dui)流以及溶(rong)質(zhi)擴散等(deng)條件有(you)關。②. 反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)：在ko<1的(de)(de)(de)(de)合(he)(he)金(jin)鑄錠中，其(qi)外層溶(rong)質(zhi)元(yuan)素高于(yu)內部(bu)(bu)，和(he)(he)(he)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)相反(fan)，故稱為反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)。③. 比重(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)：是由合(he)(he)金(jin)凝(ning)固(gu)時形(xing)成(cheng)的(de)(de)(de)(de)初晶(jing)相和(he)(he)(he)溶(rong)液之間的(de)(de)(de)(de)比重(zhong)顯著差別引(yin)起的(de)(de)(de)(de)一種宏觀(guan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)，主要存在于(yu)共晶(jing)系(xi)和(he)(he)(he)偏(pian)(pian)(pian)(pian)晶(jing)系(xi)合(he)(he)金(jin)中。如圖2-49所示(shi)，由于(yu)溶(rong)質(zhi)元(yuan)素濃(nong)度(du)相對(dui)(dui)低的(de)(de)(de)(de)等(deng)軸(zhou)晶(jing)沉積導致(zhi)在鑄錠的(de)(de)(de)(de)底(di)部(bu)(bu)出(chu)現(xian)負偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)；由于(yu)浮力和(he)(he)(he)在凝(ning)固(gu)的(de)(de)(de)(de)最后(hou)階段收縮所引(yin)起的(de)(de)(de)(de)晶(jing)間流動(dong)(dong)，在頂部(bu)(bu)會出(chu)現(xian)很嚴重(zhong)的(de)(de)(de)(de)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)（頂部(bu)(bu)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)）。A型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)是溶(rong)質(zhi)富集的(de)(de)(de)(de)等(deng)軸(zhou)晶(jing)帶，由溶(rong)質(zhi)受浮力作用(yong)流動(dong)(dong)穿過(guo)柱狀晶(jing)區，其(qi)方向(xiang)與等(deng)溫(wen)線移動(dong)(dong)速度(du)方向(xiang)一致(zhi)但速率(lv)更快所導致(zhi)。A型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)形(xing)狀與流動(dong)(dong)類(lei)型有(you)關。V型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)位于(yu)鑄錠中心(xin)，源于(yu)中心(xin)形(xing)成(cheng)等(deng)軸(zhou)晶(jing)區和(he)(he)(he)容易斷裂的(de)(de)(de)(de)連接疏(shu)松的(de)(de)(de)(de)網(wang)狀物的(de)(de)(de)(de)形(xing)成(cheng)，之后(hou)裂紋沿切(qie)應力面展開為V型，并(bing)且充滿(man)了(le)富集元(yuan)素的(de)(de)(de)(de)液相。而沿鑄錠側壁分(fen)布(bu)的(de)(de)(de)(de)帶狀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)(xi)則是由凝(ning)固(gu)過(guo)程初期的(de)(de)(de)(de)不穩定傳熱和(he)(he)(he)流動(dong)(dong)導致(zhi)的(de)(de)(de)(de)。

  對(dui)于宏(hong)(hong)觀(guan)偏(pian)析(xi)的(de)(de)研究(jiu)主要(yao)有實(shi)驗(yan)檢(jian)測和模(mo)擬(ni)計(ji)(ji)算(suan)(suan)兩(liang)種手段。實(shi)驗(yan)檢(jian)測包(bao)括硫(liu)印(yin)檢(jian)驗(yan)法(fa)、原位分(fen)析(xi)法(fa)、火(huo)花(hua)放(fang)電(dian)原子發射光譜法(fa)、鉆(zhan)孔取樣法(fa)以及化(hua)學分(fen)析(xi)法(fa)等(deng)。模(mo)擬(ni)計(ji)(ji)算(suan)(suan)是通過數值求解能(neng)量(liang)、動量(liang)以及溶質傳輸等(deng)數學模(mo)型，進而(er)探(tan)討元(yuan)素成分(fen)不均勻性(xing)的(de)(de)方法(fa)；進入20世紀后，人們對(dui)凝固過程(cheng)中的(de)(de)宏(hong)(hong)觀(guan)偏(pian)析(xi)現象進行了大量(liang)系統的(de)(de)研究(jiu)。Flemings研究(jiu)表(biao)明鑄錠中多種不同的(de)(de)宏(hong)(hong)觀(guan)偏(pian)析(xi)都可(ke)由凝固時的(de)(de)傳熱、流動和傳質過程(cheng)來定(ding)量(liang)描述(shu)，從而(er)為(wei)宏(hong)(hong)觀(guan)偏(pian)析(xi)的(de)(de)定(ding)量(liang)計(ji)(ji)算(suan)(suan)提供可(ke)能(neng)性(xing)，隨著(zhu)計(ji)(ji)算(suan)(suan)機(ji)計(ji)(ji)算(suan)(suan)能(neng)力(li)迅猛(meng)提升，宏(hong)(hong)觀(guan)偏(pian)析(xi)的(de)(de)模(mo)擬(ni)計(ji)(ji)算(suan)(suan)得到了迅速(su)發展，主要(yao)分(fen)為(wei)多區域法(fa)和連續介質法(fa)等(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。