根據相(xiang)圖，多數合(he)金元(yuan)素(su)在(zai)固(gu)相(xiang)中(zhong)的(de)(de)溶(rong)(rong)解度(du)要低于液(ye)相(xiang)，因此(ci)在(zai)凝(ning)(ning)固(gu)過(guo)程中(zhong)溶(rong)(rong)質(zhi)(zhi)原子不斷被排出到液(ye)相(xiang)，這(zhe)種固(gu)液(ye)界面兩側(ce)溶(rong)(rong)質(zhi)(zhi)濃度(du)的(de)(de)差異導致合(he)金凝(ning)(ning)固(gu)后(hou)溶(rong)(rong)質(zhi)(zhi)元(yuan)素(su)成(cheng)分(fen)(fen)不均勻(yun)(yun)性(xing)(xing)，稱作偏析(xi)(xi)。溶(rong)(rong)質(zhi)(zhi)元(yuan)素(su)分(fen)(fen)布不均勻(yun)(yun)性(xing)(xing)發生在(zai)微觀結(jie)構(gou)形成(cheng)范(fan)圍內（有10~100μm的(de)(de)樹狀(zhuang)枝晶），此(ci)時為(wei)(wei)微觀偏析(xi)(xi)。溶(rong)(rong)質(zhi)(zhi)元(yuan)素(su)通過(guo)對流傳質(zhi)(zhi)等(deng)質(zhi)(zhi)量傳輸，將(jiang)導致大范(fan)圍內成(cheng)分(fen)(fen)不均勻(yun)(yun)性(xing)(xing)，即(ji)形成(cheng)了(le)宏(hong)觀偏析(xi)(xi)。宏(hong)觀偏析(xi)(xi)可(ke)以認(ren)為(wei)(wei)是(shi)由凝(ning)(ning)固(gu)過(guo)程中(zhong)液(ye)體(ti)和固(gu)體(ti)相(xiang)對運動和溶(rong)(rong)質(zhi)(zhi)再分(fen)(fen)配(pei)過(guo)程共同(tong)導致的(de)(de)。此(ci)外，在(zai)凝(ning)(ning)固(gu)早期所形成(cheng)的(de)(de)固(gu)體(ti)相(xiang)或(huo)非金屬(shu)夾雜的(de)(de)漂浮和下沉(chen)也會造成(cheng)宏(hong)觀偏析(xi)(xi)。一般認(ren)為(wei)(wei)在(zai)合(he)金鑄(zhu)件(jian)或(huo)鑄(zhu)錠(ding)內，從(cong)幾(ji)(ji)毫米(mi)到幾(ji)(ji)厘米(mi)甚至幾(ji)(ji)米(mi)范(fan)圍內濃度(du)變化為(wei)(wei)宏(hong)觀偏析(xi)(xi)。因為(wei)(wei)溶(rong)(rong)質(zhi)(zhi)在(zai)固(gu)態中(zhong)的(de)(de)擴散系(xi)數很(hen)(hen)低，而成(cheng)分(fen)(fen)不均勻(yun)(yun)性(xing)(xing)范(fan)圍又(you)很(hen)(hen)大，所以在(zai)凝(ning)(ning)固(gu)完(wan)成(cheng)后(hou)，宏(hong)觀偏析(xi)(xi)很(hen)(hen)難通過(guo)加工處理來(lai)消(xiao)除，因此(ci)抑制宏(hong)觀偏析(xi)(xi)的(de)(de)產生主要是(shi)對工藝(yi)參數進行優化，如(ru)控制合(he)金成(cheng)分(fen)(fen)、施(shi)加外力場（磁場等(deng)）、優化鑄(zhu)錠(ding)幾(ji)(ji)何(he)形狀(zhuang)、適當加大冷(leng)卻速率等(deng)。

  宏(hong)觀偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是(shi)大范圍(wei)內的(de)(de)(de)(de)(de)(de)(de)成(cheng)(cheng)分(fen)不均(jun)勻現(xian)象，按(an)其表現(xian)形式可分(fen)為(wei)正偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)、反(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)和(he)比(bi)重(zhong)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)等。①. 正偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：對平衡分(fen)配系數o<1的(de)(de)(de)(de)(de)(de)(de)合金系鑄(zhu)(zhu)(zhu)錠(ding)(ding)先凝固(gu)的(de)(de)(de)(de)(de)(de)(de)部分(fen)，其溶質(zhi)(zhi)含量低于(yu)(yu)(yu)后凝固(gu)的(de)(de)(de)(de)(de)(de)(de)部分(fen)。對ko>1的(de)(de)(de)(de)(de)(de)(de)合金系則正好相(xiang)反(fan)，其偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)程度與(yu)凝固(gu)速率、液體對流(liu)(liu)以及溶質(zhi)(zhi)擴散等條件有(you)關。②. 反(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：在ko<1的(de)(de)(de)(de)(de)(de)(de)合金鑄(zhu)(zhu)(zhu)錠(ding)(ding)中(zhong)，其外(wai)層(ceng)溶質(zhi)(zhi)元素(su)高于(yu)(yu)(yu)內部，和(he)正偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)相(xiang)反(fan)，故稱為(wei)反(fan)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)。③. 比(bi)重(zhong)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)：是(shi)由(you)合金凝固(gu)時形成(cheng)(cheng)的(de)(de)(de)(de)(de)(de)(de)初(chu)晶(jing)(jing)相(xiang)和(he)溶液之間(jian)的(de)(de)(de)(de)(de)(de)(de)比(bi)重(zhong)顯(xian)著差(cha)別引(yin)起(qi)的(de)(de)(de)(de)(de)(de)(de)一種宏(hong)觀偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)，主要存在于(yu)(yu)(yu)共晶(jing)(jing)系和(he)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)晶(jing)(jing)系合金中(zhong)。如圖2-49所(suo)(suo)(suo)示，由(you)于(yu)(yu)(yu)溶質(zhi)(zhi)元素(su)濃度相(xiang)對低的(de)(de)(de)(de)(de)(de)(de)等軸(zhou)晶(jing)(jing)沉積(ji)導(dao)(dao)致(zhi)在鑄(zhu)(zhu)(zhu)錠(ding)(ding)的(de)(de)(de)(de)(de)(de)(de)底部出現(xian)負偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)；由(you)于(yu)(yu)(yu)浮力和(he)在凝固(gu)的(de)(de)(de)(de)(de)(de)(de)最后階段收縮(suo)所(suo)(suo)(suo)引(yin)起(qi)的(de)(de)(de)(de)(de)(de)(de)晶(jing)(jing)間(jian)流(liu)(liu)動(dong)(dong)，在頂(ding)部會出現(xian)很嚴重(zhong)的(de)(de)(de)(de)(de)(de)(de)正偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)（頂(ding)部偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)）。A型偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)是(shi)溶質(zhi)(zhi)富集的(de)(de)(de)(de)(de)(de)(de)等軸(zhou)晶(jing)(jing)帶(dai)(dai)，由(you)溶質(zhi)(zhi)受浮力作用流(liu)(liu)動(dong)(dong)穿(chuan)過柱狀(zhuang)晶(jing)(jing)區，其方向與(yu)等溫線(xian)移動(dong)(dong)速度方向一致(zhi)但速率更快所(suo)(suo)(suo)導(dao)(dao)致(zhi)。A型偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)形狀(zhuang)與(yu)流(liu)(liu)動(dong)(dong)類型有(you)關。V型偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)位(wei)于(yu)(yu)(yu)鑄(zhu)(zhu)(zhu)錠(ding)(ding)中(zhong)心，源于(yu)(yu)(yu)中(zhong)心形成(cheng)(cheng)等軸(zhou)晶(jing)(jing)區和(he)容易(yi)斷裂的(de)(de)(de)(de)(de)(de)(de)連接疏松(song)的(de)(de)(de)(de)(de)(de)(de)網狀(zhuang)物(wu)的(de)(de)(de)(de)(de)(de)(de)形成(cheng)(cheng)，之后裂紋沿(yan)切應(ying)力面展開為(wei)V型，并且充滿了(le)富集元素(su)的(de)(de)(de)(de)(de)(de)(de)液相(xiang)。而沿(yan)鑄(zhu)(zhu)(zhu)錠(ding)(ding)側壁(bi)分(fen)布的(de)(de)(de)(de)(de)(de)(de)帶(dai)(dai)狀(zhuang)偏(pian)(pian)(pian)(pian)(pian)(pian)(pian)(pian)析(xi)(xi)(xi)則是(shi)由(you)凝固(gu)過程初(chu)期的(de)(de)(de)(de)(de)(de)(de)不穩(wen)定傳熱和(he)流(liu)(liu)動(dong)(dong)導(dao)(dao)致(zhi)的(de)(de)(de)(de)(de)(de)(de)。

  對(dui)于宏(hong)(hong)觀偏(pian)(pian)析(xi)的(de)研究主(zhu)要(yao)有(you)實(shi)驗(yan)檢測和(he)模擬計(ji)算(suan)(suan)(suan)兩(liang)種手段。實(shi)驗(yan)檢測包括(kuo)硫印檢驗(yan)法(fa)、原(yuan)位(wei)分(fen)(fen)析(xi)法(fa)、火(huo)花放電原(yuan)子發(fa)射光譜(pu)法(fa)、鉆孔(kong)取樣法(fa)以及(ji)化學分(fen)(fen)析(xi)法(fa)等(deng)(deng)。模擬計(ji)算(suan)(suan)(suan)是(shi)通過數(shu)值(zhi)求解能(neng)(neng)量(liang)(liang)、動量(liang)(liang)以及(ji)溶質(zhi)(zhi)傳(chuan)輸等(deng)(deng)數(shu)學模型，進(jin)而探討(tao)元素成分(fen)(fen)不(bu)(bu)均勻性(xing)的(de)方法(fa)；進(jin)入20世(shi)紀后，人們對(dui)凝(ning)固過程中(zhong)的(de)宏(hong)(hong)觀偏(pian)(pian)析(xi)現象進(jin)行了大(da)量(liang)(liang)系統的(de)研究。Flemings研究表明鑄錠中(zhong)多(duo)種不(bu)(bu)同的(de)宏(hong)(hong)觀偏(pian)(pian)析(xi)都可(ke)(ke)由凝(ning)固時的(de)傳(chuan)熱(re)、流動和(he)傳(chuan)質(zhi)(zhi)過程來定量(liang)(liang)描述，從而為宏(hong)(hong)觀偏(pian)(pian)析(xi)的(de)定量(liang)(liang)計(ji)算(suan)(suan)(suan)提(ti)供(gong)可(ke)(ke)能(neng)(neng)性(xing)，隨著計(ji)算(suan)(suan)(suan)機計(ji)算(suan)(suan)(suan)能(neng)(neng)力迅猛提(ti)升(sheng)，宏(hong)(hong)觀偏(pian)(pian)析(xi)的(de)模擬計(ji)算(suan)(suan)(suan)得(de)到了迅速發(fa)展，主(zhu)要(yao)分(fen)(fen)為多(duo)區域法(fa)和(he)連(lian)續介質(zhi)(zhi)法(fa)等(deng)(deng)。

  對于高氮不銹(xiu)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。