根(gen)據相(xiang)(xiang)圖，多數合金(jin)元(yuan)素(su)在(zai)(zai)固(gu)(gu)相(xiang)(xiang)中(zhong)的(de)(de)溶解(jie)度要(yao)低(di)于液(ye)相(xiang)(xiang)，因此(ci)在(zai)(zai)凝(ning)(ning)固(gu)(gu)過(guo)程(cheng)中(zhong)溶質(zhi)原子不(bu)斷被排(pai)出到(dao)液(ye)相(xiang)(xiang)，這種固(gu)(gu)液(ye)界面兩側溶質(zhi)濃度的(de)(de)差異導(dao)致(zhi)合金(jin)凝(ning)(ning)固(gu)(gu)后溶質(zhi)元(yuan)素(su)成(cheng)分(fen)不(bu)均(jun)勻(yun)(yun)性，稱作偏(pian)析(xi)(xi)(xi)。溶質(zhi)元(yuan)素(su)分(fen)布不(bu)均(jun)勻(yun)(yun)性發(fa)生在(zai)(zai)微觀結(jie)構(gou)形成(cheng)范圍內（有10~100μm的(de)(de)樹狀(zhuang)枝(zhi)晶），此(ci)時為(wei)微觀偏(pian)析(xi)(xi)(xi)。溶質(zhi)元(yuan)素(su)通過(guo)對(dui)流傳質(zhi)等質(zhi)量傳輸，將導(dao)致(zhi)大范圍內成(cheng)分(fen)不(bu)均(jun)勻(yun)(yun)性，即形成(cheng)了宏(hong)觀偏(pian)析(xi)(xi)(xi)。宏(hong)觀偏(pian)析(xi)(xi)(xi)可以認為(wei)是由凝(ning)(ning)固(gu)(gu)過(guo)程(cheng)中(zhong)液(ye)體和固(gu)(gu)體相(xiang)(xiang)對(dui)運動和溶質(zhi)再分(fen)配(pei)過(guo)程(cheng)共同導(dao)致(zhi)的(de)(de)。此(ci)外(wai)，在(zai)(zai)凝(ning)(ning)固(gu)(gu)早(zao)期所形成(cheng)的(de)(de)固(gu)(gu)體相(xiang)(xiang)或非金(jin)屬夾雜的(de)(de)漂浮(fu)和下(xia)沉也會造(zao)成(cheng)宏(hong)觀偏(pian)析(xi)(xi)(xi)。一(yi)般認為(wei)在(zai)(zai)合金(jin)鑄件或鑄錠內，從幾(ji)毫米到(dao)幾(ji)厘米甚至幾(ji)米范圍內濃度變化(hua)為(wei)宏(hong)觀偏(pian)析(xi)(xi)(xi)。因為(wei)溶質(zhi)在(zai)(zai)固(gu)(gu)態中(zhong)的(de)(de)擴散(san)系數很(hen)低(di)，而成(cheng)分(fen)不(bu)均(jun)勻(yun)(yun)性范圍又很(hen)大，所以在(zai)(zai)凝(ning)(ning)固(gu)(gu)完(wan)成(cheng)后，宏(hong)觀偏(pian)析(xi)(xi)(xi)很(hen)難通過(guo)加(jia)工(gong)處理來消除，因此(ci)抑制(zhi)宏(hong)觀偏(pian)析(xi)(xi)(xi)的(de)(de)產生主(zhu)要(yao)是對(dui)工(gong)藝參數進行優化(hua)，如控制(zhi)合金(jin)成(cheng)分(fen)、施(shi)加(jia)外(wai)力場(chang)（磁場(chang)等）、優化(hua)鑄錠幾(ji)何形狀(zhuang)、適當加(jia)大冷卻速率等。

  宏觀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)是(shi)大范圍內(nei)的(de)(de)(de)(de)(de)成(cheng)分(fen)(fen)不均勻現(xian)(xian)象，按其(qi)(qi)(qi)表現(xian)(xian)形(xing)(xing)(xing)式可分(fen)(fen)為(wei)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)、反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)和(he)(he)比重(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)等。①. 正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)：對(dui)平衡(heng)分(fen)(fen)配(pei)系(xi)(xi)數(shu)o<1的(de)(de)(de)(de)(de)合金(jin)系(xi)(xi)鑄錠先凝固(gu)的(de)(de)(de)(de)(de)部分(fen)(fen)，其(qi)(qi)(qi)溶(rong)(rong)(rong)質(zhi)含量低于(yu)(yu)后(hou)凝固(gu)的(de)(de)(de)(de)(de)部分(fen)(fen)。對(dui)ko>1的(de)(de)(de)(de)(de)合金(jin)系(xi)(xi)則正(zheng)好(hao)相反(fan)，其(qi)(qi)(qi)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)程(cheng)度(du)與凝固(gu)速率、液體對(dui)流(liu)(liu)(liu)以及溶(rong)(rong)(rong)質(zhi)擴散(san)等條件有(you)關。②. 反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)：在(zai)ko<1的(de)(de)(de)(de)(de)合金(jin)鑄錠中，其(qi)(qi)(qi)外層溶(rong)(rong)(rong)質(zhi)元素高(gao)于(yu)(yu)內(nei)部，和(he)(he)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)相反(fan)，故稱(cheng)為(wei)反(fan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)。③. 比重(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)：是(shi)由(you)(you)(you)合金(jin)凝固(gu)時形(xing)(xing)(xing)成(cheng)的(de)(de)(de)(de)(de)初晶(jing)(jing)相和(he)(he)溶(rong)(rong)(rong)液之間的(de)(de)(de)(de)(de)比重(zhong)顯著差別(bie)引(yin)起的(de)(de)(de)(de)(de)一種宏觀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)，主要存在(zai)于(yu)(yu)共晶(jing)(jing)系(xi)(xi)和(he)(he)偏(pian)(pian)(pian)(pian)晶(jing)(jing)系(xi)(xi)合金(jin)中。如(ru)圖2-49所示，由(you)(you)(you)于(yu)(yu)溶(rong)(rong)(rong)質(zhi)元素濃度(du)相對(dui)低的(de)(de)(de)(de)(de)等軸(zhou)晶(jing)(jing)沉積導致在(zai)鑄錠的(de)(de)(de)(de)(de)底部出(chu)現(xian)(xian)負偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)；由(you)(you)(you)于(yu)(yu)浮(fu)力和(he)(he)在(zai)凝固(gu)的(de)(de)(de)(de)(de)最后(hou)階段收縮所引(yin)起的(de)(de)(de)(de)(de)晶(jing)(jing)間流(liu)(liu)(liu)動，在(zai)頂(ding)部會(hui)出(chu)現(xian)(xian)很嚴重(zhong)的(de)(de)(de)(de)(de)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)（頂(ding)部偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)）。A型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)是(shi)溶(rong)(rong)(rong)質(zhi)富集的(de)(de)(de)(de)(de)等軸(zhou)晶(jing)(jing)帶，由(you)(you)(you)溶(rong)(rong)(rong)質(zhi)受浮(fu)力作(zuo)用流(liu)(liu)(liu)動穿過柱狀(zhuang)(zhuang)晶(jing)(jing)區，其(qi)(qi)(qi)方向與等溫線移動速度(du)方向一致但速率更快所導致。A型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)形(xing)(xing)(xing)狀(zhuang)(zhuang)與流(liu)(liu)(liu)動類型有(you)關。V型偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)位于(yu)(yu)鑄錠中心(xin)，源于(yu)(yu)中心(xin)形(xing)(xing)(xing)成(cheng)等軸(zhou)晶(jing)(jing)區和(he)(he)容易(yi)斷(duan)裂的(de)(de)(de)(de)(de)連接疏松的(de)(de)(de)(de)(de)網狀(zhuang)(zhuang)物(wu)的(de)(de)(de)(de)(de)形(xing)(xing)(xing)成(cheng)，之后(hou)裂紋沿切應力面展開為(wei)V型，并且充滿了(le)富集元素的(de)(de)(de)(de)(de)液相。而沿鑄錠側壁分(fen)(fen)布的(de)(de)(de)(de)(de)帶狀(zhuang)(zhuang)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)則是(shi)由(you)(you)(you)凝固(gu)過程(cheng)初期的(de)(de)(de)(de)(de)不穩定傳(chuan)熱和(he)(he)流(liu)(liu)(liu)動導致的(de)(de)(de)(de)(de)。

  對(dui)于宏觀(guan)偏析(xi)的研(yan)究(jiu)(jiu)主要有(you)實(shi)驗(yan)檢(jian)測和(he)(he)模(mo)擬(ni)計算(suan)兩種手段。實(shi)驗(yan)檢(jian)測包括硫印檢(jian)驗(yan)法(fa)(fa)、原(yuan)位分(fen)析(xi)法(fa)(fa)、火花放(fang)電(dian)原(yuan)子發射光譜法(fa)(fa)、鉆孔取樣(yang)法(fa)(fa)以及化(hua)學(xue)分(fen)析(xi)法(fa)(fa)等。模(mo)擬(ni)計算(suan)是通過數值求解能量、動(dong)(dong)量以及溶質(zhi)(zhi)(zhi)傳(chuan)輸(shu)等數學(xue)模(mo)型，進而(er)探討元素成分(fen)不均勻性的方法(fa)(fa)；進入20世紀后，人們(men)對(dui)凝(ning)固過程中的宏觀(guan)偏析(xi)現象進行了大量系統的研(yan)究(jiu)(jiu)。Flemings研(yan)究(jiu)(jiu)表明鑄錠(ding)中多種不同的宏觀(guan)偏析(xi)都可由凝(ning)固時的傳(chuan)熱、流動(dong)(dong)和(he)(he)傳(chuan)質(zhi)(zhi)(zhi)過程來定量描述(shu)，從而(er)為(wei)宏觀(guan)偏析(xi)的定量計算(suan)提供可能性，隨著計算(suan)機計算(suan)能力(li)迅猛提升(sheng)，宏觀(guan)偏析(xi)的模(mo)擬(ni)計算(suan)得到(dao)了迅速發展，主要分(fen)為(wei)多區(qu)域法(fa)(fa)和(he)(he)連續介質(zhi)(zhi)(zhi)法(fa)(fa)等。

  對于高氮不銹鋼(gang)，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。