根據相(xiang)圖，多(duo)數(shu)合(he)金(jin)(jin)元素(su)在(zai)固(gu)相(xiang)中的(de)(de)(de)溶解度要(yao)低(di)于液(ye)相(xiang)，因(yin)此(ci)在(zai)凝固(gu)過程(cheng)中溶質(zhi)原子不斷被排(pai)出(chu)到(dao)液(ye)相(xiang)，這種(zhong)固(gu)液(ye)界面兩側溶質(zhi)濃度的(de)(de)(de)差(cha)異(yi)導致(zhi)合(he)金(jin)(jin)凝固(gu)后(hou)溶質(zhi)元素(su)成(cheng)(cheng)分(fen)不均勻(yun)(yun)性，稱作偏(pian)(pian)(pian)析(xi)(xi)。溶質(zhi)元素(su)分(fen)布不均勻(yun)(yun)性發生(sheng)在(zai)微觀(guan)(guan)結構形成(cheng)(cheng)范(fan)圍(wei)內(nei)（有10~100μm的(de)(de)(de)樹狀(zhuang)枝(zhi)晶(jing)），此(ci)時為(wei)微觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)。溶質(zhi)元素(su)通(tong)過對流傳質(zhi)等(deng)質(zhi)量傳輸，將導致(zhi)大(da)范(fan)圍(wei)內(nei)成(cheng)(cheng)分(fen)不均勻(yun)(yun)性，即形成(cheng)(cheng)了(le)宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)。宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)可(ke)以(yi)認為(wei)是(shi)由凝固(gu)過程(cheng)中液(ye)體和固(gu)體相(xiang)對運動和溶質(zhi)再分(fen)配過程(cheng)共同導致(zhi)的(de)(de)(de)。此(ci)外(wai)，在(zai)凝固(gu)早期所(suo)形成(cheng)(cheng)的(de)(de)(de)固(gu)體相(xiang)或非金(jin)(jin)屬(shu)夾雜的(de)(de)(de)漂(piao)浮和下沉也會造成(cheng)(cheng)宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)。一般認為(wei)在(zai)合(he)金(jin)(jin)鑄件或鑄錠(ding)內(nei)，從幾(ji)毫米到(dao)幾(ji)厘米甚(shen)至幾(ji)米范(fan)圍(wei)內(nei)濃度變化為(wei)宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)。因(yin)為(wei)溶質(zhi)在(zai)固(gu)態中的(de)(de)(de)擴散系數(shu)很低(di)，而(er)成(cheng)(cheng)分(fen)不均勻(yun)(yun)性范(fan)圍(wei)又很大(da)，所(suo)以(yi)在(zai)凝固(gu)完成(cheng)(cheng)后(hou)，宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)很難通(tong)過加工(gong)(gong)處理來(lai)消除，因(yin)此(ci)抑制宏觀(guan)(guan)偏(pian)(pian)(pian)析(xi)(xi)的(de)(de)(de)產生(sheng)主要(yao)是(shi)對工(gong)(gong)藝參數(shu)進行優化，如控制合(he)金(jin)(jin)成(cheng)(cheng)分(fen)、施(shi)加外(wai)力場(chang)（磁場(chang)等(deng)）、優化鑄錠(ding)幾(ji)何形狀(zhuang)、適當加大(da)冷卻(que)速(su)率等(deng)。

  宏觀偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)是大范圍內的(de)(de)成(cheng)(cheng)分(fen)(fen)不均勻現(xian)(xian)象，按其表現(xian)(xian)形(xing)(xing)式可分(fen)(fen)為(wei)正(zheng)(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)、反(fan)(fan)(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)和(he)(he)(he)比(bi)重偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)等(deng)(deng)。①. 正(zheng)(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)：對平衡分(fen)(fen)配系(xi)(xi)數o<1的(de)(de)合(he)金(jin)(jin)系(xi)(xi)鑄錠(ding)先(xian)凝(ning)(ning)(ning)固的(de)(de)部(bu)(bu)(bu)分(fen)(fen)，其溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)含量低(di)(di)于(yu)(yu)(yu)(yu)后凝(ning)(ning)(ning)固的(de)(de)部(bu)(bu)(bu)分(fen)(fen)。對ko>1的(de)(de)合(he)金(jin)(jin)系(xi)(xi)則正(zheng)(zheng)好相(xiang)反(fan)(fan)(fan)，其偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)程度與凝(ning)(ning)(ning)固速(su)率、液體對流以及溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)擴散(san)等(deng)(deng)條件有(you)(you)關(guan)。②. 反(fan)(fan)(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)：在(zai)ko<1的(de)(de)合(he)金(jin)(jin)鑄錠(ding)中，其外(wai)層(ceng)溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)元(yuan)素高于(yu)(yu)(yu)(yu)內部(bu)(bu)(bu)，和(he)(he)(he)正(zheng)(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)相(xiang)反(fan)(fan)(fan)，故稱為(wei)反(fan)(fan)(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)。③. 比(bi)重偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)：是由(you)(you)(you)合(he)金(jin)(jin)凝(ning)(ning)(ning)固時形(xing)(xing)成(cheng)(cheng)的(de)(de)初晶(jing)(jing)相(xiang)和(he)(he)(he)溶(rong)(rong)液之間的(de)(de)比(bi)重顯著(zhu)差別(bie)引起的(de)(de)一種宏觀偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)，主要存在(zai)于(yu)(yu)(yu)(yu)共晶(jing)(jing)系(xi)(xi)和(he)(he)(he)偏(pian)(pian)(pian)(pian)(pian)(pian)晶(jing)(jing)系(xi)(xi)合(he)金(jin)(jin)中。如圖2-49所示，由(you)(you)(you)于(yu)(yu)(yu)(yu)溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)元(yuan)素濃(nong)度相(xiang)對低(di)(di)的(de)(de)等(deng)(deng)軸(zhou)晶(jing)(jing)沉(chen)積導(dao)致(zhi)(zhi)在(zai)鑄錠(ding)的(de)(de)底部(bu)(bu)(bu)出現(xian)(xian)負偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)；由(you)(you)(you)于(yu)(yu)(yu)(yu)浮力和(he)(he)(he)在(zai)凝(ning)(ning)(ning)固的(de)(de)最后階段收縮所引起的(de)(de)晶(jing)(jing)間流動(dong)，在(zai)頂(ding)部(bu)(bu)(bu)會出現(xian)(xian)很嚴重的(de)(de)正(zheng)(zheng)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)（頂(ding)部(bu)(bu)(bu)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)）。A型偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)是溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)富集的(de)(de)等(deng)(deng)軸(zhou)晶(jing)(jing)帶(dai)(dai)，由(you)(you)(you)溶(rong)(rong)質(zhi)(zhi)(zhi)(zhi)受浮力作用流動(dong)穿過(guo)柱狀(zhuang)(zhuang)晶(jing)(jing)區，其方(fang)向(xiang)與等(deng)(deng)溫線移(yi)動(dong)速(su)度方(fang)向(xiang)一致(zhi)(zhi)但(dan)速(su)率更快所導(dao)致(zhi)(zhi)。A型偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)形(xing)(xing)狀(zhuang)(zhuang)與流動(dong)類(lei)型有(you)(you)關(guan)。V型偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)位(wei)于(yu)(yu)(yu)(yu)鑄錠(ding)中心，源于(yu)(yu)(yu)(yu)中心形(xing)(xing)成(cheng)(cheng)等(deng)(deng)軸(zhou)晶(jing)(jing)區和(he)(he)(he)容易(yi)斷裂(lie)的(de)(de)連接疏松的(de)(de)網狀(zhuang)(zhuang)物的(de)(de)形(xing)(xing)成(cheng)(cheng)，之后裂(lie)紋沿(yan)切應(ying)力面(mian)展(zhan)開(kai)為(wei)V型，并(bing)且充滿了富集元(yuan)素的(de)(de)液相(xiang)。而(er)沿(yan)鑄錠(ding)側壁分(fen)(fen)布的(de)(de)帶(dai)(dai)狀(zhuang)(zhuang)偏(pian)(pian)(pian)(pian)(pian)(pian)析(xi)則是由(you)(you)(you)凝(ning)(ning)(ning)固過(guo)程初期的(de)(de)不穩定(ding)傳熱和(he)(he)(he)流動(dong)導(dao)致(zhi)(zhi)的(de)(de)。

  對于宏(hong)(hong)觀(guan)偏(pian)(pian)(pian)析的(de)研究主(zhu)(zhu)要有(you)實驗檢(jian)測和(he)模(mo)(mo)擬計(ji)(ji)算(suan)兩種手段。實驗檢(jian)測包括硫(liu)印檢(jian)驗法(fa)(fa)(fa)、原位分析法(fa)(fa)(fa)、火花放電原子發(fa)射光譜法(fa)(fa)(fa)、鉆孔取樣法(fa)(fa)(fa)以及化(hua)學(xue)分析法(fa)(fa)(fa)等(deng)。模(mo)(mo)擬計(ji)(ji)算(suan)是通過(guo)數(shu)值(zhi)求解能量、動量以及溶質(zhi)傳輸等(deng)數(shu)學(xue)模(mo)(mo)型(xing)，進(jin)而探討元(yuan)素成分不(bu)均勻性的(de)方(fang)法(fa)(fa)(fa)；進(jin)入20世紀后，人們對凝(ning)固(gu)(gu)過(guo)程中的(de)宏(hong)(hong)觀(guan)偏(pian)(pian)(pian)析現象進(jin)行(xing)了(le)大量系統(tong)的(de)研究。Flemings研究表(biao)明(ming)鑄錠(ding)中多種不(bu)同的(de)宏(hong)(hong)觀(guan)偏(pian)(pian)(pian)析都可由凝(ning)固(gu)(gu)時的(de)傳熱、流動和(he)傳質(zhi)過(guo)程來(lai)定量描述，從而為(wei)宏(hong)(hong)觀(guan)偏(pian)(pian)(pian)析的(de)定量計(ji)(ji)算(suan)提供可能性，隨(sui)著計(ji)(ji)算(suan)機計(ji)(ji)算(suan)能力迅猛(meng)提升(sheng)，宏(hong)(hong)觀(guan)偏(pian)(pian)(pian)析的(de)模(mo)(mo)擬計(ji)(ji)算(suan)得到了(le)迅速發(fa)展，主(zhu)(zhu)要分為(wei)多區域法(fa)(fa)(fa)和(he)連續(xu)介質(zhi)法(fa)(fa)(fa)等(deng)。

  對于高氮不銹鋼，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。