根據相(xiang)圖，多數(shu)合(he)金元(yuan)素(su)(su)在(zai)(zai)(zai)(zai)固(gu)相(xiang)中(zhong)(zhong)的溶(rong)解(jie)度(du)(du)要低于液(ye)相(xiang)，因此(ci)在(zai)(zai)(zai)(zai)凝(ning)固(gu)過(guo)程中(zhong)(zhong)溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)原子(zi)不(bu)斷被(bei)排出到(dao)液(ye)相(xiang)，這種(zhong)固(gu)液(ye)界面兩側溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)濃度(du)(du)的差異導致合(he)金凝(ning)固(gu)后(hou)溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)元(yuan)素(su)(su)成分(fen)(fen)不(bu)均(jun)勻(yun)性(xing)，稱作偏(pian)(pian)析(xi)。溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)元(yuan)素(su)(su)分(fen)(fen)布不(bu)均(jun)勻(yun)性(xing)發生(sheng)在(zai)(zai)(zai)(zai)微(wei)(wei)觀(guan)結(jie)構形成范圍(wei)(wei)(wei)內（有(you)10~100μm的樹狀(zhuang)枝晶），此(ci)時為微(wei)(wei)觀(guan)偏(pian)(pian)析(xi)。溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)元(yuan)素(su)(su)通過(guo)對流傳(chuan)質(zhi)(zhi)(zhi)(zhi)(zhi)等質(zhi)(zhi)(zhi)(zhi)(zhi)量傳(chuan)輸(shu)，將導致大(da)范圍(wei)(wei)(wei)內成分(fen)(fen)不(bu)均(jun)勻(yun)性(xing)，即形成了宏(hong)觀(guan)偏(pian)(pian)析(xi)。宏(hong)觀(guan)偏(pian)(pian)析(xi)可以認為是(shi)由(you)凝(ning)固(gu)過(guo)程中(zhong)(zhong)液(ye)體和(he)固(gu)體相(xiang)對運動和(he)溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)再分(fen)(fen)配過(guo)程共同導致的。此(ci)外(wai)，在(zai)(zai)(zai)(zai)凝(ning)固(gu)早期所(suo)形成的固(gu)體相(xiang)或非金屬(shu)夾雜的漂浮和(he)下沉也(ye)會造成宏(hong)觀(guan)偏(pian)(pian)析(xi)。一般認為在(zai)(zai)(zai)(zai)合(he)金鑄件或鑄錠(ding)(ding)內，從幾毫(hao)米(mi)到(dao)幾厘米(mi)甚(shen)至幾米(mi)范圍(wei)(wei)(wei)內濃度(du)(du)變化為宏(hong)觀(guan)偏(pian)(pian)析(xi)。因為溶(rong)質(zhi)(zhi)(zhi)(zhi)(zhi)在(zai)(zai)(zai)(zai)固(gu)態(tai)中(zhong)(zhong)的擴散系(xi)數(shu)很(hen)低，而成分(fen)(fen)不(bu)均(jun)勻(yun)性(xing)范圍(wei)(wei)(wei)又很(hen)大(da)，所(suo)以在(zai)(zai)(zai)(zai)凝(ning)固(gu)完成后(hou)，宏(hong)觀(guan)偏(pian)(pian)析(xi)很(hen)難(nan)通過(guo)加(jia)工(gong)處(chu)理來消除，因此(ci)抑(yi)制宏(hong)觀(guan)偏(pian)(pian)析(xi)的產生(sheng)主要是(shi)對工(gong)藝參數(shu)進行(xing)優(you)化，如控(kong)制合(he)金成分(fen)(fen)、施加(jia)外(wai)力場(chang)（磁場(chang)等）、優(you)化鑄錠(ding)(ding)幾何形狀(zhuang)、適(shi)當加(jia)大(da)冷卻(que)速率等。

  宏觀(guan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)是(shi)大(da)范圍內的(de)(de)(de)(de)成分(fen)(fen)不均勻現(xian)象，按其(qi)表現(xian)形(xing)(xing)(xing)式可分(fen)(fen)為正(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)比重(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)等(deng)。①. 正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)：對(dui)平衡分(fen)(fen)配系(xi)數o<1的(de)(de)(de)(de)合金系(xi)鑄錠(ding)先凝固(gu)(gu)的(de)(de)(de)(de)部(bu)分(fen)(fen)，其(qi)溶(rong)質(zhi)(zhi)含(han)量(liang)低于(yu)(yu)(yu)(yu)后(hou)凝固(gu)(gu)的(de)(de)(de)(de)部(bu)分(fen)(fen)。對(dui)ko>1的(de)(de)(de)(de)合金系(xi)則(ze)正(zheng)好相反(fan)，其(qi)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)程(cheng)度(du)(du)與凝固(gu)(gu)速(su)率、液體對(dui)流(liu)以及溶(rong)質(zhi)(zhi)擴散(san)等(deng)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)合金鑄錠(ding)中，其(qi)外層溶(rong)質(zhi)(zhi)元(yuan)素(su)(su)高(gao)于(yu)(yu)(yu)(yu)內部(bu)，和(he)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)相反(fan)，故(gu)稱為反(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)合金凝固(gu)(gu)時形(xing)(xing)(xing)成的(de)(de)(de)(de)初晶(jing)相和(he)溶(rong)液之間的(de)(de)(de)(de)比重(zhong)顯著差別引起的(de)(de)(de)(de)一種宏觀(guan)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)，主要存在(zai)于(yu)(yu)(yu)(yu)共晶(jing)系(xi)和(he)偏(pian)(pian)(pian)(pian)晶(jing)系(xi)合金中。如圖2-49所示，由(you)于(yu)(yu)(yu)(yu)溶(rong)質(zhi)(zhi)元(yuan)素(su)(su)濃度(du)(du)相對(dui)低的(de)(de)(de)(de)等(deng)軸晶(jing)沉(chen)積(ji)導致(zhi)在(zai)鑄錠(ding)的(de)(de)(de)(de)底部(bu)出現(xian)負偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)；由(you)于(yu)(yu)(yu)(yu)浮(fu)力和(he)在(zai)凝固(gu)(gu)的(de)(de)(de)(de)最后(hou)階段收縮所引起的(de)(de)(de)(de)晶(jing)間流(liu)動，在(zai)頂(ding)(ding)部(bu)會(hui)出現(xian)很嚴重(zhong)的(de)(de)(de)(de)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)（頂(ding)(ding)部(bu)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)）。A型(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)是(shi)溶(rong)質(zhi)(zhi)富(fu)集的(de)(de)(de)(de)等(deng)軸晶(jing)帶，由(you)溶(rong)質(zhi)(zhi)受浮(fu)力作用流(liu)動穿過柱狀晶(jing)區，其(qi)方向與等(deng)溫線移動速(su)度(du)(du)方向一致(zhi)但速(su)率更(geng)快所導致(zhi)。A型(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)形(xing)(xing)(xing)狀與流(liu)動類型(xing)有(you)關。V型(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)位(wei)于(yu)(yu)(yu)(yu)鑄錠(ding)中心(xin)，源于(yu)(yu)(yu)(yu)中心(xin)形(xing)(xing)(xing)成等(deng)軸晶(jing)區和(he)容易斷裂的(de)(de)(de)(de)連(lian)接(jie)疏松的(de)(de)(de)(de)網狀物的(de)(de)(de)(de)形(xing)(xing)(xing)成，之后(hou)裂紋(wen)沿(yan)切應力面(mian)展開為V型(xing)，并(bing)且充滿了富(fu)集元(yuan)素(su)(su)的(de)(de)(de)(de)液相。而沿(yan)鑄錠(ding)側壁分(fen)(fen)布的(de)(de)(de)(de)帶狀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)(xi)(xi)則(ze)是(shi)由(you)凝固(gu)(gu)過程(cheng)初期的(de)(de)(de)(de)不穩定(ding)傳(chuan)熱和(he)流(liu)動導致(zhi)的(de)(de)(de)(de)。

  對(dui)于宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)析的(de)(de)研究(jiu)(jiu)主(zhu)要有(you)實驗檢(jian)測(ce)和(he)模擬(ni)計(ji)(ji)算(suan)兩種(zhong)手(shou)段(duan)。實驗檢(jian)測(ce)包(bao)括硫印檢(jian)驗法(fa)(fa)(fa)、原位分析法(fa)(fa)(fa)、火(huo)花放(fang)電原子發射光譜法(fa)(fa)(fa)、鉆(zhan)孔取樣法(fa)(fa)(fa)以(yi)及化學分析法(fa)(fa)(fa)等。模擬(ni)計(ji)(ji)算(suan)是通過(guo)數(shu)值求(qiu)解能量(liang)、動量(liang)以(yi)及溶質傳輸等數(shu)學模型，進而探討元(yuan)素成分不均勻(yun)性的(de)(de)方法(fa)(fa)(fa)；進入(ru)20世紀后，人們對(dui)凝(ning)固(gu)(gu)過(guo)程(cheng)中的(de)(de)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)析現象進行了(le)大(da)量(liang)系(xi)統(tong)的(de)(de)研究(jiu)(jiu)。Flemings研究(jiu)(jiu)表明鑄錠中多(duo)種(zhong)不同的(de)(de)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)析都可由凝(ning)固(gu)(gu)時的(de)(de)傳熱、流動和(he)傳質過(guo)程(cheng)來定(ding)(ding)量(liang)描述，從而為宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)析的(de)(de)定(ding)(ding)量(liang)計(ji)(ji)算(suan)提供可能性，隨著計(ji)(ji)算(suan)機計(ji)(ji)算(suan)能力迅(xun)猛提升，宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)析的(de)(de)模擬(ni)計(ji)(ji)算(suan)得到(dao)了(le)迅(xun)速發展，主(zhu)要分為多(duo)區域法(fa)(fa)(fa)和(he)連續介(jie)質法(fa)(fa)(fa)等。

  對于高氮(dan)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。