根據相(xiang)圖(tu)，多數合(he)金(jin)(jin)元(yuan)(yuan)素(su)在固(gu)(gu)相(xiang)中(zhong)的(de)(de)溶(rong)(rong)(rong)(rong)解度要低于液相(xiang)，因此在凝固(gu)(gu)過(guo)(guo)程中(zhong)溶(rong)(rong)(rong)(rong)質(zhi)原子不(bu)斷被(bei)排出到(dao)液相(xiang)，這種固(gu)(gu)液界面兩側溶(rong)(rong)(rong)(rong)質(zhi)濃度的(de)(de)差異(yi)導(dao)致(zhi)合(he)金(jin)(jin)凝固(gu)(gu)后溶(rong)(rong)(rong)(rong)質(zhi)元(yuan)(yuan)素(su)成(cheng)(cheng)(cheng)(cheng)分(fen)不(bu)均勻性(xing)，稱(cheng)作偏(pian)(pian)(pian)析。溶(rong)(rong)(rong)(rong)質(zhi)元(yuan)(yuan)素(su)分(fen)布不(bu)均勻性(xing)發生在微(wei)觀(guan)(guan)結(jie)構形成(cheng)(cheng)(cheng)(cheng)范圍內(nei)（有10~100μm的(de)(de)樹狀枝晶），此時為(wei)微(wei)觀(guan)(guan)偏(pian)(pian)(pian)析。溶(rong)(rong)(rong)(rong)質(zhi)元(yuan)(yuan)素(su)通(tong)過(guo)(guo)對流傳質(zhi)等質(zhi)量傳輸，將(jiang)導(dao)致(zhi)大(da)(da)范圍內(nei)成(cheng)(cheng)(cheng)(cheng)分(fen)不(bu)均勻性(xing)，即(ji)形成(cheng)(cheng)(cheng)(cheng)了宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析。宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析可以(yi)認為(wei)是(shi)由凝固(gu)(gu)過(guo)(guo)程中(zhong)液體(ti)(ti)和固(gu)(gu)體(ti)(ti)相(xiang)對運(yun)動(dong)和溶(rong)(rong)(rong)(rong)質(zhi)再分(fen)配(pei)過(guo)(guo)程共同導(dao)致(zhi)的(de)(de)。此外(wai)，在凝固(gu)(gu)早期(qi)所形成(cheng)(cheng)(cheng)(cheng)的(de)(de)固(gu)(gu)體(ti)(ti)相(xiang)或(huo)非金(jin)(jin)屬夾(jia)雜的(de)(de)漂浮和下(xia)沉也會造成(cheng)(cheng)(cheng)(cheng)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析。一般認為(wei)在合(he)金(jin)(jin)鑄(zhu)件或(huo)鑄(zhu)錠內(nei)，從幾毫米到(dao)幾厘米甚至(zhi)幾米范圍內(nei)濃度變化為(wei)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析。因為(wei)溶(rong)(rong)(rong)(rong)質(zhi)在固(gu)(gu)態中(zhong)的(de)(de)擴散(san)系數很(hen)低，而成(cheng)(cheng)(cheng)(cheng)分(fen)不(bu)均勻性(xing)范圍又很(hen)大(da)(da)，所以(yi)在凝固(gu)(gu)完成(cheng)(cheng)(cheng)(cheng)后，宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析很(hen)難通(tong)過(guo)(guo)加工處理來消除，因此抑制(zhi)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)析的(de)(de)產生主(zhu)要是(shi)對工藝參數進行優化，如控(kong)制(zhi)合(he)金(jin)(jin)成(cheng)(cheng)(cheng)(cheng)分(fen)、施加外(wai)力場(chang)（磁(ci)場(chang)等）、優化鑄(zhu)錠幾何形狀、適當加大(da)(da)冷卻速率等。

  宏(hong)觀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是大范圍(wei)內的(de)(de)(de)(de)成(cheng)分(fen)不均勻現(xian)象，按(an)其(qi)表現(xian)形式可(ke)分(fen)為正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)、反偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)和(he)(he)(he)比重(zhong)(zhong)(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)等(deng)。①. 正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：對(dui)平衡分(fen)配系數o<1的(de)(de)(de)(de)合(he)金(jin)系鑄錠(ding)先凝(ning)固(gu)(gu)的(de)(de)(de)(de)部(bu)(bu)分(fen)，其(qi)溶(rong)質(zhi)含(han)量低(di)于(yu)(yu)后凝(ning)固(gu)(gu)的(de)(de)(de)(de)部(bu)(bu)分(fen)。對(dui)ko>1的(de)(de)(de)(de)合(he)金(jin)系則正(zheng)好(hao)相(xiang)反，其(qi)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)程度(du)與凝(ning)固(gu)(gu)速(su)(su)(su)率、液(ye)體對(dui)流(liu)以及溶(rong)質(zhi)擴散(san)等(deng)條(tiao)件有(you)關。②. 反偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：在ko<1的(de)(de)(de)(de)合(he)金(jin)鑄錠(ding)中，其(qi)外層溶(rong)質(zhi)元素(su)高于(yu)(yu)內部(bu)(bu)，和(he)(he)(he)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)相(xiang)反，故稱為反偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)。③. 比重(zhong)(zhong)(zhong)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：是由(you)(you)合(he)金(jin)凝(ning)固(gu)(gu)時形成(cheng)的(de)(de)(de)(de)初(chu)晶(jing)相(xiang)和(he)(he)(he)溶(rong)液(ye)之間的(de)(de)(de)(de)比重(zhong)(zhong)(zhong)顯(xian)著差(cha)別引起的(de)(de)(de)(de)一種宏(hong)觀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)，主要(yao)存在于(yu)(yu)共晶(jing)系和(he)(he)(he)偏(pian)(pian)(pian)(pian)晶(jing)系合(he)金(jin)中。如圖(tu)2-49所示，由(you)(you)于(yu)(yu)溶(rong)質(zhi)元素(su)濃度(du)相(xiang)對(dui)低(di)的(de)(de)(de)(de)等(deng)軸(zhou)(zhou)晶(jing)沉積(ji)導致(zhi)在鑄錠(ding)的(de)(de)(de)(de)底(di)部(bu)(bu)出現(xian)負偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)；由(you)(you)于(yu)(yu)浮(fu)(fu)力和(he)(he)(he)在凝(ning)固(gu)(gu)的(de)(de)(de)(de)最后階(jie)段收縮(suo)所引起的(de)(de)(de)(de)晶(jing)間流(liu)動，在頂(ding)部(bu)(bu)會(hui)出現(xian)很嚴重(zhong)(zhong)(zhong)的(de)(de)(de)(de)正(zheng)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)（頂(ding)部(bu)(bu)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)）。A型(xing)(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是溶(rong)質(zhi)富(fu)集的(de)(de)(de)(de)等(deng)軸(zhou)(zhou)晶(jing)帶(dai)，由(you)(you)溶(rong)質(zhi)受浮(fu)(fu)力作用流(liu)動穿過(guo)柱狀晶(jing)區，其(qi)方向與等(deng)溫(wen)線(xian)移(yi)動速(su)(su)(su)度(du)方向一致(zhi)但速(su)(su)(su)率更快所導致(zhi)。A型(xing)(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)形狀與流(liu)動類型(xing)(xing)有(you)關。V型(xing)(xing)偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)位(wei)于(yu)(yu)鑄錠(ding)中心(xin)，源于(yu)(yu)中心(xin)形成(cheng)等(deng)軸(zhou)(zhou)晶(jing)區和(he)(he)(he)容易斷裂(lie)的(de)(de)(de)(de)連(lian)接疏松的(de)(de)(de)(de)網(wang)狀物的(de)(de)(de)(de)形成(cheng)，之后裂(lie)紋沿切(qie)應力面(mian)展開為V型(xing)(xing)，并且充(chong)滿了富(fu)集元素(su)的(de)(de)(de)(de)液(ye)相(xiang)。而(er)沿鑄錠(ding)側(ce)壁(bi)分(fen)布的(de)(de)(de)(de)帶(dai)狀偏(pian)(pian)(pian)(pian)析(xi)(xi)(xi)則是由(you)(you)凝(ning)固(gu)(gu)過(guo)程初(chu)期的(de)(de)(de)(de)不穩(wen)定傳熱和(he)(he)(he)流(liu)動導致(zhi)的(de)(de)(de)(de)。

  對于宏(hong)觀(guan)(guan)偏(pian)(pian)析的研究(jiu)主要(yao)有實(shi)驗(yan)(yan)檢測和(he)模(mo)(mo)擬(ni)(ni)計(ji)算(suan)兩種手段。實(shi)驗(yan)(yan)檢測包括硫印檢驗(yan)(yan)法、原(yuan)(yuan)位分(fen)析法、火花放(fang)電原(yuan)(yuan)子(zi)發(fa)(fa)射光譜(pu)法、鉆孔(kong)取樣法以及(ji)化(hua)學(xue)分(fen)析法等。模(mo)(mo)擬(ni)(ni)計(ji)算(suan)是(shi)通過(guo)數值求解能(neng)量(liang)、動量(liang)以及(ji)溶質傳輸等數學(xue)模(mo)(mo)型，進而(er)探討(tao)元(yuan)素成分(fen)不均勻性(xing)的方法；進入20世(shi)紀后，人們對凝固過(guo)程中的宏(hong)觀(guan)(guan)偏(pian)(pian)析現象進行了大量(liang)系統的研究(jiu)。Flemings研究(jiu)表明(ming)鑄錠中多(duo)種不同的宏(hong)觀(guan)(guan)偏(pian)(pian)析都可(ke)由凝固時的傳熱、流動和(he)傳質過(guo)程來定(ding)量(liang)描述(shu)，從(cong)而(er)為(wei)宏(hong)觀(guan)(guan)偏(pian)(pian)析的定(ding)量(liang)計(ji)算(suan)提供(gong)可(ke)能(neng)性(xing)，隨著(zhu)計(ji)算(suan)機計(ji)算(suan)能(neng)力迅(xun)猛(meng)提升，宏(hong)觀(guan)(guan)偏(pian)(pian)析的模(mo)(mo)擬(ni)(ni)計(ji)算(suan)得到了迅(xun)速發(fa)(fa)展，主要(yao)分(fen)為(wei)多(duo)區域法和(he)連續介質法等。

  對于高氮不(bu)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。